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Scientific Reports Volume 11, Article Number: 18442 (2021) Cite this article

The Abu Gabra and Bentiu formations are widely distributed in the Muglad Basin. Recently, the Abu Gabra Formation, a proven reservoir in the Muglad Basin, has been studied, evaluated, and characterized. However, there are few research records on the Bentiu Formation, which is the main oil and gas reservoir in the basin. Therefore, 33 core samples from the Great Moga and Keyi oil fields (NE Muglad Basin) were selected to use sedimentology and petrophysical analysis to characterize the reservoirs of the Bentiu Formation. The purpose of this research is to reduce the risk of exploration activities and increase the success rate. Composition and structural analysis revealed two main facies groups: coarse to medium-grained sandstone (braided channel sediments) and fine-grained sandstone (flood flats and fractured fan channel sediments). The porosity and permeability of coarse to medium-grained sandstone are in the range of 19.6% to 32.0% and 1825.6 mD to 8358.0 mD, respectively. On the other hand, the fine-grained clay-rich phase shows poor reservoir quality, with porosity and permeability of 1.0 to 6.0% and 2.5 to 10.0 mD, respectively. Many different processes have been identified to control the reservoir quality of the studied samples. The dissolution of feldspar and mica increases porosity and permeability, while the presence of detrital clay, kaolinite precipitation, iron oxide precipitation, siderite, overgrowth of quartz and pyrite cements negatively affects the quality of the reservoir. The intensity of the observed overgrowth of quartz increases as the depth of burial increases. At very deep places, changes in particle contact patterns were recorded, indicating the existence of moderate to high compaction conditions. In addition, scanning electron microscopy revealed the presence of micropores, which have a tendency to affect fluid flow characteristics in the sandstones of the Bentiu Formation. These evidences indicate that the quality of Bentiu Formation reservoirs is mainly inhibited by particle size, total clay content, compaction and cementation. Therefore, special attention should be paid to these inhibitory factors to reduce the risk of oil exploration in this area.

Damojia Oilfield and Keyi Oilfield in Fula Basin are located in the northeastern part of Muglad Basin (see Figure 1). The Fula Basin is about 120 kilometers long and 40 kilometers wide. The Muglad Basin covers an area of ​​about 120,000 square kilometers, covering the Republic of Sudan and South Sudan (Figure 1). It is considered to be the main oil and gas producing areas 1, 2, and 3 of the two countries. The Muglad petroliferous basin is an intracontinental rifted basin. Its formation, characteristics and oil and gas potential represent the opening of the South Atlantic during the Early Cretaceous 1, 2, 3, 4.

Shows the Great Moga and Keyi oil fields as well as the major oil fields that have been discovered, the main faults and block boundaries, and the location map of the Fula sub-basin in the Muglad Basin of Sudan (using CorelDRAW Graphics Suite 2018 v20.0.0.633 according to References 1,2 The information was created at https://www.corel.com/cn).

The Abu Gabra and Bentiu formations (Figures 2 and 3) are widely distributed in the Muglad Basin as source rocks and reservoirs 4 and 5, respectively. The source rock characteristics and hydrocarbon generation potential of the Abu Gabra Formation have been studied 1, 2, 6, 7, and 8. Like other Cretaceous source rocks in the world, especially the Persian Gulf in the Middle East, the Mediterranean Sea, the U.S. Gulf Coast and China, they have been considered to have relatively good oil and gas prospects, with Type I to Type III kerogen1,2,3 ,4. Authors 1, 6, 7, 8 report that the Lower Cretaceous shale and claystone of the Abu Gabra Formation are the main source rocks in the Muglad Basin. Their main feature is Type I kerogen, which has very good hydrocarbon generation potential. Thermal maturity assessments determined using vitrinite reflectance, biomarker parameters, pyrolysis gas chromatography, and production index (PI) data indicate that the source rock has reached the oil generation window 1, 2, 8, 9. In addition, a comprehensive study of trace/major elements and biomarkers revealed deposition conditions from hypoxia to hypoxia, which is conducive to the preservation of organic matter in the Abu Gabra formation8,9. In addition to the above research, the burial history and oil generation model of the Abu Gabra formation were also carried out10.

The main stratigraphic column of the Fula Basin. It compares the Late Jurassic/Early Cretaceous-Quaternary sedimentary sequence with 4 sequences separated by unconformities (I-IV) (using CorelDRAW Graphics Suite 2018 v20.0.0.633 https:// www.corel.com was created based on the information in References 1,2 /cn).

The elements and processes of the oil and gas system in the Fula Basin have significant oil and gas enrichment. (Created using CorelDRAW Graphics Suite 2018 v20.0.0.633 https://www.corel.com/cn based on the information in Reference 11).

However, although the Bentiu Formation is the main oil and gas reservoir in the Muglad Basin, little research has been done on it. Previous studies on this formation mainly focused on its depositional environment and general reservoir properties, without a thorough investigation of its mineral composition and diagenetic reservoir quality12, 13, 14. On a global scale, a variety of methods have been used to obtain the reservoir quality, sedimentary modeling and diagenesis of world-class reservoirs to optimize their potential4,13,14,15,16,17. Among them, it is well known that sedimentology and diagenesis are the keys to obtaining reservoir physical properties14,15,16,17. Therefore, more investigations on these aspects are needed to guide future oil and gas exploration in the Muglad Basin. To this end, based on available data, this study used samples from the Great Moga and Keyi oil fields in the Fula sub-basin (northeast of the Muglad Basin).

In the Fula Basin, the clayy shale of the Abu Gabra Formation is a very good source rock. Abu Gabra sandstone, thick Bentiu sandstone, and Aradeiba, Zarga, and Ghazal sandstones (Darfur Group) are the main reservoir rocks, while claystones in the Darfur Group are regional seals3,11,18. Therefore, the oil system in the Fula Basin is considered to be effective and can preserve a large amount of oil and gas accumulation (Figure 3). The purpose of this study is to emphasize the diagenesis process that controls the quality of the Bentiu formation. To achieve this goal, sedimentology, petrology, and petrophysical analysis were performed on selected core samples. These involve detailed sedimentology and petrographic description of the core interval, explaining the facies types and their associations to determine their depositional environment. In addition, using scanning electron microscopy (SEM) can better understand the different pore types. The influence of diagenesis on reservoir quality is studied to determine the main control of porosity and permeability on the studied reservoir facies. Other important information provided in this study includes basin evolution and sediment transport process. In addition, heavy mineral analysis was performed on selected core samples. The results of this study can be used to improve the chances of success in exploration activities in this area and other similar areas in the world.

The Late Jurassic marked the beginning of the Sudan Rift, which was caused by the movement of the right side of the Central African Shear Zone (CASZ) during the opening of the South Atlantic2,11,18. CASZ forks north-central Africa into the Bornu Basin through the Benue Trough in Nigeria to Sudan. The six major sedimentary basins in Sudan (Mugrad, Bagara, Melut, Khartoum, Blue Nile and Atbara) were shifted due to this event2,11,18. The Sudan Basin and the Benue Trough, Bongor, Doba and Ngaoundere Basins are similar in sediment distribution and evolution2,11,18. It is the product of similar tectonic events; each of the basins mentioned exhibits some unique characteristics, which are the result of local influences.

The Muglad Basin is one of the largest rift basins in North Africa. It is composed of a large number of oil accumulations represented by the Fula, Heglig and Unity oil fields2,11,19. Seismic evidence indicates that the maximum sediment thickness in the Muglad Basin is approximately 13 square kilometers. The stratum of the Fula Basin is shown in Figure 2. It is composed of about 8,200 kilometers of Late Jurassic/Early Cretaceous-Quaternary non-marine sedimentary sequence. According to seismic interpretation and drilling data, the stratigraphic column is subdivided into four series, (I-IV; Figure 2) bounded by unconformity planes 2, 3, 11, and 20. In the Fula Basin, sediments cover quartzite, schist, granitic and granodiorite gneiss (Figure 2). The age of these granitic and granodiorite gneisses is 540 ± 40 Ma, using the isotope method4. Due to the dextrorotational dynamics of CASZ5, three rift stages have been identified in the Fula sub-basin. The rift began to occur in the Early Cretaceous to the Albian period, leading to the deposition of n2, 3, and 20 in the Abu Gabra and Bentiu formations. The Abu Gabra Formation is composed of thick shale, sandstone, and claystone units. It is mainly deposited in a hypoxic to anoxic lake environment that receives a large amount of algae, bacteria and amorphous organic matter1,4. The Abu Gabra group has high total organic carbon (average 4.5 wt.%) and hydrogen index (average 640 mg HC/g TOC), indicating type I kerogen 1,7,8. In this way, the Abu Gabra Formation may be an important oil source in the Bentiu Formation reservoir. From the early Albu period to the Cenomanian period, the Bentiu Formation was deposited in braided and meandering environments3,4. According to the previous geological report, we have the Lower Bentiu Formation and the Upper Bentiu Formation 10 and 12. The lower part is characterized by the interlayer of channel sandstone and floodplain that refines the upward sequence, while the upper part is composed of thick sandstone mixed with thin layers of mudstone 4, 10, 12, which is deposited in a braided river environment.

The second rift (Figure 2) occurred from the Late Cynormanian to the Tulunian. During this period, the Darfur Group deposited 4,19, represented by 4 strata: Aradeiba, Zarga, Ghazal and Baraka formations, such as As shown in Figure 2. The Aradeiba Formation consists of large mudstones and thin sandstones. The Zarga formation coincides with Aradeiba, which was deposited by river-delta channels in a lake environment. It is identified by the upward transition from sandstone (basement) to mudstone (top) units. The Ghazal Formation has similar lithological characteristics to the Zarga Formation and is also composed of sandstone and mudstone sequences4. The Baraka Formation is the top layer of the Darfur Group, composed of sandstone and thin mudstone, deposited in the river to alluvial fan environment4,5.

At the end of the second rift, the Early Paleogene Amal Formation, which mainly contains medium-coarse sandstone sequences, was deposited as braided flows and alluvial fans4. The third rift valley formed the Kordofan Group from the late Eocene to the Miocene. The Zeraf formation is unconformably covered by thick sandstone mixed with thin clay on the Kordofan Group. The sediments of this formation are the products of braided flow and alluvial fans 3, 4, and 5.

Sedimentology and lithofacies were analyzed on 5 cores (core-1-core-5) of Moga 6 and 4 cores (core-1-core-4) of Moga 26. A total of 33 samples were selected for lithofacies research. Table 1 shows a summary of the analyzed samples, their depth intervals, facies types, analyses performed and the total length of these cores. Five sedimentological techniques (see Table 1) were used to study core samples. The following subsections briefly explain the procedures for combining techniques:

Lithofacies determination was carried out using core samples of 6.73 m, 10.79 m and 13.52 from Well Moga 62, Well Moga 6 and Well Keyi 4, respectively. These were analyzed by sedimentology and the core gamma was interpolated at a vertical scale of 1:20 (Figure 4). The number of cores and the depth interval are shown in Table 1. The lithological characteristics of the core are checked by macro (naked eye) and micro (handheld lens and binocular microscope). Rock types are classified according to their specific characteristics (such as mudstone, sandstone), and further described according to their color, composition, particle size distribution, sedimentary structure and texture. The interpretation of the sedimentary environment is based on the observation and thorough evaluation of all available evidence (for example, the interpretation of the log curves of the wells studied).

The logging responses of the Bentiu Formation in the Great Moga (a) and Keyi (b) fields show the results of wireline logging and lithology (sandstone/claystone) in the Bentiu Formation. A set of measurements includes penetration rate (ROP), gamma ray (GR) and D index (Dxc). The gamma ray response distinguishes the low gamma ray value of sand from the high gamma ray value of shale. The next column (depth trace) indicates the depth at which the measurement was made.

A total of 16 core samples, mainly from different facies types, were used for particle size analysis (Table 1). Before sieving analysis, two preparation methods were applied in order to break down the sandstone sample into individual particles without destroying or destroying them. The instrumental decomposition of the sample is carried out by adding hydrogen peroxide solution (30% w/v). The sample was then immersed in water and carefully mortared in a porcelain mortar and shaken with a shaker for about 48 hours. After that, the samples were dried at 40°C for about 24 hours. The samples were then completely decomposed and dried in order to perform mechanical wet sieving analysis on them. During the sieving analysis, 40 g samples were weighed and analyzed by OCTAGON digital (00363) at amplitude 4 and intermittent instrument shaking for 15 minutes. With the help of brushes and ultrasound, the size of each particle is collected from the sieve into a porcelain dish. These fractions are dried at 40º C. Record their weights after they are completely dry (Table 1).

In addition, 33 thin slices were made from selected core samples for lithofacies description (see Table 1). In order to identify the porosity and analyze the carbonate minerals, during the preparation of the flakes, the samples were vacuum impregnated with blue resin and stained with alizarin red-S and potassium ferric cyanide. The sample was further stained with some sodium cobalt nitrate solution to help identify alkaline feldspar. Observe the slices at different magnifications under a polarizing microscope. The optical properties of minerals such as color, shape, relief, pleochroism, extinction angle, birefringence, and twinning are used as the basis for identifying minerals. Use PETRLOG to count points to determine the percentage of minerals in each slide. Each slice is counted (250-300 points), and a summary of the petrographic results is provided according to the Dott classification scheme of the rock.

In this study, the scanning electron microscope (SEM) of the Bentiu group was involved. Before using sputtered aluminum tape to fix the sample to a standard aluminum SEM stub, treat the sample with cold chloroform to remove any hydrocarbon residues. This experiment focuses on the pore geometry, composition and morphology of the main filling authigenic minerals. The results of the SEM analysis are included in the "Clay Mineral Analysis" section. According to the method introduced by Moore and Reynolds, 20 clay parts smaller than 2 (< 2) microns were examined under the X-ray diffraction (XRD) method. The explanation is made by comparing the observations of this study with previous studies3,22. 11 core samples were analyzed for heavy minerals (Table 1). This follows the recognition technology of Hubert23. Using the techniques described by Jones and Roszelle14 and Makeen et al.18, the relative porosity and oil-water permeability are calculated as a function of water saturation.

Observations of conventional cores (such as Figure 5) reveal eight types of lithofacies. The total thickness, percentage and simple explanation of these lithofacies are shown in Table 2. All phases identified here follow the Miall classification scheme 24, 25, 26. The detailed description of these lithofacies is as follows:

Shows the conventional core photos of different wells formed by the studied cores. Eight (8) different major lithofacies types have been identified from these photos (Tables 2 and 4).

Massive sandstone (light gray to light gray) is the most important facies type (larger number) observed in all research wells (Table 2). These phases are locally huge and have weak bedding planes. The particles are very coarse, low to medium-low (vcL-mL), sub-round to round (SR-R), with medium to well sorted particles (Table 3). The porosity in these phases varies from good to very good (19.6% to 32%). These facies are thought to be formed by very rapid sedimentation, such as high-emission processes, such as flaky floods.

These sandstones consist of a grain size of 0.7 to 1.0 mm, and are usually light gray-grey, fine-down to fine-up (fL-fu) corrugated layered sandstone, with a sub-circular to circular shape (Table 4). The facies were only detected in a few cores, and the percentage did not exceed 8.43%a (Table 2). In addition, the grains are well sorted. The corrugated structure observed in these phases indicates wave/current events during low-fluid deposition deposition. Therefore, corrugated layered sandstone can be interpreted as braided channel sediments, and corrugated-marked siltstone can be interpreted as embankment sediments3,26.

A few cores present massive to massive mudstone facies with a total thickness of 0.92 m (Table 2). They appear in the form of grey-dark grey massive lumps of mudstone. These facies can be interpreted as superbank sediments in the river environment3,25.

All well cores studied have fine-bedded sandstone facies (Table 2). The color is gray to dark gray. This type of facies is very common in the high-bank area and is represented by sediments from weak traction currents, which conforms to the Miall facies classification24,25.

This phase is well distributed in the Moga and Keyi regions, and the number is considerable (Table 2). It is composed of medium-grained (mL to mU) sandstone. The color varies from gray to grayish-yellow, with round and well-sorted grains. Although it has a very good porosity (23.7% on average), some clay matrix, a small amount of carbonate and a small amount of iron oxide appear as cementing materials, which may affect the quality of its reservoir. This facies is interpreted as tongue-shaped, lateral dam or sand wave channelized sediments in the environment of river systems or delta estuarine dams.

This grey very coarse grained facies (vcU) only exists in Moga 26, with a total thickness of 0.53 m (Table 2). The clastics are sub-angular-nearly round, poor to moderately sorted particles, embedded in the silty mudstone matrix. From the point of view of its particle composition, size and shape, this phase is interpreted as a product of higher energy channels or delta estuary dams.

These phases are mostly seen in the Keyi area, with a total thickness of about 2.04 m (Table 2). It is a gray to pale yellow fine-grained sandstone (fL–fU; 0.09 to 1.0 mm). The particles are round, well sorted, and have high porosity (average 26.5%). Some siliceous cement was observed, while the oil appeared to exist as scattered points. These facies are interpreted as the products of scouring fillings, scouring dunes or anti-dunes deposited in river channels or delta diversion channels.

These phases also exist in the Keyi area, and the total thickness does not exceed 0.26 m (Table 2). It is a gray to light yellow fine-grained sandstone (fU). The particles deposited under river systems or delta estuaries are well classified and round to round sediments.

Particle size analysis was performed on selected core samples to determine potential sandstone intervals, particle size distribution and characteristics. Particle size analysis is also a supporting tool for phase analysis. In addition to the cumulative weight percentage graph, representative particle size and sieve analysis data are shown in Figure 6 and Table 3.

Representative, particle size and sieve analysis histograms show cumulative weight percentage and particle size distribution.

Particle size analysis showed that all the studied samples contained sand grains less than 0.07 mm-1.00 mm. Small amounts of silt and clay size (<0.063 mm) are present in all the samples studied. In addition, the grain size range between more than 500Im and less than 90Im (<0.07->0.5mm) represents the main components in all the studied samples. The particle size analysis showed that the sorting degree of the two analysis samples was medium, while the remaining sorting situation was poor. Table 4 summarizes the statistical data of the analyzed samples according to the phase type and correlation. In addition, the lithofacies analysis of the sandstone facies types in the cored interval by plane polarized light microscope and scanning electron microscope can divide these lithofacies into feldspar sandstone and subfeldspar sandstone (Table 5). The detailed particle size study includes particle size, sorting, shape, sorting, and accumulation described in the phase characteristics and discussion section.

Thin sections and SEM results are provided here. The main purpose of thin section analysis is to establish the classification of sandstone types by mineral composition, thereby revealing important facts about the oil reservoir source area and paleoenvironment; paleoclimate, diagenesis and tectonic history of the investigated area. On the other hand, engineers and geologists use SEM extensively to assist in information about the pore geometry of reservoir rocks.

From the glass slides, the following components of the rock were determined: detrital particles such as quartz, feldspar, mica (mainly muscovite and biotite), opacity and heavy minerals. Self-generating components, such as carbonates, overgrowth of quartz, siderite cements, Fe oxides, and authigenic clays. The minerals other than the self-produced component are detailed below.

This is the most abundant sandstone studied so far. In the flakes, the quartz looks clear or colorless, has weak birefringence, and has a lower refractive index, which is slightly higher than the mounting medium 26, 27, and 28. Single crystal quartz (Qm) and polycrystalline (Qp) were observed in most samples (for example, Figure 7a-f). Some identified quartz crystals show irregular extinction patterns. Most quartz particles contain iron oxide and heavy minerals (e.g. Figure 7a-f). The sandstone analyzed has a high percentage of single crystal quartz, ranging from 12.3% to 41.0% (Table 5). In most samples examined, the percentage of polycrystalline quartz is lower than that of single crystal quartz; the value of polycrystalline quartz is between 5.5% and 30.4%. Most quartz crystals exhibit roundness from sub-circular to sub-angular, and some are sub-circular to round or sub-angular to angular (Table 4). In addition, the quartz particles in the studied sandstone are mainly sorted from medium to good (20 samples), and seldom sorted (7 samples), medium to differential (2 samples), and differential (4 samples). Samples). This may be due to the shorter transport distance and the inspection of sandstone based on particle shape and particle roundness that experienced more wear.

(af); Massive sandstone (Sm) is characterized by medium to coarse-grained sandstone (average mU-cL), with the following properties, sorting, etc.; sub-round to sub-corner; flaky cementation is also moderately compacted; with A few long-grain contact bumps. Common polycrystalline quartz plus single crystal quartz contains a large amount of potassium feldspar (mainly orthoclase shows partial dissolution), a certain amount of plagioclase, a small amount of mica and heavy minerals (HM), which appear in the form of free particles (af); Small amounts of iron oxide appear as pore fillers, siderite cement points, and less quartz overgrowth (the euhedral crystals around the detrital quartz particles terminate). (C); The interconnection of the pores between primary particles (PBP, impregnated with blue dyed resin), secondary particle porosity (SWP) and secondary particle porosity (SBP) is very good, usually through potassium Part of the feldspar is dissolved. (e); Some pores filled with detrital clay.

Like quartz, the feldspar observed in the flakes is transparent, colorless, and has low birefringence, but is different from quartz due to its cleavage, twinning, and refractive index. However, distinguishing between unwound orthoclase and quartz can be very challenging. But in glass slides stained with sodium cobalt nitrate solution, the distinction between the two becomes much easier. In addition, compared with quartz particles, feldspar particles may be partially decomposed and appear turbid or turbid, and quartz particles always remain clear and unchanged3,28.

Among the feldspars, K-feldspar (Fk) is mainly orthoclase (Or) and anorthite (Pe), which are ubiquitous in the sandstones analyzed, and the relative abundance is between 6.4-15.0% (Table 5) . Plagioclase (PI) feldspar is found in a few samples (Figure 7a, b), and its percentage is between 1.4% and 6.4% (Table 5). The two reasons why potassium feldspar is higher than plagioclase in the sample are as follows: potassium feldspar is more stable than plagioclase, and the less stable plagioclase will change; more importantly, in continental basement rocks, potassium Feldspar minerals are more abundant than plagioclase, namely acid gneiss29. Therefore, the continental basement is considered to be the origin of many sandstones in the research area.

The flake minerals that have been determined to be extinct in parallel include muscovite (Mu) and biotite (Bi), as shown in Figure 7a and b. Muscovite is colorless under plane-polarized light, and displays second-order color under cross-polarized light. On the other hand, biotite exhibits brown to green pleochroism, which masks the interference color 27. In all the analyzed samples, the content of mica is very low. The observed biotite exists in the form of large clastic flakes along the boundary, laminae or bedding plane. A high percentage of 6.8% was recorded at a depth of 899.83 m (Table 5). Their distribution as detrital particles is controlled by sorting, which is largely caused by the hydraulic behavior of mica flakes. In addition, very few mica is crenated and may bend (Figure 7b). The noticed parallel arrangement of mica flakes in the study area indicates moderate compaction.

These auxiliary debris components are present in trace amounts (trace amounts; Table 5). The opaque ones are mainly hematite and pyrite, with very fine medium grain size and medium abrasion.

Detrital clays were identified in all the samples studied, and their abundances ranged from 0.5% to 45.0% (Table 5). XRD analysis shows that most of the interstitial clays are kaolinite and chlorite (Table 6).

These mainly include siderite (Sid) and calcite, which form punctate cements in the samples studied. Sometimes the siderite concentration can reach 21.0% (see Table 5; Well Moga 26; 807.20 m), while calcite only appears at a depth above 1510.55 m (Table 5; Well Keyi 4). Siderites can be identified by their brown staining (under the microscope) along the grain edges towards the cleavage cracks, sometimes presenting a secondary interference color of gray or white (Figure 7d).

At a depth of 902.90 m (well Moga 6), authigenic pyrite was discovered as a cementing and displacer, and it appeared in large numbers (Table 5). However, its percentage may reach 23.6%. During the core description process, pyrite nodules were recorded as cement fillings in the pores. SEM analysis showed the presence of plate-like aggregates of subcubic to cubic pyrite as microcrystals.

A small amount (0.1% to 1.8%) of syntactic quartz overgrowth was observed in all samples analyzed (Table 5). Thin sections and SEM observations show that there is a well-developed euhedral, depilated, funnel-shaped quartz surrounded by kaolinite overgrowth.

Although iron oxides did not develop well in the samples studied, they exist as cementing materials (e.g. Fig. 7e, f). Therefore, when recorded at a depth of 800.25 m (Moga 26; Table 5), their percentage may reach 3.9%.

Use XRD and Scanning Electron Microscope (SEM) analysis to study clay minerals. The characteristics of each SEM sample are further shown by two photomicrographs of the inspected sample (for example, Figure 8). This is done to clarify the diagenesis of clay minerals on the quality of the reservoir. Table 6 shows the XRD analysis results, which are further explained as follows:

(ad) SEM results show kaolinite and chlorite minerals. The kaolinite fragments partially filled some pores and partially covered some detrital particles (a: G9 and 11b: D14-15), indicating micro-pores. Well-crystallized pore-filled kaolinite brochure (a: G9), showing the worm-like texture (a: C4) of partially corroded pseudo-hexagonal base plates (a: E-F7 and d: F/G 14-15) , G3-4, J11) are observed. A small amount of poorly crystallized banded illite (b, D-E12-13, F-G2) partially replaced the clay matrix. Some disc-shaped clusters of chlorite slabs (d: Hi 6-7) were also observed.

Kaolinite is diagnosed by the 7.1 Å and 3.58 Å peaks in the dry XRD pattern. These peaks did not change when treated with ethylene glycol, although the high temperature was completely removed after heating to 550 °C 30, 31, 32. Lingshi. The range of kaolinite content observed in all samples examined was between 47.5-93.5% (Table 6). Kaolinite is mainly formed in the surface environment through a series of soil-forming events. It may also come from a lake environment as a metamorphic product of potassium feldspar in pore water rich in acidic organic matter3,31. More importantly, the hydrothermally induced alteration of aluminosilicates (such as feldspar) may still produce the mineral kaolinite30. More commonly, due to the effective consumption of cations and the availability of H ions, detrital kaolin minerals are formed33,34. In this case, the source area must have strong leaching (this requires sufficient rainfall, permeable rocks and suitable topography). Therefore, it will accelerate the consumption of calcium, magnesium, sodium and potassium ions. In addition, tropical and subtropical climates are necessary for obtaining kaolinite.

The authigenic kaolinite is usually formed as the total alteration product of potassium feldspar in the organic-rich layer, which can be obtained in most deep research samples. However, other clay minerals remain in the analyzed interval as debris obtained through the hydrolysis process. In addition, the relatively flat peaks of kaolinite observed in the SEM image confirmed the presence of such debris of kaolinite (Figure 8). It was observed that kaolinite partially filled the pores and partially covered some detrital particles (Figure 8a: G9 and 8b: D14-15). Some appear in the form of well-crystallized pore-filled kaolinite brochures (Figure 8a: G9), showing the worm-like texture of partially corroded pseudo-hexagonal base plates (Figure 8a: E-F7 and d) (Figure 8a: C4) , G3-4, J11): F/G 14-15). Other detrital clay minerals appear in the form of poorly crystallized banded illite and clay matrix replacement (Figure 8b, D-E12-13, F-G2). Disc-shaped plate clusters of chlorite were also observed (Figure 8d: Hi 6-7).

Most of the samples studied showed no (0.00%) evidence of montmorillonite. However, for a few occurrences, the highest value (1.2%) was recorded at a depth of 802.80 m (Moga 26; Table 6). It is derived during the glycolization process, and the d-spacing of its basic reflection (001) extends from 15 Å in the normal mode to 18 Å30,31 in the glycol solvation mode. According to Weaver, “The climate and topographical conditions required for the formation of montmorillonite are basically the opposite of those favorable for the formation of kaolinite”34. Under poor drainage conditions, smectite is usually formed in low-relief areas, which will prevent the rapid removal of silica and alkaline earth metal ions such as K, Na, Ca 2 and Mg 2. In addition, smectite usually develops around areas with low temperature, low precipitation, and water inflow as the product of basic and ultrabasic rock weathering or its metamorphic equivalent. Therefore, it can be inferred that we have more kaolinite than montmorillonite.

Illite does not react with ethylene glycol or when heated. Its identification is based on its basic diffraction at about 10 Å. Nevertheless, the illite mineral may be as high as 50,31. The samples studied showed a small amount of illite like the montmorillonite mineral. Its highest concentration in the samples of this study is 4.9% and the depth is 802.80 m (Moga 26 well). According to Chamley, Moore and Reynolds30,31, illite usually contains more silica, magnesium and H2O than muscovite, but there are fewer aluminum tetrahedral layers and low-K interlayers. It mainly comes from acidic igneous rocks or their metamorphic equivalents, but rarely comes from basic rocks. Abnormal rainfall and hot weather conditions are conducive to the formation of illite as a detrital material. However, diagenesis facilitates the dissolution of muscovite by illite (which is enhanced in clean sand washing), and the replacement of feldspar with illite for kaolinite is triggered by ion enrichment.

The basic spacing of the clay mineral chlorite structural unit is close to 14 Å. When absorbed in water or glycol, pseudo-chlorite will swell like montmorillonite, even if it is heat-resistant, where the d-spacing remains constant at 14 Å and 7 Å for reflections 001 and 002, respectively. The content of chlorite in the tested samples was between 6.5% and 52.5% (Table 6). Chlorite minerals can be regarded as a 2:1 layer group with a hydroxide interlayer, or a 2:1:1 layer group. In structure, chlorite usually shows negatively charged trioctahedral mica layers, alternating with positively charged octahedral sheets regularly.

Chlorite is a common component of low-grade metamorphism. These are less common in igneous rocks, and they exist as hydrothermally driven by-products of iron-magnesium minerals. Chlorite in authigenic form can be directly evolved as a by-product of montmorillonite through the transformation of illite. In this process, the iron and magnesium released from the montmorillonite are utilized, spread more closely, and then re-precipitated together with the silicon provided from the montmorillonite or other detrital silicates. The abundant chlorite in the clay indicates that the source rock is rich in iron and magnesium minerals.

To what extent do sedimentology, sedimentary processes, grain structure characteristics, diagenetic processes, and burial depth control the quality of the Bentiu formation under study? The understanding of these processes is discussed below:

Obviously, sedimentary analysis including identification and characterization of lithology, lithofacies (type/group) and sedimentary structure plays an important role in reservoir quality. The detailed facies types and groups described in the "Results and Interpretation" section describe the characteristics of the facies combination and determine the paleo-sedimentary environment, thereby clarifying the evolution of the basin. This provides clues and targets for future oil and gas exploration.

The two main facies groups that can be identified based on their composition and vertical distribution are coarse-grained and/or massive to cross-bedded sandstone (facies Sm, Sxl, Sco, Sxt, Sh) and fine-grained/shale layered sandstone (facies, Sr, Fm, Fl) phase group (tables 2 and 5). The general vertical sequence, composition and internal sedimentary structure of the lithofacies in the studied core interval indicate that the deposit is in a small braided channel dam/channel filling facies combination. The immature structure (rich in quartz and feldspar) and the overall upward trend indicate that the high-energy river source channel sand has a high deposition rate and is dominated by bed-loaded transportation. Thin pebble to conglomerate layers (usually located at the bottom of the sand layer) are interpreted as lagging sediments in the channel. The Sco/Fm facies with relatively thick coarse-grained (Sm, Sxl, Sco, Sxt, Sh) sand facies are the main dam and channel sediments. Fine-grained sandy and clay-rich facies (Facies Sr, Fl), usually overlaid on a refined sequence, are interpreted as upper bank/channel waste facies sediments. As we all know, braided channel sediments are very good high-yield reservoirs. Their total net value may be higher than that of meandering reservoirs, and is characterized by much less interbedded shale layers35. These formations respond to the rapid deposition of large amounts of coarse deposits flowing in. More importantly, because the important reservoir quality (porosity and permeability) in sandstone is usually high, the grain size is large, and the clay volume is small, it is expected that oil and gas accumulations will occur in the study area36.

Regarding the type of facies studied as a function of reservoir potential, careful examination of its porosity and permeability (Figure 9) shows that there is a clear relationship between the quality of the reservoir and the various facies present in the core interval. The best reservoir quality is related to coarse-grained, moderately sorted and high-energy channel sandstone (facies Sxl, Sm, Sxt). The porosity and permeability of these phases are 19.6% to 32.0% and 1825.6 mD to 8358.0 mD, respectively. It must be pointed out that the local low porosity and permeability of these phases are related to patchy carbonate cementation. Poorly sorted conglomerate facies show lower permeability, while fine-grained and clay-rich facies (Facies Fl, Sr) reservoirs are of poor quality. The rich shale content and large amounts of carbonate and clay cements significantly reduce the porosity and permeability of these phases (because they may prevent the interconnection of pores). Therefore, the main feature of the facies (Fl and Sr) is the ineffective microporosity (2.0% to 6.0%) and therefore are considered non-reservoir.

A clear relationship between the quality of the reservoir and the various facies present in the core interval. The best reservoir quality is related to coarse-grained, moderately sorted and high-energy channel sandstones (facies Sxl, Sm, Sxt), while fine-grained and clay-rich facies (phases Fl, Sr) show poor Reservoir quality.

About 82% of the very coarse-grained massive sandstone facies ("Results and Interpretation" section; Table 2) in the studied cores show that the porosity (19.6% to 32.0%) and permeability (1825.6 mD) are very good/ Excellent to the 8358.0 mD) area. Other series are composed of sandstone and fine-grained mudstone facies. Mudstone facies mainly appear in the upper part of the research core. This is clearly shown in the refinement-up sequence of the studied core 1 (well Moga 26; Figure 5), where the sediments range from coarse-grained to medium-grained sandstone/mudstone. Therefore, the observed relationship between sandstone and some clay content reduces the permeability value, which means that the clay content has a major control over permeability36. Similarly, the sediment transport process may also control the changes in grain size and clay content in sandstone. The grain size of the studied sandstone is composed of coarser particles, which are the product of a very fast settling rate, which may be caused by a high flow event (see the "Particle Size Analysis" section). Therefore, the Bentiu formation is expected to have very good porosity and permeability. This is consistent with the grain size analysis, which shows that most of the analyzed phase samples have a coarse grain size with a small number (3 samples) of fine grain size.

The geometric properties of sedimentary facies (grain size, shape, sorting and accumulation) have a direct impact on their primary porosity and permeability. Although the porosity does not depend on the grain size, the permeability decreases as the grain size decreases, and the coarser the grains, the higher the porosity value3,36. The study area of ​​the Bentiu group showed a high content of coarse grains and few fine-to-medium-sized grains (see the "Grain Size Analysis" and "Sedimentology and Sedimentary Processes" sections). Similarly, the total clay content ranges from 0.5% to 14.8%, although some samples are different (31.2% to 45.0%). Sxl, Sm, and Sxt have more coarse particles than the Fl and Sr phases (both of which have a high total clay content). In addition, the porosity increases as the crystal grains change from angular grains to rounded grains. Generally, higher porosity values ​​are caused by highly unequal-axed particles3,27. The samples studied showed particles with small changes in circularity, most of which were sub-circular to sub-angular in shape (Figure 7a-f). Sub-round to round, sub-corner to sub-circular, and sub-corner to angular shapes are also shown in some examples. Equidistant particles are observed in samples dominated by very fine particles and high total clay content (Fl and Sr phases). As sorting increases, porosity and permeability are generally expected to increase. In poorly classified sediments, smaller particles (clay and very fine particles) can cause clogging of the clay between the larger particles, which is known to reduce porosity and permeability3,37,38. The sorting of the studied sandstone waits until it is good, and few samples are poorly sorted (Fl and Sr phases). On the other hand, due to the deformation and accumulation of particles, the porosity and permeability are further reduced. Loosely cemented particles usually indicate high porosity and less contact between particles3,38. Most of the sandstones studied have a low degree of compaction (Figure 7a-f). Similarly, in samples with higher clay content (Fl and Sr), particle-to-particle contact is more common and therefore has a lower porosity as shown by the sample.

Diagenesis may take the form of complex interactions, and these processes may occur simultaneously over a long period of time. It can sometimes explain the characteristic distributions and patterns found in sedimentary reservoirs18,39,40.

The diagenetic characteristics of the Bentiwu Formation may be affected by compaction (mechanical and chemical), cementation (clay, quartz, and carbonate), dissolution/replacement of unstable minerals (such as feldspar and clay), and autogeneity (kaolin). The influence of stone, chlorite, montmorillonite)/illite). This diagenetic analysis is guided by lithofacies characteristics, as revealed by thin section, XRD and SEM results, and supplemented by the grain structure and geological knowledge of the study area. In summary, lithofacies evidence reveals sub-angular-sub-circular particles, such as detrital quartz, indicating that early compaction before cementation dominates (Figure 7a-f). In addition, a large number of debris particles show the advantage of non-linearity, and there is almost no linear contact in the sample. In addition, the typical combination of early diagenetic events can be determined by analyzing the common overgrowth of a few quartz, some detrital kaolinite, chlorite and montmorillonite in the samples. In most cases, carbonate, pyrite, and a small amount of iron oxide cements were found to be surrounded by detrital quartz and partly in contact (Figure 7e, f). XRD analysis showed the presence of authigenic kaolinite, illite, montmorillonite and chlorite. Illite and pyrite cement are replaceable. In this area, as the burial depth increases, as the temperature increases, late compaction (pressure dissolution) may occur, which will definitely lead to early chemical compaction. Early dissolution will affect unstable minerals such as feldspar and clay minerals. The appearance of a small number of linear bump particle contacts indicates the pressure dissolution process or the compaction/dissolution in the intermediate burial stage. The samples studied showed very little overgrowth of quartz surrounded by kaolinite, sometimes protruding into the macropores (see Figure 7c&f). This indicates that the overgrowth of quartz is older than the enclosed kaolinite, which indicates that the quartz cement is resistant to dissolution in the middle diagenetic stage 41,42. Therefore, it is believed that the diagenetic characteristics of the Sandstone of the Bediu Formation have experienced complex diagenetic and shallow mesodiagenetic stages. This sequence is consistent with the diagenetic alteration model proposed in 43, 44, 45, and 46.

Detailed XRD analysis revealed four clay mineral compositions (kaolinite, illite, montmorillonite and chlorite). Chlorite and kaolinite exhibit two main types: detrital and autogenous mode. Chlorite is widely distributed in the samples studied, appearing in the form of disc-like clusters, pore fillers (from biomass), and sometimes found as castings around the debris particles. Detrital kaolinite is identified by its layered style, while authigenic kaolinite is kaolinite that partially fills the available pores or encapsulates quartz mineral particles. As a distinguishing feature, authigenic kaolinite exhibits a worm-like texture.

The occurrence of montmorillonite in the research part is very low or zero. The Albi-Tulun period in Sudan coincided with rift activities related to warm paleogeographic conditions18, 44. According to reports, under the condition of limited atmospheric water flux, the formation of montmorillonite in river and lake facies in arid/semi-arid climates may be high. The low content recorded in the research samples may indicate that kaolinite is preferentially formed in montmorillonite under the high terrain and occasional heavy rainfall conditions that have the characteristics of the research area.

Illite also appeared in small amounts as a dysplastic ribbon crystal in the research section (Table 5). The abnormal occurrence area is related to the significant secondary porosity of the relatively elevated quartz overgrowth, which may indicate the middle diagenetic stage. The amount of montmorillonite-illite conversion occurring is small, but in the deeper part of the study part, there seems to be some correlation between the increase in chlorite content and the decrease in kaolinite content (Table 6).

The main cements include carbonate, pyrite and iron oxide, which usually fill the pores in a dispersed form. Carbonates (calcite and siderite) are generally lower because calcite shows flaky/dotted crystals limited to the deeper part, while siderite appears in the form of dotted particles. Pyrite is rare but unusually high at a depth of 902.90 m (Table 5). Pyrite is an alternative subcubic-cubic rhombus filled with pores, which is an indicator of reduction conditions. Therefore, cement indicates the post-compaction stage.

Based on the evidence of diagenesis and metasomatic characteristics, distribution, and pore filling methods recorded in this study, the evolution of the Bentiu Formation is explained by the following complex symbiotic sequence (diagenesis-shallow mid-diagenesis) (Figure 10):

The symbiotic sequence of diagenesis in the studied Bentiu Formation. Note: The order here follows the boundary discussed in 44,46 of using CorelDRAW Graphics Suite 2018 v20.0.0.633 https://www.corel.com/cn; the temperature data indicated is explanatory and should be treated with caution.

A large number of debris particles with non-linear contact (plus few linearized particles, such as bump contact) indicate mechanical compaction before cementation and a moderately early stage.

Detrital chlorite and kaolinite are believed to have evolved simultaneously. A large amount of chlorite is considered to be the main one because no signs of compaction were found. The early chlorite cements may be the result of the dissolution of feldspar or clay minerals; they may be evolutionarily similar to kaolinite. The high values ​​of abundance and retained porosity of chlorite in the studied samples indicate early diagenesis. This means that chlorite may act as a coating to improve particle compaction and prevent the early precipitation of pore-degraded cements such as overgrowth of quartz, so it is designated as a diagenetic stage. As the temperature increases with the depth of burial, it leads to early chemical compaction and later compaction (pressure solution). Early dissolution will affect unstable minerals such as feldspar and clay minerals.

It is found that authigenic kaolinite partially fills the pores and surrounds the quartz minerals, while the overgrowth of authigenic quartz is surrounded by quartz particles and kaolinite. This indicates that the quartz overgrows after the quartz particles and the early cemented kaolinite. The authigenic kaolinite is believed to originate from the early dissolution process involving mica (kaolinite)18,45. Quartz overgrowth, the amount observed in the sample is very small, the effect on porosity and permeability is negligible, and it is interpreted as shallow diagenesis. Overgrowth of quartz is considered a temperature-related event 44,45.

The authigenic chlorite may occur at the later stage of the symbiosis sequence, which may coincide with or later than the alteration of montmorillonite and illite. This can be clearly seen from the small amounts of montmorillonite and illite (Table 6). Illite coincides with the low porosity area. The observation of a large amount of chlorite may indicate that it comes from iron-magnesium minerals. Although chlorite may have been formed through smectite-illite transformation, the increase in chlorite coincides with the decrease in kaolinite levels in the deeper part of the study. Generally, the precipitation of chlorite is related to the low Fe and Mg content of carbonate formed in shallow layers (> 3 km)43. Therefore, the chlorite in this study is in the diagenetic-middle diagenetic stage.

The calcite content is low and vertically restricted, and its pore-filling properties indicate that cementation occurs earlier than chemical compaction or at the same time as chemical compaction (Table 5). The weak and restricted appearance of the calcite cement and the generally highly interconnected pores indicate that a diagenetic phase event 45 has occurred. However, the high clay matrix and reduced porosity/permeability consistent with siderite cements indicate late pore filling/displacement cementation. Therefore, the carbonate cementation studied was designated as the late diagenesis-shallow-medium diagenesis stage, because the lack of evidence of dissolved calcite ruled out the late burial stage.

Therefore, it is assumed that the symbiosis sequence (earliest to latest) of the Bentiu Formation is: (1) mechanical compaction, early dissolution (feldspar and unstable clay minerals) (2) early cementation (involving chlorite, kaolinite and smectite) Delithiation) (3) Chemical compaction (dissolution/alteration of clay, montmorillonite and illite) (4) Late cementation/precipitation (calcite, siderite, overgrowth of quartz, iron oxide and pyrite) and (5) The formation of authigenic chlorite and kaolinite.

The enrichment of chlorite, especially in mica and fold rocks, is usually related to the later stage of diagenesis. This is similar to the interpretation of Frio sandstone on the Texas coast, where the occurrence of kaolinite cements is designated as mesodiagenetic. In addition, in the Boipeba sandstone of Brazil's Reconcavo Basin, kaolinite in the form of vermiculite and pamphlet is considered to be mineralization features45, but in this case, kaolinite will overgrow cement later than quartz. The possibility of hypothetical complex interactions of symbiosis sequences that may occur simultaneously over a long period of time is sometimes used to explain the characteristic distributions and patterns found in sediment reservoirs40,47. Assuming that different diagenetic stages occur at the same time, the trend of change observed in the research sample is possible.

A diagenetic model is proposed here. The research section of the Bentiu strata represents the diagenesis of the shallow layers of the ground. It starts with the infiltration of clay-rich water through the vadose zone and/or the alteration of feldspar and clay coatings. Then, meteorite replenishment, the detrital potassium feldspar and montmorillonite layers in the illite/montmorillonite decompose, transform other clay minerals (for example, kaolinite and chlorite), and decompose from deeper buried organic-rich shale And migration provides silica and carbonate hydrocarbons (Figure 11). This model is consistent with 41,42. Coatings can reduce permeability by blocking pore throats48. Nevertheless, in this case, the clay coating together with the micro-quartz may prevent early cementation and help maintain permeability.

Diagenesis model of the Bentiu Formation. The basin has experienced diagenetic and shallow-medium diagenetic stages. The occasional atmospheric water discharge through the porous path leads to little sphalerite infiltration, dissolving to produce kaolinite. In addition, the dissolution of organic-rich shale and deeper hydrocarbon generation and migration precipitated silica, calcium, and sulfides, creating secondary pores in the process. Note: The temperature value is explained based on empirical research 44,45 using CorelDRAW Graphics Suite 2018 v20.0.0.633 https://www.corel.com/cn.

The diagenesis of the Bentiu Formation is believed to be driven by "convection force" plus meteor replenishment. In other words, the early thermal convection between the high-pressure underlying shale of the Abu Gabra Formation and atmospheric water (responsible for mass transfer/hydraulic) requires observable diagenetic changes 43,49,50. This model is similar to the symbiosis sequence reported by the Frio Group 51 along the lower Texas coast. Below the Frio Formation is the Wilcox sandstone (Early Tertiary), on which there are feldspar from medium-component igneous rocks and quartz-rich clastic particles, and overlying with Plio-Pleitocene sandstone. The Frio group is rich in montmorillonite and kaolinite, but has a low content of chlorite, and has a symbiotic sequence of calcite-quartz overgrowth-calcite-dissolved kaolinite-Fe-carbonate. This is similar to the above-mentioned Bentiu formation rock sequence.

In this case, the focus is only on controlling the main process of sandstone diagenesis. The diagenesis of the Bentiu Formation seems to be mainly affected by sedimentary facies, climate, oil and gas generation and migration. The Bentiwu Formation is a sandy facies of river and lake facies, rich in quartz (mainly microcrystalline quartz), feldspar, mica (muscovite and biotite), kaolinite and chlorite are widely distributed, and the content of montmorillonite is low. Sandstone exhibits well-sorted particles, with a total porosity of 32% retained. Compared with the average initial porosity of 40% and 26% retained porosity of coarse-grained sandstone after shallow physical compaction, approximately 80% is retained in the maximum case. Although the quartz overgrowth cement precipitates, it has little effect on the porosity and permeability of the sandstone. The appearance of calcite and siderite also exhibits similar minimum porosity-permeability effects. In addition, secondary pores were created in some parts of the sequence.

The Moga 26 sample showed higher porosity at shallower depths, while the deeper Moga 6 and Keyi 4 samples showed lower total porosity. This is an indication of the inverse relationship between depth and porosity values. The abnormally low porosity observed corresponds to elevated K-feldspar, siderite and pyrite cements and extremely low permeability values. The high value of the retained porosity of the Bentiu Formation is consistent with the high paleotopography suggested by the Bentiu Formation. Generally, due to limited atmospheric water inflow, continental river and lake facies in warm semi-arid/arid climates are expected to be rich in montmorillonite and iron oxides44. However, the initial high porosity of sandstone combined with the high topography of the area is conducive to the easier penetration of sporadic atmospheric water and the dissolution of unstable clastic components. This applies to the formation of early kaolinite and chlorite at the expense of montmorillonite. More importantly, microcrystalline quartz and chlorite are considered as coatings to prevent early precipitation of cements with reduced porosity/permeability (such as quartz 53) and may play a role in maintaining better reservoir quality in shallow layers. .

Note that calcite (as cement at the buried level) begins to form at 1510 m and usually increases downward (Table 5). In addition, the overgrowth of quartz, the initial formation of siderite, pyrite, and Fe oxide start from the shallow layer. Higher quartz overgrowth corresponds to higher calcite cement zone. Quartz overgrowth also has an upper and lower concentration zone, the latter coincides with the montmorillonite-illite transition.

Regarding the source of carbonate and quartz cements, some people believe that diagenesis began with the heating of organic-rich shale in the underlying Abu Gabra Formation (see Figure 11). The decomposition/decarboxylation of organics leads to the formation of organic carboxylic acids, CO2 and silica, which originate from the maturation of organics and the montmorillonite-illite conversion. These processes provide fluids for upward movement of the carbonate and quartz cement components. This occurs at approximately 80–100 °C43 or 80–140 °C54. Organic acids will reduce the stability of carbonates and aluminosilicates, and cause the discharge of silica-rich water, resulting in the formation of secondary pores and overgrowth and cementation of quartz. The increase in secondary porosity starts at 860 m, corresponding to the top of the oily Moga 6, and gradually increases downward. However, abnormal secondary porosity was observed at 1510 m, but no systematic trend was shown (because some porosity values ​​are very high, while others are very low). This is also related to those parts with higher calcite, siderite and pyrite cements.

As a result, quartz overgrowth and carbonate cement precipitate as pore fillers. The limited occurrence of calcite is considered to be an indicator of the proximity of the carbonate source. The supply of calcium in Keyi Oilfield has been exhausted. Carbon dioxide seems to be released all the time. Compared with the Frio group, the amount of siderite is better distributed in the Bentiu group. The water discharged by convection is rich in iron, which is beneficial to the precipitation of siderite and iron oxide.

The most common kaolinite in the central part of the study profile (Moga 6) may have originated from feldspar alteration. It is worth noting that the Moga 26 and Khoi oil fields have the highest precipitation of chlorite and relatively low pyrite content (Table 5). Previously, it has been assumed that minerals with lower water content will preferentially precipitate in the presence of hydrocarbons that provide reducing conditions. Therefore, compared with montmorillonite, the content of illite is relatively high. In the deeper part of the study profile, the relative increase in cementation has no significant effect on the porosity. Coincidentally, it is also the area with the largest secondary porosity. This is a sign of a late dissolution event (which may affect K-feldspar).

The mixing of atmospheric water with silica-rich fluids facilitates the precipitation of iron oxides. This provides an oxidation reaction consistent with the warm semi-arid environment of the region. The "convective force" plus the transport of water masses in the atmosphere is supported (weakly) by the shallower layers of cement shown by petrography. Alternative sources of silica and carbonates such as marine fossils may be another possibility. In order to better constrain the diagenetic model of the Bentiu Formation, more data, such as geochemical isotope studies, are needed.

The detailed lithofacies analysis of the samples from the Bentiwu Formation shows that the sedimentary sequence has undergone several diagenesis, which has a significant impact on its porosity and permeability. Diagenesis leads to a decrease or increase in porosity and permeability (that is, to increase or inhibit reservoir quality) 36,39. In the research samples, the diagenetic dissolution of feldspar and mica, the partial dissolution of carbonate (calcite) cement and clay have increased the porosity and permeability. On the other hand, the availability of detrital clay, kaolinite and Fe oxide precipitation, siderite and pyrite cementation, and compaction and overgrowth of quartz leads to the loss of porosity and permeability in the sediment3,18.

The percentage of detrital clay minerals that can reduce the porosity and permeability of the Bentiu formation varies from 0.5% to 45.0% (Table 5). According to XRD analysis (Table 6), kaolinite and chlorite are the main detrital clay minerals in the studied sediments. Kaolinite and chlorite can disaggregate in pores and throats, thereby reducing porosity and permeability. However, the dehydration process seems to further improve the quality of the reservoir36,39. The absence or very low content of montmorillonite and illite minerals observed in all samples (see Section 4.4) and relatively high secondary porosity (0.5% to 10.0%), with an average of 5.9%, support this Hypothesis.

In addition, overgrowth of well-developed euhedral quartz (Figure 8a: F-G11-12) and a small amount of microcrystalline siderite diamonds (Figure 8b: C-D14) and rare subcubic to cubic pyrite crystals were also detected . Quartz overgrowth, pyrite and siderite are the main types of cementation observed in Bentiu sandstone. As we all know, overgrowth of quartz is a key factor leading to the decline of reservoir quality3,55,56,57. Although the percentage of some mineral cements is low, their quartz overgrowth leads to a decrease in pore space, thereby reducing porosity and permeability by preventing the interconnection of pores3. In this study, quartz overgrowth occurred in a small amount (not more than 1.5%), because some nucleated cells around the quartz particles grew into large pores (for example, Fig. 7c, f), which actually resulted in a reduction in the macroscopic amount of porosity.

In general, the pyrite cement content in the studied samples ranges from 0.2% to 3.0%, but anomalies (23.6%) appear at a depth of 902.90 m (see the section "Slice Petrography"). Compared with other low pyrite cement samples, very low porosity and permeability values ​​were also observed near this depth (909.90 m; Moga 6 well). This indicates that pyrite cementation may have some significant negative effects on porosity and permeability. The same influence on the porosity and permeability of the analyzed samples is siderite, which appears as flaky pore-filled crystals in the particles (Figure 7d). Together with iron oxide, it is considered to be a secondary cement in the Bentiu formation (see Figure 7d). 7e). In addition, the degree of compaction increases with the increase of the burial depth, because the overburden pressure and the cementation of the sediment cause the porosity and permeability to decrease with the increase of the burial depth3,56. Most of the coarse-grained samples studied have poor compaction and no stitched particle network, which reflects the higher degree of compaction of the coarse-grained samples. However, in a small number of samples, there are few long grains and bumps in contact (for example, Figure 7a, e, respectively). On the other hand, the observation of small intergranular pores left after the compaction process indicates that compaction may have a dominant control over porosity and permeability. This is consistent with the previous discussion, which shows that sedimentology and grain structure characteristics have a direct impact on the reservoir quality of the Bentiu Formation studied. Therefore, it can be concluded that the precipitation of detrital clay, kaolinite and iron oxide, the cementation of siderite and pyrite, compaction and overgrowth of quartz all have a negative impact on the quality of the Bentiu Formation reservoir.

In addition, secondary porosity is a common diagenetic process, similar to the disintegration of mica, feldspar minerals, and calcite cements3,58. The dissolution of these minerals can increase porosity and permeability. In this study, potash feldspar showed varying degrees of alteration and dissolution, present as fresh uniform particles (for example, Figure 7a-f), and partially to almost completely dissolved (skeletal) particles (Figure 7a-d). At the same time, there is no evidence of the presence of calcite cement in the study samples (Table 5). Alteration and dissolution are two processes that form secondary pores in the studied sandstone of the Bentiu Formation. Nevertheless, these processes usually lead to the development of kaolinite and subsequent overgrowth of quartz. Among the inspected samples, kaolinite (clay) and quartz overgrowth cements have no significant impact on the quality of the reservoir, because most of the samples have relatively high porosity and permeability (Table 5), except for the high clay The content of Fl and Sr phases. In support, the detection of chlorite in all research samples can improve the resistance to particle compaction, prevent overgrowth of quartz and maintain porosity and permeability. Similarly, since the dissolution of mica will increase secondary porosity, sediments with high mica content exhibit pressure solution compaction, destroying intergranular porosity. A small amount of mica was observed in all the samples studied (Table 5), further supporting the existence of high-quality reservoirs.

The study area includes the Damoga Oilfield and the Khoi Oilfield in the Fula Basin in the northeast of the Muglad Basin. Tables 1 and 4-6 show the depth intervals and well names of the samples. Wells Moga 26 and Moga 6 are from the Great Moga oil field, and Well Keyi 4 is from the Keyi oil field. Compared with the samples from the Khoi Oilfield (the deepest depth is 1695.70 m), the samples of the Bentiu group studied have a relatively shallow depth in the Great Moga (797.07 to 910.93 m). It can be seen from Table 5 that the original porosity and permeability of the Great Moga sample is slightly lower than that of the Keyi 4 well sample, and as the burial depth increases, the porosity and permeability gradually decrease. This is consistent with the original porosity and permeability values ​​of these samples and the burial depth (Figure 12a, b) curves, which are down along the normal trend. On the other hand, the secondary porosity showed a positive trend with the increase of burial depth (Figure 12c). As the burial depth increases, leading to the dissolution of feldspar and other minerals and the dehydration of sediments, the formation of secondary pores usually increases36,47. The leaching and dissolution of a relatively large number of particles (partial cracks and micro cracks) were observed in the studied samples (for example, Figure 13a-d). The observed increase in overgrowth of quartz with increasing burial depth (Figure 12d) can be attributed to the increase in temperature and pressure associated with the increase in burial depth. In addition, different types of particle contact were observed at deeper depths, indicating moderate to high compaction and porosity loss (Figure 13a-d). Therefore, the reservoir quality of the Bentiu Formation shows evidence of burial depth control, as shown by reduced porosity and permeability. Therefore, as the burial depth increases, the overgrowth of quartz derived from compaction and cementation increases significantly.

(a) The intersection of the original porosity and permeability values ​​and the buried depth (b) The porosity and permeability decrease as the buried depth increases. (C); The secondary porosity shows a positive trend with the increase of buried depth. (d); The number of overgrowth of quartz increases with the increase of burial depth.

Thin section photomicrographs of medium-grained (average mL-mU) sandstone samples, well sorted, sub-round to round, partially cemented, with moderate to highly compacted points, bumps (photos a and b: H 5) , A few long (photos a and b: FD 5-6) and fewer suture particles in contact. Mainly polycrystalline quartz (photo b: iJ 2-4 and photo d: CE 5-7 and GF 8-9) and more single crystal quartz (photo a: A 8-10; photo b: DE 6-7 ; Photo c: F –H 1–4) and a large amount of potassium feldspar (photo a: EF; photo b: 7-10; photos c and d: EF 6-11), a minor amount of plagioclase (photo a : CD 10-11). Some detrital clays occupy very little pore space (photo a: GH 4-7; photo c: FG 4-5). Beside some iron oxide blocks, overgrowth of quartz (photo ad) occurs as cement. The leaching and dissolution of relatively large amounts of particles, parts and micro-cracks were observed (e.g. photo advertisements).

The obvious relationship between porosity and permeability and changes in sedimentary facies control the migration and accumulation of oil and gas. These in turn depend on the grain structure characteristics and diagenesis processes during or shortly after deposition,]. Therefore, it is necessary to use porosity and permeability to quantitatively determine the quality of the reservoir in order to obtain sufficient hydrocarbon accumulation 58, 59.

Although the porosity of the studied samples is affected by some mineral cements (pyrite, siderite, clay, iron oxide and quartz overgrowth), it is still characterized by high intragranular porosity and a large amount of secondary porosity related to diagenesis. Mainly (partial/complete dissolution of feldspar, mica, carbonate cement and clay). The total porosity values ​​of phases Sxl, Sm, and Sxt are 19.6% to 32.0%, while the porosity values ​​of phases F1 and Sr are 1.0 to 6.0 (Table 5). On average, the pore space varies greatly (20-350 microns), and the interconnected pore spaces range from poor to excellent. The absence or very weak appearance of calcite and pyrite cements helps to maintain relatively large pore sizes and good pore interconnection in the studied samples. Likewise, most of the samples studied showed high permeability values. The permeability values ​​of the Sxl, Sm, and Sxt phases range from 1271.6 to 7069.0 mD, while the permeability values ​​of the Fl and Sr phases range from 2.5 mD to 10.0 mD (Table 5). Therefore, it is inferred that the Sxl, Sm, and Sxt phases have very good reservoir quality, while the Fl and Sr phases show the worst reservoir quality. The observed positive correlation between porosity and permeability indicates that there is a close relationship; if all other factors meet the requirements, then high porosity corresponds to high permeability in the study area (Figure 14a).

(a) An intersection plot showing the positive correlation between porosity and permeability. (b and c) The relationship between the content of detrital quartz and porosity and permeability, respectively. The higher the detrital quartz content, the greater the porosity and permeability.

Sedimentary environment is another main factor considering the control of particle shape, size, sorting, sedimentary structure, sand body geometry, and diagenetic process. The combination of these factors affects the porosity and permeability of sediments3,58,59. The Bentiu Formation is deposited in a braided flow of thick sandstone layers with interbedded shale layers3,18. The reservoir was formed due to rapid sedimentation, and therefore deposited two main types of sediments: (1) large amounts of coarse-grained sediments, and (2) fine-grained ashore sediments. The two types of sediments have huge differences in porosity and permeability values. Since then, a large amount of coarse sediments have been deposited. The porosity and permeability values ​​of coarse-grained sediments (19.6% to 32.0% and 1271.6 mD to 7069.0 mD) are compared with riparian sediments (1.0% to 6.0% and 2.5 mD to 10.0 mD) (Table 5). In addition, the coarse-grained sediments are characterized by good sorting, with particles ranging from sub-round to round. Compared with fine-grained sediments, these characteristics explain that the studied coarse-grained sandstone has very good reservoir quality3,18. This is consistent with previous studies, indicating that braided river sediments have greater primary porosity and better reservoir quality than riparian sediments3,4,10,12. In addition, the structure and composition of the two deposits are also different. For example, the detrital quartz content of coarse-grained sediments (36.9%-49.4%) is higher than that of fine-grained sediments (23.3%-26.2%). Detrital quartz is more mature and has higher resistance to compaction, which is conducive to the preservation of the original porosity47. This is consistent with Figure 14b-c, which shows that the higher the detrital quartz content, the greater the porosity and permeability.

In addition to the sedimentary environment, the reservoir quality of the Bentiwu Formation is also affected by diagenesis such as compaction, authigenic clay (overgrowth of quartz, iron oxide, siderite and pyrite cements), dissolution of feldspar and other rock fragments. control. The clay minerals in the Bentiu reservoir are mainly chlorite and kaolinite, which appear as pore-filling cements. Therefore, these minerals reduce the porosity and permeability of the studied samples by blocking the pore space (Figure 7e, 15). Therefore, samples with higher total clay content tend to have the lowest porosity and permeability values ​​(Table 5). Similarly, siderite, pyrite, and iron oxide cements also tend to clog the pore throats, resulting in a decrease in the porosity and permeability of the sample. This allows the sample with the highest cement value to have the lowest porosity and permeability values. Figure 16 illustrates the good inverse correlation between the sum of authigenic mineral cements and porosity and permeability. Correlation indicates that these cements are the main problem of reservoir quality degradation. Quartz overgrowth or quartz cementation is very important to control the quality of reservoirs, especially in moderate to deep buried reservoirs55,56. Quartz overgrows during the late diagenesis process, forming complete or incomplete quartz grain rings (for example, Figures 7a, c, 16b). As the buried depth increases, compaction and overgrowth of quartz tend to reduce the porosity by changing the grain contact from absolutely no contact to point contact or from point contact to uniform linear and uneven contact (e.g. Figure 7a, 12a- d). The implication is that compaction and cementation are still the most important diagenesis processes. As the burial depth increases, the quality of the reservoir decreases. Therefore, the deposition method of sediments (especially clastics) has a strong control over the shape, size, structure and type of sediments. In addition, this has an adverse effect on the primary porosity and permeability of the sandstones of the Bentiu Formation.

Photomicrographs obtained by SEM show open pore throats, pore-filled illite (micropores) and kaolinite, as well as overgrowth of quartz and quartz.

An intersection graph showing a good negative correlation between authigenic mineral cement and porosity and permeability. The presence of this cement reduces the quality of the reservoir.

In summary, the quality control factors of the sandstone facies reservoirs of the Bentiwu Formation mainly include: (1) sedimentary morphology; (2) diagenesis process; (3) level of sediment burial depth. Therefore, due to these factors, the porosity and permeability of the studied sandstone vary greatly. Based on detailed sedimentology and petrological analysis, the following general conclusions are drawn:

The samples studied can be divided into two main phase groups. This is determined by the composition and vertical distribution of the sediments: (1) Alternating coarse-grained to medium-grained (blocky) sandstone, representing a series of braided channel sediments, and (2) fine-grained corrugated markings or layered sandstone It is related to some mudstone, indicating floodplain and open channel sediments. Coarse to medium-grained sandstone has a low clay content and therefore has better porosity and permeability than clay-rich sandstone.

Quartz overgrowth, pyrite, siderite and iron oxide, as well as kaolinite and chlorite are the main cementitious materials observed in Bentiu sandstone. These minerals block the pore throats, thereby reducing the porosity and permeability of the sandstone. However, the dissolution and dehydration processes of feldspar, mica, and carbonate cements seem to further improve the quality of the reservoir. This assumption is supported by relatively high secondary porosity values ​​(0.5% to 10.0%), averaging 5.9%.

As the burial depth increases, the porosity and permeability of the reservoir are significantly reduced due to the compaction and cementation of the research samples.

The quality of the reservoir is mainly controlled by the particle size distribution, texture, and total clay content (depending on the depositional environment) of the entire study area, as well as compaction, cementation, and dissolution (as the burial depth increases).

Taking all these factors into consideration, this study helps to identify sweet spots with higher reservoir quality in the area. Therefore, the success rate of oil and gas exploration can be improved and the inherent risks can be reduced.

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Thanks to the Sudanese Ministry of Energy and Mining, Sudan Petroleum Corporation (SPC) and Central Petroleum Laboratory (CPL) for providing data and analysis reports. This research was supported by Jilin University Research Fund Nos. 41908051907 and 451190330022.

School of Earth Sciences, Jilin University, Changchun 130061

Yousif M. Makeen, Xuanlong Mountain & Siyuan Su

Department of Geology, University of Malaya, 50603, Kuala Lumpur, Malaysia

Mutari Lawal & Nura Abdulmumini Yelwa

Department of Geology, Usmanu Denver Dieu University, PM B 2346, Sokoto, Nigeria

Mutari Lawal & Nura Abdulmumini Yelwa

Department of Geology, Federal University of Lokoja, PM B 1154, Lokoja, Nigeria

Jiangxi Engineering Laboratory of Radioactive Geoscience and Big Data Technology, East China University of Science and Technology, Jiangxi, China

Pan African University-Institute of Life and Earth Sciences, University of Ibadan, Ibadan, Nigeria

School of Earth Sciences, Northeast Petroleum University, Daqing 163318

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YMM, XS, ML, and HAA designed the research, SS, improved the revised copy, and NAY reviewed the methods and results. YL, NEA, and XD designed the numbers. All authors reviewed the manuscript.

Correspondence with Yousif M. Makeen or Siyuan Su.

The author declares no competing interests.

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Makeen, YM, Shan, X., Lawal, M. etc. Reservoir quality of the Bentiu Formation in the northeastern part of the Muglad Basin in Sudan and its controlling diagenesis factors[J]. Scientific Representative 11, 18442 (2021). https://doi.org/10.1038/s41598-021-97994-x

DOI: https://doi.org/10.1038/s41598-021-97994-x

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