Showing papers on "Petrography published in 2021"

Modeling the Depositional Environment of the Sandstone Reservoir in the Middle Miocene Sidri Member, Badri Field, Gulf of Suez Basin, Egypt: Integration of Gamma-Ray Log Patterns and Petrographic Characteristics of Lithology

Abstract: Understanding the depositional setting of siliciclastics reservoir is fundamental process to exploration and development of hydrocarbon reservoirs and to the multi-phase cycle of the oil and gas industry. Typically, core samples from existing or potential reservoirs can be used for interpretation of depositional environment. However, the lack of core samples in certain reservoirs represents a challenge for reservoir development plans and further exploration. To overcome the absence of core samples in the middle Miocene Sidri Member in the Badri field, Gulf of Suez, this study aimed to reconstruct its depositional settings by coupling well logging patterns and petrographic characterization of ditch cuttings. Consequently, 30 thin section samples representing the reservoir section of four wells were described in detail and standard petrographic characteristics were determined. Then, gamma-ray (GR) log patterns of the studied reservoir wells were used for interpretation of depositional environment. Petrographic analysis indicates that the sandstone reservoir is fine- to medium-grained arkose with dominant siliceous cement and composed mainly of quartz, feldspars, and lithic fragments. Pores reflecting primary and secondary porosity as well as inter-granular pores are dominant. The facies analysis indicates that the studied member has a heterogeneous nature and rapid facies change; its main lithofacies are blocky sandstones, intercalated sandstones and shales, and shales. Three electrofacies were interpreted in the studied section: (1) electrofacies-A (blocky sandstones), which is characterized by a cylindrical-shaped (right boxcar) GR trend; (2) electrofacies-B (intercalated sandstones and shales), which is characterized by an irregular log trend with serrated shape; and (3) electrofacies-C (shales), which is characterized by irregular GR trend and has no character. The interpreted results indicate a tidal channel depositional setting for electrofacies-A, mixed tidal flat depositional setting for electrofacies-B, and low relief offshore mudstone depositional setting for electrofacies-C. Finally, the results illustrate how the coupling of GR patterns with the analysis of petrographic characteristics can be used to understand the depositional setting of a hydrocarbon reservoir that lacks core samples. This work could be useful for assessment of reservoir distribution and quality, for reduction of uncertainty during field development, as well as for providing useful insight to similar hydrocarbon reservoirs elsewhere.

Diagenesis and its controls on reservoir quality of the Tambar oil field, Norwegian North Sea

Abstract: A study of the diagenetic evolution of the Late Jurassic sandstones of the shallow marine facies in the Tambar oil field, Norwegian North Sea was carried out to understand its controls on reservoir quality. Core samples and a set of wireline logs obtained from three wells were used to carry out petrographic studies including light optics and scanning electron microscopy, well log analysis, fluid inclusions, X-ray diffraction (XRD) for whole rock samples, clay mineral extracts and stable isotopes. The Tambar reservoir sandstones range mainly from siltstone to fine-grained sandstones, exclusively arkoses cemented mainly by microcrystalline quartz, euhedral quartz overgrowth, dolomite cement, illite and chlorite. Early diagenetic/eodigenetic minerals include pyrite, calcite, and microcrystalline quartz and late diagenetic/mesodiagenetic minerals include quartz overgrowth, dolomite, illite and chlorite. The δ18O values of dolomite cement in the Tambar sandstone reservoir range from −11.77‰ to −3.57‰ PDB, while the δ13C values of the dolomite cement are from −5.07‰ to −1.12‰ PDB. Homogenization temperature for fluid inclusions trapped in authigenic quartz fall between 130 °C and 169 °C. The reservoir quality is controlled mainly by early formed grain-coating microcrystalline quartz that precipitated from the dissolution of sponge spicules which significantly inhibits quartz overgrowth precipitation. Oil emplacement do not show significant control on reservoir quality because considerable quartz cementation has taken place before the onset of major oil charge. Only the coarse-grained sands show a correlation between quartz cement volume and water saturation. However, the interpreted temperature for dolomite cementation is significantly higher for water leg-suggested effect of oil emplacement on the late dolomite. Understanding these controls on the reservoir quality will enhance the exploration strategy for the Tambar oil field.

Understanding the influence of petrographic parameters on strength of differently sized shale specimens using XRD and SEM

Abstract: Ground failure is a major contributor to fatalities in underground mines in the US. Underground coal mines in the Northern Appalachian have weak roof rock composed of shale, which is prone to failure under high horizontal stress. Understanding the relationship among strength, specimen size and rock petrographic parameters is essential for developing an effective ground control plan. Size effect studies have found that rock strength varies with specimen size. This paper attempts to understand this strength variation using three specimen sizes (254-mm, 508-mm, and 762-mm). The specimen strength was measured and the major petrographic parameters affecting the strength, namely grain size, grain shape, quartz content, clay content, etc. were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The petrographic parameters were then correlated with the strength of the three differently sized specimens. The results showed that 508-mm specimen had the lowest strength. Quartz content of the 508-mm specimen was lower than that of 254-mm and 762-mm specimens. Clay content and average grain size of the 508-mm specimen were higher than those of 254-mm and 762-mm specimens. These results clearly show that grain size, quartz content and clay content contribute to strength variation observed in differently sized shale specimens.

Trace-elemental and petrographic constraints on the severity of hydrographic restriction in the silled Midland Basin during the late Paleozoic ice age

Abstract: Enrichment of redox-sensitive trace elements in ancient marine shales is conventionally believed to be controlled by marine benthic redox conditions, whereas the influence of hydrographic conditions on trace element enrichment pattern has been rarely considered. Here, we present newly obtained data sets from the Upper Pennsylvanian organic-rich Cline Shale in the Midland Basin, Texas, to illustrate the influence of hydrographic circulation on the trace-element enrichment pattern and the stratigraphic record of mudrocks. Various lithofacies, including siliceous mudrocks, argillaceous mudrocks, skeletal-bearing argillaceous mudrocks, calcareous mudrocks, and wackestone, are identified in the Cline Shale. Significant changes in the trace-element enrichment pattern, mineral composition, texture of framboidal pyrite, and other bulk geochemical parameters in different lithofacies are interpreted to have been caused by high-amplitude and high-frequency glacio-eustatic sea-level oscillations in the silled Midland Basin during the late Paleozoic ice age. Specifically, glacio-eustatic sea-level falls generally resulted in the severe isolation of the Midland Basin from the Panthalassic Ocean, highly restricted hydrographic circulation, long deep-water renewal time, euxinic bottom-water conditions, depleted seawater Mo (molybdenum) in the silled basin, and low sediment Mo/TOC (total organic carbon), coupled with significant extrabasinal detrital quartz input, forming siliceous mudrocks. Enhanced phosphorus cycling and excellent preservation conditions are considered to be responsible for the high TOC observed in siliceous mudrocks. In contrast, glacio-eustatic sea-level rises substantially enhanced water exchange between the basin and the Panthalassic Ocean, created overall suboxic to anoxic bottom-water conditions, resupplied seawater Mo, elevated sediment Mo/TOC, and increased platform carbonate production in the basin, forming calcareous mudrocks and wackestone.

Sedimentology and petrophysical analysis of pre-salt lacustrine carbonate reservoir from the Santos Basin, southeast Brazil

Abstract: Lacustrine carbonate deposits mark the rift-sag sedimentation stage of the so-called pre-salt section of the Campos and Santos Basins, in the Southeast Brazilian continental shelf. These salt-sealed carbonate reservoirs form deep-water, giant and prolific oil fields that are currently responsible for approximately 70% of the Brazilian oil and gas production. In the Santos Basin, the sag stage carbonates are represented by the Aptian Barra Velha Formation, characterized by exotic textural and compositional features whose origin, development, and correlation with petrophysical properties have been broadly discussed in the last decade. A geological and petrophysical study has been performed based on core samples, plugs, and thin-sections of a well in the Barra Velha Formation from the eastern Santos Basin. The analyzed core interval was grouped into six depositional facies constituted by the association of five key textural components: (1) crystal shrubs; (2) calcite spherulites; (3) hybrid forms; (4) mud-grade carbonates and dolomite; and (5) intraclasts. The relationship between the depositional framework and the poroperm system, as evidenced by petrographic image analysis and CT-scan data, is highly complex. The depositional control over porosity is erratic, mainly due to the extensive diagenetic and possible hydrothermal overprint. Dolomite cementation, dolomitization, pervasive silicification, and dissolution processes are the main primary porosity modifiers. Petrophysical analysis indicates that intraclastic and spherulitic grainstones–rudstones, affected by matrix dissolution, may present the best reservoir properties, in certain intervals, even surpassing the shrubby framestones, which are widely regarded as the best reservoir facies. This study seeks to contribute to an emerging body of research about the Brazilian pre-salt lacustrine carbonates, broadening the geological knowledge about these deposit, while providing new data to improve hydrocarbon forecast and recovery.

Behavioral differences between weathering and pedogenesis in a subtropical humid granitic terrain: Implications for chemical weathering intensity evaluation

Abstract: Continental weathering plays a crucial role in the evolution of the Earth’s surface by linking Earth's spheres, shaping landscapes and regulating chemical cycles and global climate. Regolith weathering studies in some cases ideally assume successive and progressive bottom-up physicochemical variation trends. To validate this assumption, we target a granitic regolith profile in a subtropical monsoon climate-dominated region (southeast China). We present mineralogical, petrographic and geochemical data from soil, saprolite and bedrock samples to characterize physical and chemical alterations. The petrographic and mineralogical results indicate that both plagioclase and K-feldspar are depleted and kaolinite is the major neoformed mineral phase in top-soil samples, revealing intensive chemical weathering on the regolith. However, these top-soil samples have much lower chemical index of alteration (CIA, ca. 64–67) values than the underlying oxidized-soil samples (CIA values ranging 65–80). Some chemically immobile elements, such as Al, Ti, Zr and Hf, are depleted in top-soil samples but are comparatively enriched in oxidized-soil samples. This is attributed to vertical leaching and translocation of fine-grained minerals (e.g., kaolinite, illite and zircon) through the soils. Furthermore, the analyzed top-soil samples are comparatively rich in elements K and P, which are most likely due to the evident biological fixation process. These physical and biological processes in the unconsolidated, porous and biotic top-soil layer can be responsible for the unexpected low CIA values. Our findings demonstrate the complex physical, chemical and biological alterations on soils and the differential behaviors between weathering and pedogenic processes, especially for those tropical–subtropical high rainfall regions. This case study also highlights the importance of petrographic and mineralogical proxies to chemical weathering intensity evaluation for regolith profiles and siliciclastic sediments.

Formation of the crater suevite sequence from the Chicxulub peak ring: A petrographic, geochemical, and sedimentological characterization

Abstract: This study presents a new classification of a ∼100-m-thick crater suevite sequence in the recent International Ocean Discovery Program (IODP)-International Continental Scientific Drilling Program (ICDP) Expedition 364 Hole M0077A drill core to better understand the formation of suevite on top of the Chicxulub peak ring. We provide an extensive data set for this succession that consists of whole-rock major and trace element compositional data (n = 212) and petrographic data supported by digital image analysis. The suevite sequence is subdivided into three units that are distinct in their petrography, geochemistry, and sedimentology, from base to top: the ∼5.6-m-thick non-graded suevite unit, the ∼89-m-thick graded suevite unit, and the ∼3.5-m-thick bedded suevite unit. All of these suevite units have isolated Cretaceous planktic foraminifera within their clastic groundmass, which suggests that marine processes were responsible for the deposition of the entire M0077A suevite sequence. The most likely scenario describes that the first ocean water that reached the northern peak ring region entered through a N-NE gap in the Chicxulub outer rim. We estimate that this ocean water arrived at Site M0077 within 30 minutes after the impact and was relatively poor in rock debris. This water caused intense quench fragmentation when it interacted with the underlying hot impact melt rock, and this resulted in the emplacement of the ∼5.6-m-thick hyaloclastite-like, non-graded suevite unit. In the following hours, the impact structure was flooded by an ocean resurge rich in rock debris, which caused the phreatomagmatic processes to stop and the ∼89-m-thick graded suevite unit to be deposited. We interpret that after the energy of the resurge slowly dissipated, oscillating seiche waves took over the sedimentary regime and formed the ∼3.5-m-thick bedded suevite unit. The final stages of the formation of the impactite sequence (estimated to be <20 years after impact) were dominated by resuspension and slow atmospheric settling, including the final deposition of Chicxulub impactor debris. Cumulatively, the Site M0077 suevite sequence from the Chicxulub impact site preserved a high-resolution record that provides an unprecedented window for unravelling the dynamics and timing of proximal marine cratering processes in the direct aftermath of a large impact event.

Radiological Hazard Evaluation of Some Egyptian Magmatic Rocks Used as Ornamental Stone: Petrography and Natural Radioactivity

Abstract: Magmatic rocks represent one of the most significant rocks due to their abundance, durability and appearance; they can be used as ornamental stones in the construction of dwellings. The current study is concerned with the detailed petrography and natural radioactivity of seven magmatic rocks. All are commercial granitic rocks and are identified as black Aswan, Nero Aswan, white Halayeb, Karnak, Verdi, red Hurghada and red Aswan. Their respective mineralogical compositions are classified as porpheritic granodiorite, granodiorite, tonalite, monzogranite, syenogranite, monzogranite and syenogranite. A total of nineteen samples were prepared from these seven rock types in order to assess their suitability as ornamental stones. Concentrations of 226Ra, 232Th and 40K radionuclides were measured using NaI (Tl) scintillation gamma-ray spectrometry. Among the studied magmatic rocks, white Halayeb had the lowest average values of 226Ra (15.7 Bq/kg), 232Th (4.71 Bq/kg) and 40K (~292 Bq/kg), all below the UNSCEAR reported average world values or recommended reference limits. In contrast, the other granitic rocks have higher values than the recommended limit. Except for the absorbed dose rate, other radiological hazard parameters including radium equivalent activity, annual effective dose equivalent, external, and internal hazard indices reflect that the White Halyeb rocks are favorable for use as ornamental stone in the construction of luxurious and high-demand residential buildings.

Deposition, Diagenesis, and Sequence Stratigraphy of the Pennsylvanian Morrowan and Atokan Intervals at Farnsworth Unit

Abstract: Farnsworth Field Unit (FWU), a mature oilfield currently undergoing CO2-enhanced oil recovery (EOR) in the northeastern Texas panhandle, is the study area for an extensive project undertaken by the Southwest Regional Partnership on Carbon Sequestration (SWP). SWP is characterizing the field and monitoring and modeling injection and fluid flow processes with the intent of verifying storage of CO2 in a timeframe of 100–1000 years. Collection of a large set of data including logs, core, and 3D geophysical data has allowed us to build a detailed reservoir model that is well-grounded in observations from the field. This paper presents a geological description of the rocks comprising the reservoir that is a target for both oil production and CO2 storage, as well as the overlying units that make up the primary and secondary seals. Core descriptions and petrographic analyses were used to determine depositional setting, general lithofacies, and a diagenetic sequence for reservoir and caprock at FWU. The reservoir is in the Pennsylvanian-aged Morrow B sandstone, an incised valley fluvial deposit that is encased within marine shales. The Morrow B exhibits several lithofacies with distinct appearance as well as petrophysical characteristics. The lithofacies are typical of incised valley fluvial sequences and vary from a relatively coarse conglomerate base to an upper fine sandstone that grades into the overlying marine-dominated shales and mudstone/limestone cyclical sequences of the Thirteen Finger limestone. Observations ranging from field scale (seismic surveys, well logs) to microscopic (mercury porosimetry, petrographic microscopy, microprobe and isotope data) provide a rich set of data on which we have built our geological and reservoir models.

Mapping hydrothermal alteration zones using ASTER images in the Arabian–Nubian Shield: A case study of the northwestern Allaqi District, South Eastern Desert, Egypt

Abstract: In order to map and differentiate ore-bearing hydrothermal alteration zones at the northwestern part of Allaqi-Heiani, we use a combined method of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image processing, field relationship, mineralogical features, and rock chemical compositions. Hydrothermal alteration zones are identified on the basis of Advanced Spaceborne Thermal Emission and Reflection Radiometer data. Propylitic, phyllic, argillic, and silicification zones are recorded in different types of igneous and metamorphic rocks typically with chromite, goethite, ilmenite, galena and gold. Crosta and CEN techniques using ASTER are successfully applied to highlight the hydrothermal alteration zones and the structural elements represented by lithologic contacts and faults/fracture zones. Combining data of lineaments, alteration zones and ore mineralogy studied after obtaining a consequence of each map allowed predicting and mapping areas of probable high mineral resources. According to petrographic and remote sensing studies, types of alteration include sericitization, chloritization, epidotization, kaolinitization, carbonatization and silicification. The results of remote sensing in the study area are confirmed by X-ray diffraction (XRD), petrography, and SEM. In the studied district, there are ophiolitic ultramafic rocks, island-arc assemblage of metasediments, volcaniclastics and metagabbro-diorite, syntectonic granite and dykes. Accordingly, to the present study can identify the most potential areas of mineralization taking us into consideration the overlapping of high lineament density areas with abundant alterations.

Geochemical signatures of mineralizing events in the Juomasuo Au–Co deposit, Kuusamo belt, northeastern Finland

Abstract: The Juomasuo Au–Co deposit, currently classified as an orogenic gold deposit with atypical metal association, is located in the Paleoproterozoic Kuusamo belt in northeastern Finland. The volcano-sedimentary sequence that hosts the deposit was intensely altered, deformed, and metamorphosed to greenschist facies during the 1.93–1.76 Ga Svecofennian orogeny. In this study, we investigate the temporal relationship between Co and Au deposition and the relationship of metal enrichment with protolith composition and alteration mineralogy by utilizing lithogeochemical data and petrographic observations. We also investigate the nature of fluids involved in deposit formation based on sulfide trace element and sulfur isotope LA-ICP-MS data together with tourmaline mineral chemistry and boron isotopes. Classification of original protoliths was made on the basis of geochemically immobile elements; recognized lithologies are metasedimentary rocks, mafic, intermediate-composition, and felsic metavolcanic rocks, and an ultramafic sill. The composition of the host rocks does not control the type or intensity of mineralization. Sulfur isotope values (δ34S − 2.6 to + 7.1‰) and trace element data obtained for pyrite, chalcopyrite, and pyrrhotite indicate that the two geochemically distinct Au–Co and Co ore types formed from fluids of different compositions and origins. A reduced, metamorphic fluid was responsible for deposition of the pyrrhotite-dominant, Co-rich ore, whereas a relatively oxidized fluid deposited the pyrite-dominant Au–Co ore. The main alteration and mineralization stages at Juomasuo are as follows: (1) widespread albitization that predates both types of mineralization; (2) stage 1, Co-rich mineralization associated with chlorite (± biotite ± amphibole) alteration; (3) stage 2, Au–Co mineralization related to sericitization. Crystal-chemical compositions for tourmaline suggest the involvement of evaporite-related fluids in formation of the deposit; boron isotope data also allow for this conclusion. Results of our research indicate that the metal association in the Juomasuo Au–Co deposit was formed by spatially coincident and multiple hydrothermal processes.

Gold occurrences of the Woumbou–Colomine–Kette district, eastern Cameroon: ore-forming constraints from petrography, SEM–CL imagery, fluid inclusions, and C–O–H–S isotopes

Abstract: The Woumbou–Colomine–Kette district is located in the East Metallogenic Province of the Republic of Cameroon. Northeast- to E-striking splays of the regional Pan-African crustal-scale Sanaga Shear Zone control widespread gold mineralization. Gold occurs within local shear zones developed along granite-gneiss contacts, as well as within the margins of the Pan-African intrusions. Field investigations coupled with mineralogical and textural studies reveal two stages of gold mineralization. Stage 1 led to pyrite, chalcopyrite, sphalerite, pyrrhotite, bismuthinite, tellurobismuthite, galenobismuthite, and native gold deposition with quartz, sericite/muscovite, chlorite, calcite, and ankerite as gangue minerals. Stage 2 included the precipitation of pyrite, hematite, tellurobismuthite, and native gold. A SEM–CL study distinguished the two stages of quartz, as well as an early remnant barren quartz depositional stage. Two fluid inclusion assemblages (FIAs) with four types of fluid inclusions can be observed. The FIA 1 is aqueous-carbonic (±N2±CH4), with carbonate daughter minerals, which is related to stage 1 of the gold mineralization and composed of low salinity (~7.5 wt% NaCl equiv) H2O-CO2-NaCl (type 1), CO2-rich (type 2), and H2O-rich (type 3a) fluid inclusions. The inclusions provide evidence of episodic phase separation, with unmixing at P~2 kbar, T~300 °C, and at a paleodepth of ~7 km. The FIA 2 consists of aqueous (type 3b) fluid inclusions with a salinity of 0.2 to 11.7 wt% NaCl equiv and trapping temperatures between 205 and 245 °C. The δ18Oquartz (+11.0 to +11.7‰), δDfluid inclusions (−46.2 to −40.9‰) and δ13Cfluid inclusions (−4.9 to −4.1‰) for gold-bearing quartz veins suggest a metamorphic source for the ore-forming fluids, although some contribution from the mantle and/or magmatic source(s) cannot be ruled out. The δ34S (+6.5 to +7.0‰) for gold-bearing pyrites suggests a metamorphic source for the sulfur that transported gold and was involved in the precipitation of sulfide minerals. These data are consistent with a mesozonal orogenic gold deposit model for the Woumbou–Colomine–Kette district, which is an important part of the emerging greenfields exploration region for Pan-African orogenic gold and with the more significant targets defined by granite-gneiss contacts along NE- and E-striking shear zones.

Quantification of Lithological Heterogeneity Within Opalinus Clay: Toward a Uniform Subfacies Classification Scheme Using a Novel Automated Core Image Recognition Tool

Abstract: The Opalinus Clay is notable in Switzerland as being the selected host rock for deep geological disposal of radioactive waste. Since the early 1990’s, this argillaceous mudstone formation of Jurassic age has been intensively studied within the framework of national and international projects to characterize its geological, hydrological, mechanical, thermal, chemical, and biological properties. While there is no formal stratigraphic subdivision, the Opalinus Clay lithology is classically divided into several, dam- to m-scale sub-units (or facies), depending on location. Recent multi-proxy studies (combining petrographic, petrophysical, geochemical and mineralogical analyses) have however demonstrated that high, intra-facies, lithological heterogeneity occurs at the dm- to cm-scale. To constrain this small-scale heterogeneity into distinct lithological units (subfacies), the present study aims at defining and presenting a convenient subfacies classification scheme covering the overall Opalinus Clay lithology across northern Switzerland. Petrographic (macro- and microfacies), mineralogical (X-ray diffraction) and textural (image analysis, machine learning and 3D X-ray computed tomography) analyses are performed on diverse drill cores from the Mont Terri rock laboratory (northwestern Switzerland), and results are extended further to the east (Riniken, Weiach and Benken). Most of the investigated Opalinus Clay can be described by the use of five distinctive subfacies types (SF1 to SF5), which are visually and quantitatively distinguishable by texture (grain size, bedding, fabric and color) and composition (nature and mineralogy of components). The five subfacies types can be further refined by additional attributes and sedimentary characteristics (biogenic, diagenetic and structural). Eventually, the widespread and consistent use of standardized Opalinus Clay subfacies types provides the means to harmonize petrographic descriptions within multidisciplinary research projects, enhance reproducibility of in-situ experiments, and further evidence the tight relations between lithology and various rock properties.

Geochemistry and mineralogy of the Silurian Akkas Formation, Iraqi western desert: implications for palaeoweathering, provenance and tectonic setting

Abstract: The mineralogical and geochemical composition of the Silurian Akkas Formation consisting of interbedded shales, sandstones, siltstones and rare marls from the western desert of Iraq was investigated based on geochemical and petrographic data to constrain their weathering condition, palaeoclimate, provenance and tectonic setting. Based on petrographic and SEM analysis, sandstones are dominated by quartz with rare rock fragments, mica and feldspar, whereas clay minerals assemblage is dominated by illite and kaolinite with a lesser amount of chlorite in shales and chlorite with a minor amount of illite, kaolinite and mixed layer illite/smectite in sandstones. The chemical index of alteration (high CIA), the index of compositional variability (ICV < 1), plagioclase index of alteration (high PIA) values and A–CN–K plot suggest mainly intensely chemically weathered source area. The elemental ratios critical for palaeoclimate (Sr/Cu and Ga/Rb) and Ga/Rb vs Sr/Cu plot in addition to clay mineral evidence indicate fluctuating climate between humid and arid conditions, which was affected by the extensive Late Ordovician–Early Silurian glaciation (Hirnantian glaciation event) during the deposition of the Akkas sediments. The geochemical data from major, trace and REE elements, as well as bivariate discrimination provenance diagrams, suggest mainly felsic–intermediate igneous source rocks. The mineralogical composition of sandstones (high quartz content, rare feldspar and rock fragments) and the high geochemical maturity of shales and sandstones indicate that they were derived from stable cratonic source and recycled orogeny with the possible contribution from quartz-rich crystalline provenance. These sources were likely the plutonic–metamorphic complexes of the Arabian Shield located to the southwest of the Akkas basin. The detrital mode tectonic provenance diagram, discriminant function-based major element diagrams and REE concentrations suggest continental margin/intracratonic depositional setting mixed with an older collisional source, which is consistent with the general geology of the study area.

Sedimentology, petrography, and reservoir quality of the Zarga and Ghazal formations in the Keyi oilfield, Muglad Basin, Sudan.

Abstract: The Zarga and Ghazal formations constitute important reservoirs across the Muglad Basin, Sudan. Nevertheless, the sedimentology and diagenesis of these reservoir intervals have hitherto received insignificant research attention. Detailed understanding of sedimentary facies and diagenesis could enhance geological and geophysical data for better exploration and production and minimize risks. In this study, subsurface reservoir cores representing the Zarga formation (1114.70–1118.50 m and 1118.50–1125.30 m), and the Ghazal formation (91,403.30–1406.83 m) were subjected to sedimentological (lithofacies and grain size), petrographic/mineralogic (thin section, XRD, SEM), and petrophysical (porosity and permeability) analyses to describe their reservoir quality, provenance, and depositional environments. Eight (8) different lithofacies, texturally characterized as moderately to well-sorted, and medium to coarse-grained, sub-feldspathic to feldspathic arenite were distinguished in the cored intervals. Mono-crystalline quartz (19.3–26.2%) predominated over polycrystalline quartz (2.6–13.8%), feldspar (6.6–10.3%), and mica (1.4–7.6%) being the most prominent constituent of the reservoir rocks. Provenance plot indicated the sediments were from a transitional continental provenance setting. The overall vertical sequence, composition, and internal sedimentary structures of the lithofacies suggest a fluvial-to-deltaic depositional environment for the Ghazal formation, while the Zarga formation indicated a dominant deltaic setting. Kaolinite occurs mainly as authigenic mineral, while carbonates quantitatively fluctuate with an insignificant amount of quartz overgrowths in most of the analyzed cores. Integration of XRD, SEM, and thin section analysis highlights that kaolinite, chlorite, illite, and smectite are present as authigenic minerals. Pore-destroying diagenetic processes (e.g. precipitation, cementation, and compaction etc.) generally prevailed over pore-enhancing processes (e.g. dissolution). Point-counted datasets indicate a better reservoir quality for the Ghazal formation (ɸ = 27.7% to 30.7%; K = 9.65 mD to 1196.71 mD) than the Zarga formation (17.9% to 24.5%; K = 1051.09 mD to 1090.45 mD).