Sedimentology, diagenesis and reservoir quality of the upper Abu Gabra Formation sandstones in the Fula Sub-basin, Muglad Basin, Sudan

Abstract

Successful hydrocarbon exploration of the Lower Cretaceous Abu Gabra Formation has established the need to understand the diagenesis and reservoir quality of the sandstones reservoirs in the area. Therefore, this study contains a subsurface facies analysis based on description of three conventional cores from the Moga21-2 well. Methods include wireline log analysis, a petrographic study with both thin sections and scanning electron microscope investigations, grain-size, X-ray diffraction (XRD), and heavy minerals analysis of the Abu Gabra sandstone.Sedimentary composition and facies sequences of the well logs indicate a complex fluvial channel paleo-environment. Grain-size analysis revealed that the sandstone is a mixture of mainly sand-size (<0.01–2.00 mm) along with minor amount of silt and clay size (<0.063 mm). Alternation of coarse to medium-grained, massive to cross-bedded sandstones with fine-grained ripple laminated sandstones shows a sequence of fluvial channel fill or sand deposits of channel bars. The finer-grained facies association, predominantly mudstone and siltstone, represents overbank deposits or floodplain sediments.Petrographically, the sandstones are classified (based on standard rock classification schemes) into, subfeldspathic arenites, and feldspathic arenite lithofacies. Quartz grains are the most abundant detrital components in the samples (both polycrystalline and monocrystalline), followed by feldspars (mainly K-feldspar), micas, and detrital and authigenic clays with carbonatic detritus. Although, lithic fragments, iron oxides and heavy minerals occur in minor amounts, presence of sillimanite, epidote and hornblende (compared to zircon, tourmaline and rutile), as well as the presence of chlorite, the dominance of polycrystalline quartz and high crystallinity of kaolinite could be responsible for the low grade metamorphism. Kaolinite is an authigenic component, while quartz overgrowths occur in variable quantity and carbonates (calcite and siderite) in a relatively minor amount. Pore types are basically primary and secondary interparticle, with few secondary intraparticle pores due to the dissolution of feldspars. Although porosity ranges from 9.20 to 31.20%, the presence of kaolinite, carbonate, and quartz overgrowth cement reduce the porosity of the samples.Analyzed samples are characterized by poor to very good pore interconnectivity. Despite the fact that most of the sandstones display point, concavo-convex, and long contacts with poor to good compaction, few sutured grain contacts (which reflect a higher degree of compaction) still exist. XRD and SEM analyses show that kaolinite, chlorite, illite, smectite, and illite/smectite mixed layer clay are the common clay minerals in the studied samples. The presence of vermiform morphology, besides the sharp peak pattern of the kaolinite, in most of the XRD pattern indicates that part of the kaolinite was authigenically formed.