College of Earth Sciences, Jilin University, Changchun, 130061, China.
Department of Geology, Federal University Lokoja, P. M. B 1154, Lokoja, Nigeria.
Sci Rep. 2021 Jan 12;11(1):743. doi: 10.1038/s41598-020-80831-y.
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).
扎尔加和加扎勒河地层构成了苏丹穆格莱德盆地重要的储层。然而，这些储集层段的沉积学和成岩作用迄今尚未得到充分的研究关注。详细了解沉积相和成岩作用可以增强地质和地球物理数据，以更好地进行勘探和生产，并最大限度地降低风险。在这项研究中，对代表扎尔加组（1114.70-1118.50 m 和 1118.50-1125.30 m）和加扎勒河组（91,403.30-1406.83 m）的地下储层岩心进行了沉积学（岩相和粒度）、岩矿（薄片、XRD、SEM）和岩石物理（孔隙度和渗透率）分析，以描述其储层质量、物源和沉积环境。在岩心段中区分出了 8 种不同的岩相，其结构特征为中等到良好分选，中等到粗粒，次长石质到长石质砂岩。单晶体石英（19.3-26.2%）占主导地位，多晶石英（2.6-13.8%）、长石（6.6-10.3%）和云母（1.4-7.6%）是储层岩石的最主要成分。物源图表明，沉积物来自过渡大陆物源区。岩相的总体垂直序列、组成和内部沉积结构表明，加扎勒河组为河流到三角洲沉积环境，而扎尔加组则指示主要为三角洲环境。高岭石主要以自生矿物的形式存在，而碳酸盐在大多数分析岩心的定量波动中与少量石英增生相伴。XRD、SEM 和薄片分析的综合表明，高岭石、绿泥石、伊利石和蒙脱石为自生矿物。破坏孔隙的成岩作用过程（如沉淀、胶结和压实等）普遍超过增强孔隙的过程（如溶解）。点计数数据集表明，加扎勒河组的储层质量更好（φ=27.7%至 30.7%；K=9.65 mD 至 1196.71 mD），而扎尔加组的储层质量较差（17.9%至 24.5%；K=1051.09 mD 至 1090.45 mD）。
