Depositional Environment and Reservoir Characterization of the Cretaceous Bentiu & Aradeiba Formations in Diffra-Hamam oil fields, Muglad Basin- Sudan

Abstract

Abstract Yousif Abuobida Yousif Mohamed This work was conducted in the area of Diffra and Hamam oil fields, which are located in Muglad Rift Basin. Muglad Rift Basin is the largest sedimentary petroliferous basin in Sudan. The overall objective of this study is to investigate the evolution and hydrocarbon resources in the Diffra-Hamam sub-basin. The study in particular, deals with the subsurface facies sedimentology of the Cretaceous siliciclastic sediments of Bentiu and Aradeiba Formations within Diffra- Hamam oil fields. Within this scope the study is designed to highlight many sedimentological and petro physical aspects, which include: facies analysis that based on conventional core description and analysis, interpretation of wire line logs from seven wells within the study area as well as interpretation of seismic cube across the area; petrographical investigation for 71 chip core samples using the plane polarized microscope besides 40 chip core samples under the scanning electron microscope (SEM); clay minerals analysis for 62 samples using X-ray diffraction and scanning electron microscopy techniques. The study includes petro physical analysis and interpretation for the 7 wells using the software IP (Interactive Petrophysics). The subsurface lithofacies analysis has revealed that: the maximum thickness of the sediment strata is about 3300m towards the North Eastern direction, whereas the minimum thickness is about 2000m in the South Western part of the study area. Seven lithofaceis have been recognized in the study area these are: Massive sandstone facies (Sm), Trough cross bedded sandstone facies (St), Planner cross bedded sandstone facies (Sp), Ripple cross lamination facies (Sr), Horizontal bedded sandstone facies (Sh), Fine lamination siltstone/mudstone facies (Fl) and Massive mudstone/siltstone facies (Fsm). The sediments of the Bentiu and Aradeiba Formations have been subdivided into three units of first order sequence, which are fluvial-dominated-sequence unit, deltaic-dominated-sequence unit and lacustrine-dominated-sequence unit. However, the Diffra- Hamam sub-basin has been found to be dominated by three major i sedimentation cycles, which were controlled by the autocyclic and the allocyclic processes. The Petrographical analysis has shown that: the sandstone of Bentiu Formation is dominantly quartz areintes, whereas, the sandstone of Aradeiba Formation is dominantly subfeldspathic arenites. The provenance of the reservoir rocks in both of the studied formation seemed to be derived from igneous/metamorphic origin. In addition, the petrographical analysis has revealed that the main factors which have decreased the porosity of the sandstone of both of the studied formations are; kaolinite precipitation, compaction and quartz overgrowths. However, the dissolution of the feldspars and mica gave rise to increase porosity. The clay mineral analysis allowed the subdivision of both of the studied formations into two clay mineral zones, which reflect mainly different environmental and diagenetic conditions. The lower clay mineral zone consists of kaolinite, chlorite, illite and smectite whereas; the upper zone consists of kaolinite, chlorite and illite. The higher amount of the kaolinite in the two zones suggests intensity of chemical weathering and leaching processes under a warm-humid climate. However the presence of the smectite in the lower zone suggests that the warm-humid climate was interrupted by dry periods. The well logs analyses have indicated that: the porosity of Bentiu Formation sandstone (21% - 15%) is better than that of Aradeiba Formation sandstone (16% - 12%). This could be due to effects of the kaolinite precipitation and compaction diagenetic processes. These results have been already confirmed by the point counted results. Furthermore, the volume of clay within the Bentiu Formation does not exceed 30% in average whereas, that of Aradieba Formation ranges between 30% and 50%