Abstract:

The Muglad Basin in Sudan is rich in oil and gas resources and currently is under intensive hydrocarbon exploration. The western slope of the southern-central part of the basin in the western slope zone of the Kaikang depression is one of the important exploration areas in the Muglad Basin. At present, the thickness of the remaining strata is about 5000 m as a result of severe denudation during the structural transformation period. Influenced by regional tectonic activities, the evolutionary development of the sedimentary reservoir in the study area is closely related to the tectonic evolution. In the study area, as exploration for tectonic traps entering the middle and late stages, finding new exploration fields and targets has become an urgent task, for which litho-stratigraphic trap exploration becomes an important approach. Currently, the major factor limiting oil and gas exploration is lacking a systematic understanding of the basic issues, such as the sequence stratigraphic structure of the western slope of the southern-central Muglad Basin. Here, we focused on the evolutionary characteristics of the sedimentary sequence of the western slope zone and their controlling factors through a comprehensive analysis of seismic, well logging and core data using the principle of sequence stratigraphy. We discussed the favorable source-reservoir-cap assemblage and predicted and evaluated the favorable stratigraphic traps in the study area. The main results and achievements are as follows: 1) The basin experienced three large rift movements and three post-rift thermal subsidence depression movements, which resulted in six secondary sequences. In this secondary sequence stratigraphic framework, the target formations, from Abu Gabra to Adok, were divided into thirteen third-order sequences. The filling and evolution of each sequence was controlled by regional tectonic background. 2) Stratigraphic development in the study area experienced three stages of rift-depression while the basin maintained its sedimentary characteristics under the control of structural background in each stage, with obvious difference in sedimentation controls. Based on the analyses of core, logging, earthquake, and paleocurrent in the western slope zone of the southern-central Muglad Basin, it is considered that the lake and braided river delta deposition system mainly developed during the rift period, whereas the braided river and delta mainly developed during the depression period. Due to multistage expansion and contraction of the lake basin, the sedimentary systems exhibited inherited and diverse spatiotemporal distributions. 3) According to the formation sequence of the Cretaceous-Paleogene source rocks, reservoirs and caprocks in the western slope and their spatial assemblage relationships, four sets of source-reservoir-cap assemblages could be identified in the study area featuring two main rock-forming types: the lower generation - upper reservoir type, and the self generation - self reservoir type. In the isochronous sequence stratigraphic framework of the study area, we identified two types of stratigraphic traps: stratigraphic erosion unconformity trap and stratigraphic overlap unconformity trap. The top interface of the sedimentary facies SQ5 in the western part of the study area is a regional erosive unconformity surface. It forms an erosive unconformity triangle with the underlying strata, creating a favorable stratigraphic erosive unconformity trap developing area. The sedimentary facies SQ4 in the northwestern part of the study area overlaps the unconformity surface, forming an overlap unconformity triangle zone—a favorable stratigraphic overlap unconformity trap developing area.

Key words: sedimentary sequence, stratigraphic trap, the western slope of Muglad Basin, Sudan