Study On Tectonics, Sedimentology And Reservoirs In Melut Basin, Sudan

Melut basin is a weakly-inversed passive rift basin within extension tectonic background and demonstrates typical tectonic features of intra-cratonic rift basin. Comprehensively applying many kinds of data, this paper established the stratigraphical framework from Cretaceous to Paleogene formations in the basin after divided eight sequence boundaries from SB1 to SB8 in Cretaceous -Paleogene ages. The paper also set up four long-term base-level cycles, including Early Cretaceous, Late Cretaceous, period from Samma Fm to Lau Fm and period from Jimidi Fm to Agor Fm, which correspond with the three rifting stages and one depressing stage during Cretaceous -Paleogene ages in the basin.Five sedimentary systems were identified based on drilling data, log facies, seismic facies and results of core analysis, together with results of heavy mineral and grain-size analysis. These systems include braided river delta, fan delta, river-alluvial plain, collapse turbidite fan and lacustrine facies. Sedimentary features of the basin and its filling evolution history are resulted from low intra-basin relief, indistinct differentiation of macro-facies, thick source rocks and cap rocks, relatively stable tectonic architecture and successive provenance with simple ingredient composition; consequently, distribution of major sand bodies in the basin was controlled by the structural accommodation zones.The intense rifting activities from Early Cretaceous to Paleogene had resulted in a quite favorable framework for development of deep lacustrine lakes and source-rich depressions. Both are fundamental elements for hydrocarbon-rich depressions. North depression is a depression of such kind in the basin. Exploration practices demonstrate that one set of source rock deposited in a given depression during a rifting stage controls its hydrocarbon distribution. The predominant source rock in the basin is Cretaceous shales based on tectonic evolution history, characters of stratigraphical sequences and geochemistry analysis.Era-spanning pattern of hydrocarbon enrichment was established based on tectonic development and sedimentary evolution of the basin and its Petroleum geologic properties . The pattern is distinguishable from that in Muglad and Doba basin. Several new viewpoints have been put forward based on the distinct features of hydrocarbon enrichment in the basin, where hydrocarbon was sourced by Cretaceous lacustrine shales and accumulated in Paleogene reservoir. These viewpoints include that "late geothermal increment" led to delay of thermal evolution and controlled late hydrocarbon migration and enrichment subsequently, and hydrocarbon was trapped predominately in the structural accommodation zones after migrating through a series of strata with giant thickness vertically and long distance horizontally. These new viewpoints have broadened the knowledge of petroleum geology of rift basin and reached international advanced level.The north depression has been recognized as an oil-rich petroleum system according to results of stratigraphical, structural studis and accumulation assemblage analysis. Ten favorable plays have been identified in the basin. These plays are rich of sand bodies in Yabus and Samma formations with high reserves per unit volume and possess anticlines as the main structure style. These conclusions have laid a sound foundation for the new discoveries and put forward subsequent prospective areas for the future exploration campaigns.