关键词: Termit盆地, 古近系构造样式, 走滑伸展, 构造调节带, 油气聚集

Abstract: Niger Termit Basin experienced the first and second rifting cycles during the Early Cretaceous and EoceneOligocene, respectively. The two cycles originated through different geodynamic and kinematical mechanisms; as a result, they have different structural styles and basin features. This paper analyzed the geodynamic origin, kinematical features and structural styles of the second rifting cycle, as well as implications of these factors in hydrocarbon accumulation in the Termit Basin, Niger. The findings are the following. During the EoceneOligocene, and under the regional compression stress field caused by the AfricaArab and Eurasia plate collision, the Termit Basin moved as an extrusion from the inner Africa plate in the NEESWW direction and acted as typical strikeslip with extensional characteristics. Building on the NWSE fault system in the first rifting cycle, the second rifting cycle developed the NNWSSE fault system in the Termit Basin. The basin has primarily grabenhorst and half graben structures, with graben style in the north, and Domino half graben style in the south. The EoceneOligocene transtension formed a series of en echelon normal faultsconnected by soft linkageand developed a large number of strike slip transfer structures at both basin and subbasin levels. The Paleogene structure transfer zones could be divided into three basic styles, i.e., parallel, antiparallel and back to back styles, of which, overlapping parallel, antiparallel and back to back overlapping transfer zones are the main structural styles observed in the Termit Basin. The second rifting cycle controlled the EoceneOligocene depositional system of the Termit Basin. During the Eocene initial rifting stage, delta system was formed in Sokor1 Fm.; and during the OligoceneEarly Miocene deeprifting stage, lacustrine mudstone was deposited in the Sokor2 Fm., which generated the most advantage reservoirseal assemblage in the Termit Basin. Meanwhile, structural transfer in the second cycle helped to form many fault blocks and noses and faulted anticline structures, which account for the main types of traps in the Termit Basin. Furthermore, during the OligoceneEarly Miocene deeprifting stage, the main active fault became connected to the Cretaceous source kitchen, this led to extensive vertical and lateral hydrocarbon migrations in the Termit Basin. The growth indices of the main faults implied that the Paleogene hydrocarbon distribution and accumulation were controlled by the fault activity during the OligoceneEarly Miocene deeprifting stage. Based on above analysis, the lithological traps of the Sokor1 and midlower Sokor2 Fms. —located on the strike slope of transfer zones, and new petroleum system in the Upper Sokor2 Fm., all have hydrocarbon potentials and could be further explored.

Key words: Termit Basin, Paleogene structure styles, strikeslip and extension, structure transfer zone, hydrocarbon accumulation