129 / 2022-02-24 16:27:11

Sedimentary response and characteristics to the boundary fault activity during post⁃rift period of continental lacustrine rift basin:A case of Jiufotang formation in Ludong sag，Kailu Basin

post-rift fault activity , sedimentary response, sedimentary characteristics, Ludong sag

During the post-rift filling period of continental faulted lacustrine basin, high quality oil and gas exploration reservoirs can be formed by clastic sandbodies deposited from the basin slope to the subsag. The tectonic evolution stage of early Cretaceous in Ludong Sag, Kailu Basin can be divided into four stages: strong fault-depression stage, rapid subsidence stage, stable subsidence stage and fault-depression transition stage. As the main oil-bearing strata in the early Cretaceous of the sag, jiufotang Formation was deposited in the stage of rapid subsidence. Among the deposition process, jiufotang Formation experienced a complete set of three system tracts, namely, lowstand system tract(LST), transgressive system tract(TST) and highstand system tract(HST). Based on the theory of sequence stratigraphy and sedimentology, 3D seismic data, drilling core data and well-logging data, Sag steep slope boundary fault activity, paleogeomorphology, lithofacies and sedimentary microfacies were comprehensively analyzed. The study shows that the types of sedimentary system is controlled by paleogeomorphy, which is mainly influenced by the intensity of fault activity at the boundary of steep slope zone. In the early HST of the Jiufotang Formation, the strongly active boundary depression-controlled faults leading to the rapid maximization of the accommodable space and the formation of the short-axial provenance system. A large number of detrital sources quickly accumulated in the sedimentation center of the basin, forming a large scale alluvial fan and nearshore underwater fan. With the weakening of the boundary fault activity and the filling of debris, the lake basin gradually showed a shallow basin setting from LST to TST. The provenance was still sufficient due to the inheritance of the post-rift source system. A large area of "sand-rich" pattern was formed in the basin, which was dominated by fan-delta and shallow lake sedimentary system. At the end of TST, the lake basin rapidly entered the deep lake environment due to the recovery of the boundary fault activity. Meanwhile, the provenance began to shrink, and small nearshore underwater fans and large sets of shale were deposited under the deep lake system sedimentary environment in HST period. According to 27 lithofacies types and 11 lithofacies associations were identified for different sedimentary systems, which mainly reflect three transport types of nearshore rapid accumulation, long-source transport deposit and far-source turbidity current deposit, and five types of fluid including traction flow, clastic flow, slide and slump block, turbidity current and in-situ settlement. The sandbodies developed in different sedimentary systems have obvious differences in hydrodynamic origin. Among all the different types of sand bodies, the traction flow sand body of fan-delta front face is the most important oil-bearing reservoir in Jiufotang formation.