Depositional characteristics and reservoir potential of Paleogene sediment gravity flow deposits on a faulted slope of the Zhanhua Sag, Bohai Bay Basin, China

Abstract

Deep-water sandstones related to sediment gravity flows are becoming an increasingly important exploration domain in continental rift basins in China. However, ambiguities remain regarding the initiation, evolution, and deposition of sediment gravity flows on faulted slopes, where step faults are commonly developed. To address the uncertainties related to these processes, grain size analysis, lithofaciesanalysis, logging interpretation, and seismic interpretation were employed to study Paleogene sediment gravity flow deposits from well cores on a faulted slope in the Zhanhua Sag of the Bohai Bay Basin. Twelve lithofacies related to sediment gravity flows were recognized and attributed to intrabasinalsediment gravity flows (ISGFs) induced by slope failures and extrabasinal sediment gravity flows (ESGFs) generated by floods. Active faults were important triggers for ISGFs on the faulted slope in the Zhanhua Sag. The fault activity rate had a prominent role in determining the volume of re-transported sediments, which further influenced the evolution of the resulting flows. Travelling across multiple faults downslope, ISGFs generally underwent considerable velocity losses, with the result that almost all sediments were deposited in shallow water. Moreover, unconsolidated ISGF deposits could be transported again to generate sediment gravity flows, obscuring the depositional characteristics of ISGFs. ESGFs originated from rivers during floods and involved the transformation of cohesive into turbulent hyperpycnal flows. In humid and warm climates, active sag-border faults favoured the generation of long-lived ESGFs. These flows usually travelled along intra-sag faults and therefore considerable volume of sediments could be transferred into deep water. The resulting thick-bedded hyperpycnites contributed substantially to the reservoirs of Bonan Oilfield. Meanwhile, short-lived ESGFs usually generated in the regions where sag-border faults were extinct. They generally travelled down the faulted slope and deposited sediments in shallow water. These features suggest that the initiation, evolution, and deposition of sediment gravity flows on the faulted slope are mainly controlled by faults and climate. Hyperpycnites can serve as excellent reservoirs for hydrocarbon accumulation and they are probably common in other sags of the Bohai Bay Basin.