Fault-sand combination modes and hydrocarbon accumulation in Binhai fault nose of Qikou Sag, Bohai Bay Basin, East China

Abstract

Based on seismic and logging data, taking the downthrow fault nose of Binhai fault in Qikou Sag as the object of study, we analyzed fault characteristics, sand body distribution, fault-sand combinations and hydrocarbon accumulation to reveal the hydrocarbon enrichment law in the fault-rich area of fault depression lake basin. The results show that the Binhai Cenozoic fault nose is characterized by east-west zoning, the main part of the western fault segment is simple in structure, whereas the broom-shaped faults in the eastern segment are complex in structure, including several groups of faults. The difference of fault evolution controls the spatial distribution of sand bodies. The sand bodies are in continuous large pieces in the downthrow fault trough belt along the Gangdong Fault in the middle segment of the fault nose, forming consequent fault-sand combination; whereas the fault activity period of the eastern part of the fault nose was later, and the sand bodies controlled by paleogeomorphology are distributed in multi-phase north-south finger-shaped pattern, forming vertical fault-sand combination pattern matching with the fault. The configuration between faults and sand bodies, and oil sources and caprocks determine the vertical conductivity, plane distribution and vertical distribution of oil and gas. Two oil and gas accumulation modes, i.e. single main fault hydrocarbon supply–fault sand consequent matching–oil accumulation in multi-layers stereoscopically and fault system transportation–fault sand vertical matching–oil accumulation in banded overlapping layers occur in the middle and eastern segments of the fault nose respectively, and they control the difference of oil and gas distribution and enrichment degree in the Binhai fault nose. Key words: Bohai Bay Basin, Qikou Sag, Binhai fault nose, sandstone development mechanism, fault-sand combination, oil and gas accumulation, reservoir control mechanism