Gravity flow sedimentation and tight oil exploration in lower first member of Shahejie Formation in slope area of Qikou Sag, Bohai Bay Basin

Abstract

Based on observation of cores and thin slices and analysis of drilling, log and test data, the gravity flow deposits characteristics and formation conditions and characteristics of tight oil in lower first member of Shahejie Formation (Es1x) were systematically analyzed in Qikou Sag, Huanghua depression, Bohai Bay Basin. There mainly developed sandy debris flow, muddy debris flow and classical turbidity flow in the study area, and sandstone with boulder-clay is mostly sandy debris flow origin, muddy debris flow deposits are mudstone or silty mudstone with sandy lumps, the normal graded bedding is a reliable sign of classical turbidite. Affected by multi-source supply, slope break, different trigger mechanism and fluctuation of lake level, multi-gravity flows formed multi-level fans in mid-low slope to main sag, which can be divided into five micro-facies including submerged main channel, submerged distributary channel, lateral channel margin, mud between channels and turbidite sheet sand. Multi-level and multi-stage gravity flow sediments provide material foundations for tight oil and gas in the deep mud and sand-rich lake, where wide spread tight reservoirs and high quality source rocks contact closely vertically, forming multiple sets of sandwich-like source-reservoir-seal assemblages. Exploration shows that the submerged distributary channel and turbidite sheet sand of sandy debris flow are two main exploration facies of gravity flow in Es1x and important area of scale reserve increase in slope zones of Qikou Sag. Key words: gravity flow sedimentation, tight oil, Shahejie Formation, slope area, Qikou Sag, Bohai Bay Basin