Geochemical characteristics and factors controlling the deep lithologic reservoirs in Puwei Sag, Dongpu Depression—A Case Study of Well PS20

As the advances of the exploration in shallow layers, petroleum exploration moves toward deep layers has become an inevitable choice. However, the hydrocarbon accumulation mechanism in deep reservoir remains a big challenge for the petroleum exploration. The Dongpu Depression is one of the major petroliferous basins in the Bohai Bay Basin of eastern China. The deep Well PS 20 discovered in the Puwei Sag of Dongpu Depression suggests that there is great prospecting in deep reservoir. In this paper, the geochemical characteristics and main controlling factors of deep lithologic reservoir in the third member of the Paleogene Shahejie Formation (Es3) of Well PS20 are analyzed by means of geological and organic geochemical methods including rock-eval, Gas Chromatgraphy-Mass Spectrometry (GC/MS) and Scanning Electron Microscopy (SEM). The results show that the main source rocks are situated in the middle and lower parts of the third member in the Shahejie Formation of Eocene (Es3M-Es3L). They are characterized by higher organic content (>0.6%), good organic matter (most of them are type II, and some are type I) and higher maturity (Ro>1.0%). The oil-source comparison shows that the origin of the Es3U is a mixture from both local and off-sited sources. The origin of the hydrocarbon in Es3M is from a remote source, and the hydrocarbons in Es3L are indigenous. The reservoir properties reveal that the deep strata have been compacted with increasing depth, and higher oil content only occurred in less tight strata with good physical properties. The deep hydrocarbon reservoir in Puwei Sag is controlled by the source rock quality, reservoir properties and sand body thickness. The reservoir physical properties control the oil and gas accumulation and distribution. Sand thickness controls the oil content and hydrocarbon accumulation. The ultra-thin and thin layers in the lower member of Es3 are characterized by multiple layers (>85% of total layer numbers）and thick reservoir layers (>65% of total thickness). The oil saturation in the ultra-thin and thin layers is much higher than that in the thick layers. Numerous micro-fractures produced by abnormally high pressure should be responsible for the higher oil content. Besides, both sandstone and mudstone layers have high brittle mineral content, which also provides advantageous condition for micro-fractures. The results expand the space for petroleum exploration, strengthen the confidence of the new field to achieve a major