Organic matter enrichment and hydrocarbon accumulation models of the marlstone in the Shulu Sag, Bohai Bay Basin, Northern China

Abstract

Hybrid unconventional systems are composed of juxtaposed organic-rich and organic-lean intervals. Organic matter enrichment, hydrocarbon migration, and the spatial relationship of source rocks and reservoirs have important influences on differential hydrocarbon accumulation within the system. Most studies have ignored the complexity of hydrocarbon accumulation in unconventional oil reservoirs. A typical hybrid lithology system with a combination of marlstone and rudstone has been discovered in the lower part of the third member of the Shahejie Formation of the Shulu Sag, Bohai Bay Basin, Northern China. The basin showcases a strong heterogeneity in unconventional hydrocarbon production and is an appropriate case to study hydrocarbon accumulation within hybrid unconventional systems. However, relevant work in the study area was scarce. A study of the mechanisms of organic matter enrichment and hydrocarbon accumulation was required to understand the heterogeneity of the oil reservoirs. Nine units in ascending order, namely IL, IU, IIL, IIU, IIIL, IIIU, IV, VL, and VU, were proposed in this study. A detailed geochemical analysis was carried out to distinguish the organic matter sources and preservation conditions among the different units, and assess the migration patterns of the hydrocarbons. The results show that Unit IIL has the highest total organic carbon (TOC) content. Unit IIL has a higher quantity of aquatic sourced organic matter, with relatively high abundance of short chain n-alkanes, C27/C29 sterane, and 4-methyl steranes. Based on the low ratios of pristane to phytane, the varve-like lamination structure of the lithology and suitable salinity, we can speculate that good organic matter preservation conditions existed in Unit IIL. Maturity of the source rocks increases with depth. Biomarkers indicate that hydrocarbons within the marlstones were retained in situ, whereas hydrocarbons within the rudstones were derived from source rocks via short- or long-distance migrations. Fractures can act as effective pathways for hydrocarbon migration. Organic matter sources, preservation conditions, maturity, and hydrocarbon migration ways jointly controlled the formation of oil reservoirs in the Shulu Sag. The results of this study provide a better understanding of organic matter enrichment and hydrocarbon accumulation and guide further exploration in the basin.