Geology of shales in prolific shale-oil well BYP5 in the Jiyang Depression, Bohai Bay Basin

Shales in the lower sub-member of the 3^rd member of the Paleogene Shahejie Formation (Es^3l section) in prolific shale-oil well BYP5 in the Jiyang Depression are of typical carbonate-rich type with high maturity. Research on their geological characteristics is of analogy and reference significance for the exploration of similar shales. We delve into the basic characteristics of these shales in terms of mineral composition, thin layer structure, hydrocarbon-generating condition, hydrocarbon fluid property, and reservoir space type. Based on the anomalies of geochemical parameters, we discuss the micro-migration adjustment and accumulation mechanism of shale oil, determine the lower limit of the oil saturation index (OSI), total organic carbon (TOC) content, and porosity for shale oil mobility. Therefore, the geological conditions favorable for high shale oil production are concluded. As revealed by this study, shales in the Es^3l section in well BYP5 is predominantly of carbonate-rich type, characterized by thin layer structure dominated by argillaceous and micritic calcite thin layers. With TOC content ranging from 0.58 % to 7.98 % (average: 4.52 %) and Type Ⅰ organic matter predominating, the shales in the study area are at the stage of light oil and condensate gas generation. With porosity between 2.2 % and 6.9 % (average: 3.22 %), the dominant storage spaces are matrix pores, followed by inter-layer and cross-cutting fractures. The lower limit of the shales’ OSI for oil prodution is less than 50 mg/g, while that of their TOC content and porosity for oil prodution is 1 % and 2.2 %, respectively. The geological conditions favorable for high shale oil production are as follows: (1) High organic matter abundance and high hydrocarbon-generating potential as a result serve to lay a solid material foundation for oil enrichment and flow; (2) High hydrocarbon mobility significantly reduces the lower limit of effective reservoir properties for hydrocarbon storage; (3) Abnormally high pressures provide sufficient natural energy for oil production; (4) The lamellar/layered structures of shales determine the high efficiency of hydrocarbon generation, storage, and permeability of the reservoir; (5) Multiple types of fractures like inter-layer and cross-cutting fractures can effectively connect matrix pores on both sides of the fractures, facilitating the oil recovery from the matrix pores.