Shale oil accumulation conditions in the second member of Paleogene Funing Formation, Gaoyou Sag, Subei Basin

Abstract

"Carbonate sandwich shale oil" is a main exploration target in the Subei Basin. The deep depression-inner slope zone of the Gaoyou Sag is a favorable area for the exploration of "carbonate sandwich shale oil" in the second member of Funing Formation (E1f2). The source rock and reservoir quality, engineering conditions and movability of each shale section of E1f2 were analyzed in order to further clarify "sweet spot" sections by using scanning electron microscopy, mercury intrusion at elevated temperature, nitrogen adsorption, CT scanning, X-ray diffraction, triaxial mechanics, multi-temperature pyrolysis and other research methods. The E1f2shale1 and E1f2shale2 in the deep depression-inner slope zone of Gaoyou Sag were deposited in a dry, hot, reducing and brackish water environment, developing favorable shale with high organic matter content. The TOC contents are 2.33% and 1.63%, respectively Kerogens are mainly of types Ⅰ and Ⅱ1, with the most favorable conditions for hydrocarbon generation. The interlamellar pores were well developed, and the reservoir space is dominated by macropores and fractures. The brittle mineral content of E1f2shale1 to E1f2shale5 is 59.1%-63.6%, and the clay minerals are dominated by illite/montmorillonite mixed layer and illite. The brittleness index of E1f2shale2 is 64.25% at most. In the range of burial depth greater than 3 500 m, the conversion of the illite/montmorillonite mixed layer to illite is obvious, and the shale brittleness and fracturing transformation conditions are better. The E1f2shale1 and E1f2shale2 in the deep depression-inner slope zone of Gaoyou Sag are exploration directions for shale oil in the Subei Basin.

Keywords

• carbonate rock interlayer
• shale oil
• reservoir-forming conditions
• second member of funing shale
• gaoyou sag
• subei basin