Shiyou shiyan dizhi (Sep 2022)
Significance and formation mechanism of abnormally pressured compartments of shale gas in the Ordovician Wufeng-Silurian Longmaxi formations, southeastern Sichuan Basin
Abstract
Overpressure is commonly existed in shale gas pools of Jiaoshiba and Dingshan structures in the southeastern part of Sichuan and Huangjinba structure in Changning area of the southern region of Sichuan. From the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation, there are two abnormally pressured compartments of shale gas separated by three sealing layers. The lower compartment is in accordance with the superior shale layer in the Wufeng-Longmaxi formations, sealed by tight marl-bearing limestone and nodular limestone in the Upper Ordovician Linxiang and Baota formations at the bottom and the interbeds of volcanic ash-altered clay rock, thin carbonate rock and mudstone in the central segment of the first sub-member of the first member of Longmaxi Formation on the top, which also works as the sealing layer at the bottom of the upper compartment. Between the first and second members of the Longmaxi Formation, sealing layer for the top of the upper compartment exitsed, which is mainly composed of mudstone or mudstone interbedded with thin carbonate rocks.Researches show that the sealing layers have better sealing ability when they contain tight carbonate rock or thin carbonate rock and are interbedded with mudstone. There are differences in logging curves such as acoustic wave, resistivity, natural gamma, etc. between the top and bottom sealing layers and the compartments. In the upper compartment, the microstructure of shale shows strong anisotropy with low quartz content, high clay content, low organic matter content, poor organic pore connectivity, low total hydrocarbon content, and low porosity. In the lower compartment, the microstructure of shale shows isotropy, with high quartz content, low clay content, high organic matter content, interconnected organic matter, interconnected organic pores, good connectivity, high total hydrocarbon content, and high porosity. These compartments have experienced the early stage of rapid burial, the embryonic stage in which organic matter is immature or low-mature and is dominated by normal fluid pressure system, the formation stage in which a large quantity of hydrocarbon is generated after fast and deep burial and is dominated by overpressure system, the formation stage in which a large quantity of gas is generated after deep burial and high-degree thermal evolution and is dominated by overpressure system, and finally the late stage of the formation of abnormally pressured compartments for shale gas, resulting in two isolated and abnormally pressured compartments for shale gas with three sealing layers on the top or at the bottom, lateral dipping of formation, and lateral sealing of reverse fault. The lower compartment with high porosity, high organic matter content, good connectivity and high pressure coefficient is the priority target for the efficient development of shale gas.
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