川东南地区奥陶系五峰组—志留系龙马溪组页岩气异常压力封存箱剖析与形成机制

Significance and formation mechanism of abnormally pressured compartments of shale gas in the Ordovician Wufeng-Silurian Longmaxi formations, southeastern Sichuan Basin

刘伟新 卢龙飞 叶德燎 何陈诚 刘旺威 俞凌杰 张文涛 申宝剑

LIU Weixin LU Longfei YE Deliao HE Chencheng LIU Wangwei YU Lingjie ZHANG Wentao SHEN Baojian

中国石化 石油勘探开发研究院 无锡石油地质研究所, 江苏 无锡 214126 中国石化油气成藏重点实验室, 江苏 无锡 214126

Wuxi Research Institute of Petroleum Geology, SINOPEC, Wuxi, Jiangsu 214126, China SINOPEC Key Laboratory of Petroleum Accumulation Mechanisms, Wuxi, Jiangsu 214126, China

川东南地区焦石坝构造、丁山构造以及南部长宁黄金坝构造页岩气普遍存在超压现象，上奥陶统五峰组—下志留统龙马溪组页岩气存在由3个隔板构成的2个页岩气异常压力封存箱。下封存箱位置与五峰组—龙马溪组优质页岩层段相当，由上奥陶统临湘组与宝塔组致密含泥灰岩、瘤状灰岩形成页岩气封存箱的底部封隔层；顶板位于龙马溪组龙一段一亚段的中部，由火山灰蚀变黏土岩夹层、薄层碳酸盐岩及泥岩互层组成，同时形成上封存箱的底板；上封存箱顶板位于龙马溪组二段与龙一段分界处，由泥岩或泥岩夹薄层碳酸盐岩互层组成。研究认为页岩气封存箱顶、底板封隔层含致密碳酸盐岩或碳酸盐岩薄层并与泥岩互层时具有更好的封盖能力。封存箱顶底板与封存箱内部在测井曲线上(如声波、电阻率、自然伽马等)存在差异。上、下封存箱内部特征也不同：上封存箱内页岩微观结构显示强的各向异性，石英含量低、黏土含量高，有机质含量低，有机质孔隙连通性差，全烃含量低，孔隙度低；下封存箱内页岩微观结构显示各向同性，石英含量高、黏土含量低，有机质含量高，有机质相连，有机孔相通，连通性好，全烃含量高，孔隙度高。该页岩气异常压力封存箱经历了早期快速埋藏、有机质未熟低熟、以正常流体压力系统为主的封存箱雏形阶段，快速深埋大量生烃的液态烃类流体超压封存箱形成阶段，深埋及高过成熟大量生气的超压封存箱形成阶段，以及晚期页岩气异常压力封存箱改造定型阶段，最终形成由3个隔板组成页岩气底封顶盖、地层侧向倾伏及逆断层侧向封堵、空间上相互叠置的上、下2个互相独立的页岩气异常压力封存箱。孔隙度高、有机质含量高、连通性好及压力系数高的下封存箱是页岩气高效开发的优先目标。

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.

国家自然科学基金项目 U19B6003-02;国家自然科学基金项目 41972164

https://doi.org/10.11781/sysydz202205804