渝东南盆缘转换带常压页岩气地质特征及富集高产规律

四川盆地东南部及其盆缘转换带(以下简称渝东南盆缘转换带)是中国常压页岩气勘探的热点地区。为了总结该区页岩气地质特征及其富集高产规律,利用物探、钻井、测井及分析测试等资料,从沉积建造、构造改造、生产特征等方面分析了该转换带南川—武隆地区的页岩气地质特征,并与焦石坝区块的超压页岩气进行对比,探讨了该转换带常压页岩气的富集高产主控因素和成藏模式。研究结果表明:(1)相对于焦石坝区块,该转换带页岩孔隙度偏低、微裂缝更发育、吸附气占比高、两向应力差异大、地温梯度较低、地层压力系数低、初期产液量大、返排率高;(2)该区常压页岩气富集高产主要受富碳富硅富笔石页岩、有机孔隙和构造应力场"三因素"的控制——受深水陆棚相控制的富碳富硅富笔石页岩是页岩气富集的基础,有机孔隙是页岩气富集的主要控制因素,构造应力场是页岩气高产的关键因素;(3)该转换带常压页岩气可分为背斜型、向斜型、斜坡型、逆断层断下盘型等4种成藏模式,并明确了不同模式页岩气富集高产特征。结论认为,该研究成果丰富了常压页岩气富集高产地质理论,为复杂构造区常压页岩气的勘探开发提供了支撑。

Geological characteristics and enrichment laws of normal-pressure shale gas in the basin-margin transition zone of SE Chongqing

The southeastern Sichuan Basin and its basin-margin transition zone (hereinafter referred to as the basin-margin transition zone of SE Chongqing) is the focus of normal-pressure shale gas exploration in China. In order to summarize the geological characteristics and enrichment laws of shale gas in the basin-margin transition zone of SE Chongqing, we analyzed the geological characteristics of shale gas reservoirs in the Nanchuan–Wulong area of this transition zone from the aspects of sedimentary formation, tectonic reworking and production characteristics by using geophysical, drilling, logging and testing data, and then we compared it the overpressure shale gas reservoirs in Jiaoshiba Block. Finally, we explored the main factors controlling the enrichment & high yields of normal-pressure shale gas in this transition zone and their hydrocarbon accumulation patterns. And the following research results were obtained. (1) Different from the overpressure shale gas reservoirs in Jiaoshiba Block, the normal-pressure shale gas reservoirs in this transition zone are characterized by lower organic porosities, more developed micro-fractures, higher ratios of adsorbed gas, greater differences of stresses in two directions, lower geothermal gradients, lower formation pressure coefficients, higher initial fluid production rates and higher fluid flowback rates. (2) The enrichment & high yields of normal-pressure shale gas in this area is mainly controlled by three factors, i.e., carbon-rich, silicate-rich and grapholite-rich shale, organic pores, and tectonic stress field, among which, the first factor controlled by deepwater continental shelf facies is the basis of shale gas enrichment, the second is the main controlling factor of shale gas enrichment, and the third is the key factor of high-yield shale gas. (3) The hydrocarbon accumulation patterns of normal-pressure shale gas reservoirs in the transition zone can be divided into four types, including the anticline type, the syncline type, the slope type and the reverse fault type. And the enrichment & high-yield characteristics of shale gas in different hydrocarbon accumulation patterns are also clarified. In conclusion, the research results enrich the geological theory of enrichment & high-yield laws of normal-pressure shale gas and provide a support for the exploration and development of normal-pressure shale gas in complex structures.