焦石坝五峰组-龙马溪组页岩裂缝脉体成因及流体包裹体捕获古温压信息

The overpressure phenomenon is widespread occurrence in Wufeng-Longmaxi shales, Jiaoshiba area. The overpressure of shale gas zone is not only a favorable evidence for the long-term effective preservation conditions, but also a key factor for the enrichment of shale gas in Wufeng-Longmaxi shales. The scientific question that the relation between the preservation and enrichment of shale gas and the overpressure evolution in Wufeng-Longmaxi shales requires to be studied deeply, and one of the fundamental issues is to find the paleotemperature-pressure conditions in Wufeng-Longmaxi shales and the genetic of the related information carrier of fracture veins. In this project quartz and calcite veins formed by cements filling shale fractures in the Wufeng-Longmaxi shales is as the research object. Using comprehensive approaches including mineralogy, element and isotope geochemistry, geochronology and fluid inclusion techniques, this study focus on the palaeo-fluid sources of shale fracture veins, vein formation times and paleotemperature-pressure information captured by fluid inclusions. The relationship between the trapped paleotemperatures and paleopressures of fluid inclusion in fracture veins and the paleotemperature-pressure environments of the shale layers at the time of the formation of fracture veins will be analyzed, the paleotemperatures and paleopressures conditions and the corresponding geological time of the Wufeng-Longmaxi shale gas zone in the processes of bury and tectonic uplift erosion will be proposed. The research results can provide scientific evidence and reference for the studies about the preservation and enrichment of shale gas associated with the evolution of overpressure in Jiaoshiba area and other similar shale gas area.

焦石坝地区上奥陶统五峰组-下志留统龙马溪组页岩气层为超压发育系统，该页岩气层超压是其具有长期有效封存条件的有利证据，是页岩气田富集高产的有利因素。焦石坝五峰组-龙马溪页岩气保存和富集与超压演化的关系如何是需要深入研究的科学问题，其中基础性研究问题之一是五峰组-龙马溪组页岩气层古温压条件及相关信息载体的裂缝脉体成因研究。该选题以焦石坝五峰组-龙马溪组页岩裂缝充填的石英和方解石脉体为研究对象，综合运用矿物学、元素和同位素地球化学、地质年代学以及地质流体的研究思路和技术，聚焦页岩裂缝脉体古流体来源、脉体形成时间以及流体包裹体捕获古温压信息开展深入研究，剖析页岩裂缝脉体流体包裹体捕获古温压与脉体形成时五峰组-龙马溪组页岩气层古温压环境的关系，提出页岩气层埋藏、抬升剥蚀过程中古温压条件与对应地质时间的信息。该研究成果可为焦石坝地区和类似地区页岩气保存和富集与超压演化关系研究提供科学的依据和借鉴。

项目研究以焦石坝地区为重点，包括涪陵页岩气田的焦石坝背斜区块、平桥区块和梓里场区块，研究对象为五峰组-龙马溪组一段页岩裂缝石英脉和方解石脉。项目研究过程中，将提出的研究思路和方法推广到中、下扬子地区的鄂西宜昌地区下震旦统陡山沱组、下寒武统水井沱组、彭水地区五峰组-龙马溪组一段和鄂西恩施地区、湘中坳陷和江西萍乐凹陷上二叠统大隆组等页岩裂缝脉体研究。在深入研究页岩裂缝脉体成因及流体包裹体捕获古温压的基础上，进一步提出了页岩含气性动态研究方法和计算模型，延伸研究了页岩埋藏-抬升过程中，随温压变化的页岩含气性、页岩气散失量和页岩气赋存状态的动态演化，以及页岩裂缝脉体形成的氧化-还原环境与页岩气保存条件。..项目研究综合运用矿物学、稀土元素和同位素地球化学、同位素年代学以及地质流体研究方法和技术，深入开展了页岩裂缝脉体成因、成脉古流体来源、脉体形成时间以及流体包裹体捕获古温压研究。主要创新性成果有：（1）阐明了焦石坝地区五峰组-龙马溪组一段页岩裂缝脉体成因，成脉古流体来源主要为自身页岩层系的成岩流体；（2）测得了焦石坝地区五峰组-龙马溪组一段页岩裂缝石英和方解石脉流体包裹体均一温度范围分别为198.5 - 229.9 ℃和196.2 - 221.7 ℃，对应古埋深约5500m；（3）发现了焦石坝地区五峰组-龙马溪组一段页岩脉体中发育大量的高密度甲烷包裹体，提出了依据激光拉曼技术和超临界甲烷状态方程计算甲烷包裹体密度和捕获古压力的方法，得到石英脉甲烷包裹体密度为0.245 - 0.302 g/cm3，古压力为 91.8 - 139.4 MPa，压力系数为 1.63 - 2.18；（4）获得了焦石坝地区14块页岩方解石脉样品Sm-Nd同位素年龄，构成了两组年龄等时线，五峰组和龙马溪组一段页岩方解石脉年龄分别为距今133±15 Ma和160±13 Ma，该年代学信息揭示了焦石坝地区页岩裂缝脉体主要在燕山运动中期多幕侧向挤压构造运动期间形成。.