扬子地区二叠系页岩气赋存地质条件研究

Research on Shale Gas Occurrence Geological Condition of Permian in Yangtze Region

扬子地区古生界下寒武统牛蹄塘组、上奥陶统五峰组—下志留统龙马溪组、上二叠统大隆组为我国页岩气重点勘探层位,关于下古生界两套地层页岩气赋存地质条件的研究较多,而二叠系相关的研究则较薄弱。结合前人研究成果,运用地球化学测试、XRD分析、扫描电镜、等温吸附等实验方法,对二叠系泥页岩的生烃条件、储集特征、含气性能等进行分析,并对页岩气发育有利区进行预测,对扬子地区二叠系页岩气赋存地质条件展开深入的研究。结果表明,扬子地区二叠系泥页岩有多个厚度中心,最大厚度可达175~200 m;TOC在四川盆地及其周缘较高,最高可达4.0%~5.0%;有机质类型以Ⅱ2型为主,整个扬子地区泥页岩成熟度较高,Ro可高达3.0%~5.0%。二叠系泥页岩中普遍发育原生晶间孔、粒间孔等原生孔隙和次生晶间孔、溶蚀孔、有机质孔等次生孔隙,此外还发育原生层间缝或页理缝、矿物收缩缝、有机质收缩缝、溶蚀缝等成岩缝和层间裂缝、节理缝、构造裂缝等构造缝。矿物分析表明,其脆性指数1和脆性指数2分别为8.8%~67.4%、9.4%~74.4%,平均值分别为34.43%、46.9%,显示扬子地区二叠系泥页岩脆性较强而有利于页岩气压裂开采。等温吸附实验显示,页岩全岩和干酪根样品吸附气量分别为2.0~4.3 cm^3/g、3.5~5.3 cm^3/g,平均值分别为2.75 cm^3/g、4.25 cm^3/g,表明该区二叠系泥页岩吸附性能较强。页岩气发育有利区预测结果显示,研究区存在成都—宜宾—泸州—重庆—广安、芜湖—南京—镇江、张家界—宜昌—常德—石首一共3个有利区。

Niutitang Formation in the Lower Cambrian,Wufeng Formation in the Upper Ordovician and Longmaxi Formation in the Lower Silurian,and Dalong Formation in the Upper Permian of Paleozoic in Yangtze region are the main shale gas exploration formations in China. The research on the two formations in the Lower Paleozoic is more,but the research on the Permian is rather less. Combining the previous study results,geochemistry tests,XRD analysis,scanning electron microscope, isothermal adsorption and other experimental method are used to analysis the hydrocarbon generation conditions,reservoir characteristics and gas propertiesof the Permian shale. The results indicate that the Permian shale exists several thickness centers with the biggest thickness ranging from 175 meters to 200 m eters; the organic matter abundance is much higher in Sichuan Basin and its peripheral regions; the value of TOC is 4. 0%- 5. 0%; the main type of organic matter isⅡ2;the maturity of whole Yangtze area is high and the value of Rois from 3. 0% to 5. 0%. The primary pores such as primary intercrystalline pore,intergranular pore,and the secondary pores such as secondary intercrystalline pore,dissolution pore,organic pore are widely spreaded in the Permian shale. In addition,it develops diagenetic gaps such as primary interlayer fissure,lamellation fissure,mineral shrinkage fissure,organic shrinkage fissure,dissolution fissure,and tectonic gaps such as interlayer fissure,joint fissure,and tectonic fissure.The mineral analysis indicates that the index 1 and index 2 of brittleness are 8. 8%- 67. 4% and 9. 4%-74. 4% with average value of 34. 43% and 46. 9%,respectively. It is revealed that the brittleness of the Permian shale is high and beneficial for fracturing exploitation of shale gas. The isothermal adsorption experiment shows that the adsorbed gas content of whole rock and kerogen are 2. 0- 4. 3 cm3/ g and 3. 5- 5. 3 cm3/ g with average value of 2. 75 cm3/ g and 4. 25 cm3/ g,respectively,indicating that the methane adsorption capacity of the Permian shale is strong. The favorable area of shale gas development in the study area is forecasted as follows,Chengdu-Yibin-Luzhou-Chongqing-Guang＇an,Wuhu-Nanjing-Zhenjiang and Zhangjiajie-Yichang-Changde-Shishou areas.