渝南地区龙马溪组笔石页岩相与页岩气成藏关系探讨

发布时间：2018-09-12 09:44

【摘要】：渝南地区龙马溪组发育笔石页岩相,且具有较好的页岩气成藏地质条件。本文以渝南地区龙马溪组笔石页岩为研究对象,通过资料收集-野外地质调查-实验测试-理论分析,对研究区龙马溪组古生物化石发育特征、页岩气成藏条件及笔石生物对页岩气成藏贡献等问题展开系统研究,结果表明:研究区构造简单,地层出露较好。龙马溪组下部泥质深水陆棚沉积,发育炭质页岩、泥质页岩、钙质页岩,上部为灰泥质浅水陆棚沉积,发育钙质页岩、粉砂岩,夹透镜状灰岩。綦江观音桥剖面共识别出20属71种笔石化石,其中奥陶系五峰组5属8种,志留系龙马溪组17属63种,划分7个笔石带。奥陶纪晚期的冰期海退事件及志留纪初的全球气候变暖事件,影响笔石的发育和保存条件,最终导致龙马溪组下部笔石丰度高,上部笔石丰度低,与实测剖面有机碳含量自下而上逐渐降低的特点相对应,两者具有良好的相关关系。且研究区笔石体碳元素含量极高,围岩碳元素含量低。因此笔石丰度为控制源岩有机质丰度的主要因素,笔石为页岩气形成提供了物质基础,为页岩气主要生烃母质。研究区五峰组内腕足类化石以矿化立体-半立体保存,形成于有机质含量低的层位中;龙马溪组笔石化石多以有机质压扁薄膜状态保存,形成于高有机质含量储层中;龙马溪组中上部发现黄铁矿化笔石化石,发育于有机质含量相对较低的还原层位。因此,炭质薄膜的笔石化石可反映研究区具有高有机碳含量的特征。研究区五峰组内腕足类化石和龙马溪组的黄铁矿化的笔石化石由于填充及交代作用,有效孔隙少,对储层孔隙贡献有限。有机质压扁薄膜保存的笔石化石表面发育大量微孔-过渡孔,呈圆-不规则孔穴状,笔石体内胞管边缘、中轴两侧仍存保留有一定的空间。总体看来,笔石的有效孔隙度相对较好,开放性好,可形成一定的孔隙空间及内表面积;层间间隙发育且连通性较好,可提供一定的储集空间和渗流通道。笔石生物化石残余有机碳可发育大量笔石有机质孔,主要为纳米级的孔隙,保存较好,很难被充填,且可形成巨大的内表面积,为页岩气的吸附提供大量的吸附点位,从而成为吸附气的重要载体。同时笔石通过控制TOC的含量影响孔隙整体发育,尤其是微孔-过渡孔的发育,为游离气和吸附气提供了大量的赋存空间。因此,笔石化石是有利于页岩气赋存的重要因素。本论文研究成果对笔石与页岩气成藏关系及渝南地区笔石页岩相页岩气的勘探具有参考价值。
[Abstract]:Graptolite shale facies are developed in Longmaxi formation and have good geological conditions for shale gas accumulation. Taking graptolite shale of Longmaxi formation in Yunan area as the research object, the development characteristics of paleontological fossils of Longmaxi formation in the study area were studied by data collection, field geological survey, experimental test and theoretical analysis. The conditions of shale gas reservoir formation and the contribution of graptolite organisms to shale gas accumulation are systematically studied. The results show that the structure of the study area is simple and the strata are well exposed. In the lower part of Longmaxi formation, shaly shales, calcareous shallow water shelf deposits, calcareous shale, siltstone and lenticular limestone are developed in the lower part of Longmaxi formation. A total of 71 species of graptolites belonging to 20 genera have been identified in the Guanyinqiao section of Qijiang, including 5 genera and 8 species of Ordovician Wufeng formation and 63 species of 17 genera of Silurian Longmaxi formation, which are divided into 7 graptolite zones. The late Ordovician sea retreat event and the early Silurian global warming event affected the development and preservation conditions of graptolite, which resulted in high graptolite abundance in the lower part of Longmaxi formation and low graptolite abundance in the upper part. The results show that the organic carbon content of the measured profile decreases gradually from the bottom to the top, and there is a good correlation between the two. The content of carbon element is very high and the content of carbon element in surrounding rock is low in the study area. Therefore graptolite abundance is the main factor to control the organic matter abundance of source rocks and graptolite provides material basis for shale gas formation and is the main hydrocarbon source material of shale gas. The brachiopod fossils in the Wufeng formation of the study area are mineralized stereoscopically and semi-stereoscopically, and formed in the low organic layer, while the graptolite fossils of the Longmaxi formation are mostly preserved as organic matter flattened thin films and formed in the reservoirs with high organic matter content. Pyrite graptolite fossils were found in the middle and upper part of Longmaxi formation. Therefore, the graptolite fossils of carbonaceous films can reflect the characteristics of high organic carbon content in the study area. The brachiopod fossils in the Wufeng formation and the pyrite graptolite fossils in the Longmaxi formation have little effective pores due to filling and metasomatism and limited contribution to the reservoir porosity. A large number of microvoids and transition pores are formed on the surface of graptolite fossils preserved by organic matter flattened thin films, which are circular and irregular holes, with the cell margins in the graptolite body, and there is still some space on both sides of the central axis. As a whole, the effective porosity of graptolite is relatively good and open, which can form a certain pore space and internal surface area, and the interlayer gap is developed and connectivity is good, which can provide a certain reservoir space and seepage passage. The residual organic carbon of graptolite fossils can develop a large number of graptolite organic pores, mainly nanoscale pores, which are well preserved and difficult to be filled, and can form a huge internal surface area, which provides a large number of adsorption sites for the adsorption of shale gas. Thus it becomes an important carrier of adsorption gas. At the same time, graptolite influences the development of pore by controlling the content of TOC, especially the development of micropore-transition pore, which provides a lot of space for free gas and adsorption gas. Therefore, graptolite fossils are important factors conducive to the occurrence of shale gas. The results of this paper have reference value for the relationship between graptolite and shale gas accumulation and the exploration of graptolite shale gas in Yunan area.
【学位授予单位】：中国矿业大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P618.13

【参考文献】