川中磨溪构造寒武系龙王庙组多期流体充注的地球化学表征与油气成藏过程

Longwangmiao formation of Cambrian in Moxi structure in central Sichuan Basin has made a breakthrough in exploration of hydrocarbon, and Dengying formation also has gained industrial capacity, both marking the beginning of a new phase in oil and gas exploration of Sinian, Lower Paleozoic, Sichuan Basin. .Research shows that the geologic conditions for hydrocarbon accumulation of Longwangmiao formation is complex and it experienced several tectonic movements and multistage fluid charge. The phase state of hydrocarbon in reservoir also experienced the evolution process, as follows: ancient oil-pools→ancient oil and gas pools→ancient gas pools→present gas pools. The complex evolution processes lead to the usual research ideas would be difficult to reveal the hydrocarbon accumulation process and accumulation mechanism..In this research, through the analyzing of the characteristics of dolomite, calcite, quartz and other minerals and bitumen or filled in holes, and cracks in the reservoir of the Longwangmiao formation in Cambrian of the Moxi structure, a relationship between the timing of the filling minerals will be established. By means of the geochemical analysis and testing, the geochemical characteristics of the multistage charged fluid will be conducted, which will help to reveal its hydrocarbon accumulation event and geological significance; The research will explore the hydrocarbon accumulation process and accumulation mechanism of the large-gas pools in Longwangmiao formation, Cambrian .This research will provide some reference for hadrocarbon exploration of Sinian-Cambrian in Sichuan Basin.

川中磨溪构造寒武系龙王庙组的油气勘探获得重大突破，震旦系灯影组也获得了工业产能，标志着四川盆地震旦系-下古生界的油气勘探进入了新的阶段；初步成果表明，寒武系龙王庙组油气成藏的地质条件复杂，经历了多期构造演化和多期流体充注过程，储层中烃类流体相态也经历了古油藏→古油气藏→古气藏→现今气藏的演变，成藏过程漫长；常规的油气成藏的研究思路难以揭示其油气成藏过程和成藏机理；本研究拟通过分析该构造寒武系龙王庙组储层胶结物及孔、洞、缝充填的白云石、方解石、沥青及石英等矿物的特征，建立所充填矿物的时序关系；利用丰富的地球化学分析测试手段，重点关注多期充注的流体地球化学特征及演化，揭示其所表征的油气成藏事件及地质意义；探索寒武系龙王庙组大气田的油气成藏过程和成藏机理，为四川盆地震旦系—寒武系的油气勘探提供借鉴。

从岩芯和薄片观察入手，利用阴极发光分析、流体包裹体分析、激光拉曼分析、充填物的稳定同位素分析、磷灰石裂变径迹年代学分析等技术手段，确定了龙王庙组储层的胶结充填序列、油气成藏历史。龙王庙组储层中可以识别出7个期次的胶结充填序列：海底环境纤维状白云石胶结→浅埋藏环境粒状白云石胶结→表生成岩环境渗流粉砂白云石充填→再次浅埋藏—中埋藏环境白云石充填→深埋藏环境白云石充填→深埋藏环境方解石充填→抬升埋藏环境石英充填；不同成岩环境下的方解石或白云石矿物的阴极发光具一定差异性、微量元素含量（Fe、Mn）及稀土元素的分布模式均有一定的区别。.通过对所识别出的胶结充填物的流体包裹体特征以及和沥青之间的关系，认为龙王庙组存在至少五期油气成藏。第Ⅳ期白云石—第Ⅶ期石英中捕获的流体包裹体的相态也从液烃包裹体→气液两相烃类包裹体→气相烃类包裹体演化，反映了龙王庙组经历了古油藏形成、古油藏破坏、再次形成古油藏、油裂解为天然气、古气藏形成和喜马拉雅期构造隆升期气藏调整定型的过程；磷灰石裂变径迹分析表明，安岳构造喜马拉雅期的隆升幅度约1865~2030m，为龙王庙组气藏的调整和定型期，这一成藏事件被晚期充填的第Ⅶ期石英所记录。最晚期的石英流体包裹体的40Ar-39Ar年龄为125±8.2Ma，气藏定型时间为早白垩世；龙王庙组储层中第Ⅳ期白云石充填物的87Sr/86Sr、δ13C、δ18O同位素比值反映了该期白云石与围岩基质的重溶沉淀有关，且是在相对温度较高的埋藏条件下沉淀的。晚期孔洞缝中充填石英的87Sr/86Sr、δ30S、δ18O同位素表明其主要源于含硅质的成岩流体，与深部幔源热液流体活动无关，龙王庙组油气成藏过程中未见外来破坏性流体的参与。