中扬子当阳复向斜带流体地球化学示踪及保存条件研究

RESEARCH ON FLUID GEOCHEMISTRY AND CONDITION OF TRACE AND HYDROCARBON PRESERVATION IN DANGYANG SYNCLINORE,CENTRAL YANGTZE

将构造演化与古流体研究相结合,对当阳复向斜带三叠统地层以下的古油气保存条件进行研究,结果表明,当阳复向斜带下二叠统—下三叠统中存在多期流体充注。一部分流体的87Sr/86Sr变化于0.7100～0.7104间,为外源富锶流体,流体可能来源于中、上寒武统含泥质碳酸盐岩和泥质膏盐岩;充注于不同层位不同深度外源流体的均一温度十分相近,集中于120℃～140℃之间。另一部分流体的87Sr/86Sr介于0.7075～0.7081间,与寄主围岩和同时代海水的87Sr/86Sr相近,属于内源流体。无论是内源还是外源流体包裹体测试,其流体包裹体中均不含有机包裹体,说明下伏地层中的油气没有在上覆地层中显示。燕山早中期,研究区以中寒武统膏盐岩层作为推覆滑动面发生多期冲断滑脱,来自中、上寒武统不含有机组分的富锶流体,沿断层向上运移充注于滑动面以上不同层位的海相地层,滑动面下的古气藏未受到改造和破坏。燕山中期的冲断作用切穿了中寒武统膏盐岩层,古气藏被破坏,被破坏的古气藏向上运移到新的储层中重新聚集成藏,上组合具有相对较好的保存条件。

Hydrocarbon preservation conditions in the Dangyang synclinore were studied on the basis of its tectonic evolution with paleo-fluid.Infillings of multi-stage paleo-fluid of multiple stages occurred in lower Permian-lower Triassic strata in the Dangyang synclinore.87Sr/86Sr of part of fluids changes from 0.710 0 to 0.710 4,suggesting its allochthonous origin,probably came from the mud carbonate rocks or mud gypsum rock of middle Cambrian and upper Cambrian.The homogenization temperature determined by the allochthonous fluid inclusions from different depth and strata is similar and ranges from 120 ℃ to 140 ℃ respectively.87Sr/86Sr of other fluids is between 0.707 5 and 0.708 1,similar to that of host rock and contemporary seawater,suggesting an endogenous fluid origin.No organic inclusions were found in above mentioned fluids.In early-middle Yanshanian period,multi-stage thrusting along middle Cambrian gypsum rock took place,fluids rich in 87Sr/86Sr from the middle and upper Cambrian migrated upwards along fault and filled in different overlying marine strata,and the paleo-gas reservoir under the detachment plane was not destroyed.In the middle Yanshanian period,middle Cambrian gypsum layers were truncated by overthrusting faults and paleo-gas reservoir under gypsums rock was destroyed,and then the gas migrated upwards to accumulate in new and young reservoirs,and therefore,the upper assemblage has a good preservation condition for gas formation and accumulation.