Tectonically Driven Organic Fluid Flow in Dabashan Foreland Belt: Recorded by fibrous calcite veins contained hydrocarbon bearing inclusions
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摘要: 含烃包裹体纤维状方解石脉被认为是超高压下油气流体形成和排泄的标志。大巴山前陆构造带一些断裂和下古生界黑色泥岩和泥灰岩烃源岩微裂隙中分布有含烃包裹体纤维状方解石脉，成分分析表明其为低镁方解石。纤维状方解石脉δ13CVPDB和δ18OVPDB比围岩碳酸盐岩的明显变轻，前者δ13CVPDB和δ18OVPDB变化范围分别为-1.9%～-4.8‰和-8.4%～-12.8‰，后者分别为-1.7%～+3.1‰和-8.7%～-4.5‰，且δ13C与δ18O具有明显的线性关系，反映出纤维状方解石脉具有成岩有机流体与浅部流体混合的流体特征。纤维状方解石脉含有共生的固体沥青包裹体、含甲烷液相包裹体和气液二相盐水包裹体等多相态包裹体，其中沥青包裹体为油气运移的残余沥青。气液二相盐水包裹体均一温度主要位于140℃和196℃之间（峰值为179℃），盐度较高（平均为9.7wt% NaCl）。分别应用盐水包裹体和甲烷包裹体等溶线P—T相图确定出含烃包裹体纤维状方解石脉形成的流体压力为150~200 MPa，属于异常超高压流体。地质和地球化学特征分析认为，大巴山含烃包裹体纤维状方解石脉不具有泥岩因压实成岩作用而形成的超高压流体特征。结合沉积和构造演化历史分析认为，印支碰撞造山运动和燕山前陆构造作用导致大巴山褶皱隆起并伴随天然气藏破坏和改造，挤压环境下的超高压构造应力驱动天然气流体排泄，大巴山前陆构造带含烃包裹体纤维状方解石脉就是超高压构造应力驱动天然气排泄的产物。Abstract: Fibrous calcite veins with organic inclusions were considered as indication to oil/gas generation and expelling under overpressure. Abundant fibrous calcite veins contained hydrocarbonbearing inclusions filled in fractures were found in lower Paleozoic hydrocarbon source of black mudstone, clay shale and limestone as well as some faults in Dabashan Foreland Belt. Composition analysis shows that fibrous calcite veins are kind of low magnesium calcite which formed during later diagenesis rather than sedimentary period. 13CVPDB and 18OVPDB of fibrous calcite veins, ranging from -1.9‰ to -4.8‰ and -8.4‰ to -12.8, respectively, are lighter than that of surrounding carbonate rocks, ranging from -1.7‰ to +3.1‰ and -8.7‰ to -4.5‰, show obvious linear relationship, which indicates that fibrous calcite veins were mixture fluid from different source fluids. Fibrous calcite veins contain immiscible fluid inclusions such as solid bitumen, methane bearing liquid fluid inclusion and vaporliquid aqueous inclusions. Bitumen inclusions are residue organic during oil/gas migration. Homogeneous temperatures of vaporliquid aqueous inclusions are from 140 to 196 ℃ with peak of 179℃. Salinities of vaporliquid aqueous inclusions are high with average of 9.7 wt% NaCl. Mehtods of IsoPT phase diagrams of aqueous fluid inclusions and methane inclusions were used to get fluid pressure when fibrous calcite veins formed. The pressure of fibrous calcite vein with hydrocarbonbearing inclusions in Dabashan Foreland Belt is from 150 to 200 MPa, which indicates of abnormal overpressure fluid. Geology and geochemistry features indicate that fibrous calcite veins contained hydrocarbonbearing inclusions are not fluid formed by overpressure of mudrocks source rocks at shallow buried phase. However, combined with the regional sedimentary characteristics and tectonic evolution, it indicates that fibrous calcite veins contained hydrocarbonbearing inclusions were fluid of natural gas drove to expel under overpressure of tectonic stress during InidanChina tectonic movement and Yanshan Foreland tectonics.
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