米仓-大巴山海相地层古流体地球化学特征与油气保存

Geochemical characteristics of paleofluid and hydrocarbon preservation in marine strata of Micang-Daba piedmont and its adjacent areas

米仓-大巴山前带及周缘海相地层形成时间早、变形期次多、油气保存条件差，是油气勘探的主要制约因素。通过研究构造裂隙充填方解石脉碳、氧同位素及流体包裹体特征，分析古流体成因来源、活动期次及大气水混入情况，评价该区海相地层的油气保存条件。研究结果表明：从盆地内经山前带至盆地边缘，大气水下渗作用逐渐增强，山前带古大气水下渗深度普遍在1200～3000 m；根据方解石脉包裹体均一温度和盐度特征，推测该区存在两期古流体活动，分别对应燕山期和喜马拉雅期构造运动。其中，燕山期大气水下渗影响较弱，海相地层封闭性相对较好，有利于烃类的保存；喜马拉雅山期海相地层遭受强烈的抬升剥蚀，大气水下渗作用强烈，保存条件变差。

Exploration for oil and gas in the marine strata of the Micang-Daba piedmont and its adjacent area turned out to be not so encouraging due to poor preservation conditions caused by multiple tectonic deformations .By characterizing car-bon and oxygen isotopes and fluid inclusions in calcite veins in tectoclases ,we tried to evaluate conditions for hydrocarbon preservation in these strata based on analyses of paleo-fluid,including origin,phases of activity and mixing of meteoric water .Results show that meteoric water infiltration increased progressively from the center to the edge of the basin and the infiltrating depth of the water was mostly between 1 200 and 3 000 meters in the piedmont .The homogenization tempera-ture and salinity measurements of fluid inclusions indicate two phases of paleofluid activity corresponding respectively to the Yanshanian and Himalayan tectonic movements .During the Yanshanian movement ,the meteoric water infiltration was weak and the marine strata in the study area had favorable preservation conditions for hydrocarbons .While during the Hi-malayan movement ,the meteoric water infiltration was intensive .Together with a strong tectonic uplift and erosion ,the in-filtration made preservation conditions for hydrocarbon worse .