桑植—石门复向斜及邻区古流体特征与油气保存意义

Paleo-fluid characteristics and preservation of hydrocarbons in the Sangzhi-Shimen synclinorium and adjacent areas

通过对桑植—石门复向斜及相邻地区震旦系—三叠系方解石脉和围岩的碳氧同位素、锶同位素对比,结合方解石脉中流体包裹体测试分析,研究了桑植—石门地区古流体特征及其油气保存意义。下古生界围岩87Sr/86Sr部分高于同期海水,也高于方解石脉,表明围岩在裂隙形成之前受到高^(87)Sr/^(86)Sr流体改造。桑植—石门复向斜北翼温泉附近的方解石脉体具高87Sr/86Sr、低δ13C、低δ18O特征,显示大气水参与了方解石脉沉淀,逆断层可能在形成之初就不具封闭性。流体包裹体盐度和均一温度指示,桑植—石门地区上寒武统娄山关组上覆盖层残余厚度小于3 km,不具封闭能力;中寒武统孔王溪组方解石脉流体包裹体高盐度表明膏盐盖层在研究区广泛分布,在埋深超过2 km条件下对下伏地层具有较好的封盖能力。桑植—石门复向斜田二垭、向家垭、车坊背斜圈闭、桥头、中坪隐伏圈闭保存条件较好,除田二垭背斜外,可以同时将震旦系灯影组、寒武系清虚洞组白云岩作为勘探目的层。

The paleo-fluid characteristics and hydrocarbon preservation in the Sangzhi-Shimen synclinorium and adjacent areas are discussed using comparisons of C,O,and Sr isotopes between the calcite veins and surrounding rocks in the Sinian and Triassic formations and combined with the tests of fluid inclusions in calcite veins. The87 Sr /86 Sr values of surrounding rocks in the Lower Paleozoic are higher than those of contemporary seawater and calcite veins,which suggests that the surrounding rocks had been reformed by fluid with high87 Sr /86 Sr values before fracturing. The calcite veins close to thermal springs in the north of Sangzhi- Shimen synclinorium have high87 Sr /86 Sr values,low δ13C and low δ18O values,which can be explained by the mixing of meteoric water during calcite vein deposition and suggests that reverse faults might have been open when they were formed. The salinity and homogenization temperature of fluid inclusions shows that the residual cap formations above the Upper Cambrian Loushanguan Formation are less than 3 km thick with poor sealing capacity. The high salinity of calcite inclusions in the Middle Cambrian Kongwangxi Formation showed that gypsum salt cap layers are distributed widely in the study area,and the gypsum salt layers buried over 2 km deep have excellent sealing capacity. The preservation conditions of the Tianerya,Xiangjiaya,Chefang anticline traps,and the Qiaotou,Zhongping buried structural traps are favorable. The dolomites in the Sinian Dengying Formation and Cambrian Qingxudong Formation are exploration targets except for those in the Tianerya anticline.