古流体恢复及在储集层形成机理研究中的应用——以塔里木盆地奥陶系为例

Paleofluid restoration and its application in studies of reservoir forming: A case study of the Ordovician in Tarim Basin, NW China

基于缝洞-溶洞充填泥的硼元素分析,充填方解石中包裹体均一温度-盐度、液相成分中阴离子、氢氧同位素测定,并结合区域地质背景,恢复塔里木盆地奥陶系岩溶储集层古流体,在此基础上分析岩溶储集层成因。缝洞-溶洞充填泥中硼元素含量总体小于80μg/g;充填方解石中包裹体均一温度存在地区差异,盐度分布范围大,包裹体液相成分中HCO3-含量总体较高而Cl-、SO42-含量分布离散,包裹体氢氧同位素组成较为离散、δD值偏负、δ18O值偏正。这些证据表明,塔里木盆地奥陶系古流体来源于表生期大气淡水、被埋藏封存的淡水-海水混合水、海水及浓缩海水,以及沿断裂带上窜的寒武系白云岩层系、蒸发岩系的埋藏卤水;孔洞-溶洞型储集层形成的最主要建设性成岩作用是表生期大气淡水的侵蚀、溶蚀,在埋藏期经历了深部热流体不同程度的改造。

Karst reservoir paleofluid types of the Ordovician formation in the Tarim Basin are restored based on the analysis of element boron of filled mud, the test of fluid inclusions homogenization temperature and salinity, liquid anion and hydroxyl isotope of filled calcite in vug-fractures, and regional geologic background. The origin of the karst reservoirs are analyzed on this basis. The element boron contents of mud filled in vug-fractures are less than 80 μg/g generally; fluid inclusions have different homogenization temperatures in different regions, and the salinities are in a large range. The HCO3^- contents are high, and the Cl^- and SO4^2- contents are dispersive in the liquid component of the fluid inclusions. The hydroxyl isotope contents are relatively dispersive, with relatively negative δD value and positive δ^18O value. This evidence shows that the paleofluid of the Ordovician was from supergene atmospheric freshwater, buried fresh-brackish mixed water, seawater or concentrated seawater, and buried brine from underlying dolomite or evaporate rock of the Cambrian formation. The main constructive diagenesis for the formation of vug-cave type reservoirs is erosion and dissolution caused by atmospheric freshwater, and the reservoirs have been subjected to thermal fluid reformation from underlying evaporate rock of the Cambrian during burial stage.