塔河地区奥陶系碳酸盐岩缝洞充填物的地球化学特征及其形成流体分析

Geochemical characteristics and origins of fracture- and vug-fillings of the Ordovidan in Tahe oilfield, Tarim basin.

塔河地区奥陶系方解石脉和胶结物^(87)Sr/^(86)Sr值多高于围岩与同期海水,显示具有外来的、富^(87)Sr的流体的贡献。富^(87)Sr的流体可来自奥陶系抬升时期淡水对碎屑岩的淋滤作用,但也可来自深部的热液流体。不同成因的流体所沉淀的方解石Sr含量具有明显的差异。地层水δD-δ^(18)O关系证实了存在淡水的混合作用。淡水的混合导致了原油的厌氧生物降解,产生低δ^(34)S的黄铁矿与有机酸;有机酸促进了不整合面附近的岩溶作用。而方解石的均一化温度可达150～190℃,有意义地高于地层所经历的最高温度,支持了存在深部热液流体的活动,说明Sr很可能都来自寒武系或前寒武碎屑岩地层;该热液流体促进了寒武系碳酸盐岩发生热化学硫酸盐还原作用(TSR),产生H_2S气体和黄铁矿,其δ^(34)S值为18‰～22‰。热液流体的活动,导致了碳酸盐矿物的溶解-再沉淀作用。

Fracture-filling calcite and huge crystalline calcite from Ordovician carbonate reservoirs of Tahe Oilfield were measured to have87Sr/86Sr mainly higher than bulk limestone and contemporaneous seawater, indicative of a significant contribution from external87Sr-rich fluid in this area.87Sr-rich fluid may have derived from paleo-freshwater flowing through detrital strata during Hercynian Orogeny or from deep hydrothermal fluid. Paleo-freshwater is indicated by low Sr concentrations and the relationship between λD and λ18O. Infiltration of paleo-freshwater resulted in biodegradation of early emplaced oil and generation of organic acids and anions (OAAs) and pyrite with λ34down to -25‰. Hydrothermal fluid is indicated by relatively high Sr concentrations and significantly higher homogenization temperatures of fluid inclusions (up to 150 ˜ 190°C ) than what ambient limestone experienced, suggesting that radiogenic Sr may have derived from deeper pre-Cambrian detrital strata. Mixing of hydrothermal, basinal and likely fresh fluids led to dissolution of calcite, thus improvement of porosity in local areas.