准噶尔盆地玛湖凹陷区下三叠统百口泉组砾岩储层成岩流体演化

The diagenetic fluid in reservoirs is not only involved in components, but also played a role of delivering matter and energy during the diagenetic evolution and hydrocarbon accumulation. Geochemical information from diagenetic cements which were precipitated and dissolved is widely used to investigate the pore fluid evolution. The Mahu Sag conglomerate oil reservoirs , the largest conglomerate oil pool found in the world at present, has been chosen as an ideal target for studying the mechanism of diagenetic fluid evolution. Based on detailed geological and lab study, using petrographic, fluid inclusions and geochemical analyses of stable O and C isotopes, Sr isotopes from calcite cements, combined with buried history, thermal and hydrocarbon maturity history of sandy conglomerate reservoirs in the Triassic Baikouquan Formation of the Mahu Sag, Junggar Basin, the precipitation mechanism of diagenetic cements and the geochemical characteristics of diagenetic fluid were discussed. The main objective of this project is to reestablish the origins, causes and evolution of diagenetic fluid that altered the properties in the process of water-rock interactions within geological and reservoirs diagenetic evolution. the project will reveal the mechanism of coarse deposits reservoirs development, which could provide a scientific basis for oil and gas exploration in the study area.

储层成岩流体在储层成岩演化及油气成藏过程中不仅是参与组分，还是物质和能量的运输载体。储层中成岩自生矿物的沉淀及组分的溶解均是对地层流体性质变化的响应。本项目以目前世界上发现的最大砾岩油田—准噶尔盆地玛湖大油田为研究对象，在详细的野外调查及室内研究的基础上，对百口泉组砾岩储层成岩自生矿物岩石学、矿物学、流体包裹体以及碳酸盐胶结物碳、氧、锶同位素系统分析的基础上，结合储层热埋藏史和有机质热演化史，探讨储层成岩流体的性质及演化。旨在重塑储层地球化学环境的演变过程，及其由此产生的各种矿物的溶解、蚀变、迁移和沉积作用的机理。有助于进一步查明粗粒沉积储层形成条件和发育机理，为研究区及类似地区油气勘探提供科学依据。

通过对准噶尔盆地玛湖凹陷区下三叠统百口泉组砾岩储层成岩自生矿物岩石学、矿物学、流体包裹体、碳酸盐胶结物碳、氧、锶同位素等分析的基础上，探讨储层成岩演化过程中成岩矿物的沉淀机制及成岩流体的演化特征。结果表明：（1）根据成岩矿物共生组合关系，可以确定成岩矿物由早到晚形成的相对顺序为：少量粘土膜→早期方解石（泥晶菱铁矿）或沸石类→石英次生加大→长石、火山岩屑或沸石强烈溶解→高岭石、自生石英、绿泥石填充粒间孔→方解石溶解→连晶（含铁）方解石→白云石→铁（方解石）白云石→自生伊利石。（2）根据碳酸盐胶结物的赋存状态，可知研究区碳酸盐胶结物主要分三个期次沉淀，从早到晚依次Ⅰ期泥晶方解石、Ⅱ期含铁方解石、Ⅲ期铁白云石。（3）根据碳酸盐胶结物的碳、氧同位素特征，可知研究区成岩流体主要为机烃类流体的充注，同时有深部热液流体混入（δ13C=-47.23‰）和无机碳源或者生物气成因碳源（δ13C ca.3‰）。δ18OPDB值总体呈现偏负的特征，随着深度增加，δ18OPDB值更加偏负，说明储层有外来流体的注入引起水介质的盐度降低，或者沉积环境中有热液流体的入侵。（4）流体包裹体表明研究区存在多期油气充注。综上可知，研究区百口泉组的成岩体系主要为半封闭-开放流体系统。早期主要为铝硅酸盐的水化作用导致孔隙流体 pH 由中性或中偏碱性向碱性转变，并提供Fe2+,K+，Na+和Mg2+等各种金属离子，随后油气大量充注，流体保持酸性环境，发生强烈溶蚀作用，黏土矿物转化释放Ca2+，Fe3+和Mg2+离子，有机酸不断被消耗，导致水中二氧化碳的分压增高，且伴随着深埋藏、高水温、高压强及缺氧等条件，变为富碱性的环境。