深层白云岩多期白云石化及其对孔隙演化的影响——以川中地区下寒武统龙王庙组为例

Multi-stage dolomitization process of deep burial dolostones and its influence on pore evolution: A case study of Longwangmiao formation in the Lower Cambrian of central Sichuan Basin

白云岩是深层—超深层油气的主要储层,其孔隙的成因一直存在争议.本文基于岩石学观察、扫描电镜、流体包裹体、电子探针以及稳定同位素(δ13C,δ18O,87Sr/86Sr)测试,确定了川中超深层龙王庙组白云岩的白云石化过程及其对储层的影响,认为基质白云岩主要形成于蒸发环境咸化-半咸化海水向台缘流动而发生的渗透回流白云石化作用,并且早期的白云石化差异控制着区域孔隙的分布,后期多期白云石化进一步促进了孔隙分布的非均质性.研究结果表明:1)龙王庙组白云岩基质中有较高的Fe,Al,Ba含量,以及较高的87Sr/86Sr比值(0.709 2~0.710 4),显示早期陆源输入;同时基质白云石化程度很高,δ18O值(-8.60‰^-5.54‰)显示为海源灰岩白云石化成因,并可见膏模孔岩相特征;2)在进一步埋藏成岩过程中,依次发生了外源混合流体Cd4期白云石化和深源热液流体Cd5期白云石化,以及两期埋藏白云石化(Cd3,Cd6).其中Cd4期白云石阴极发光下为亮红色,富Mn(质量分数均值为0.1%),87Sr/86Sr值变化大(0.709 0~0.710 4)等特征指示发生临层淡水混合;Cd5期白云石具异常高的Fe(质量分数均值为3%)、Mn(质量分数均值为0.3%)以及极高的87Sr/86Sr值(0.712 7),并充填裂隙,显示深源热液活动;两期(Cd3,Cd6)地层水埋藏成因的白云石与基质白云岩碳、锶同位素范围相近,并多充填孔洞;3)早期白云石化作用控制着孔隙的分布,研究区同一相带在距离台内蒸发潟湖较近的东部区域发生"过白云石化"减孔,而距离较远的西部区域则发生白云石化增孔,在此基础上后期白云石(Cd3,Cd4)充填和油气充注会造成孔隙进一步差异,储层中还可见有机酸溶蚀和深部热硫酸盐还原作用(TSR)等晚期次生溶蚀现象.

Dolostones are the main reservoirs of deep to ultra-deep oil and gas, but the pore origin of these reserviors is always controversial. In this study, petrology observation, scanning electron microscope, inclusions observation, electron probe and stable isotope(δ13C, δ18O, 87Sr/86Sr) analysis were used to study the multi-stage dolomitization in Longwangmiao formation and its influences on reservoirs. It is considered that matrix dolomite is mainly formed as seepage reflux dolomitization model casused by brackish seawater, and the degree of dolomitization differences in early stage controls the pore distribution in the region and later multi-stages dolomitization further promotes the heterogeneity of pore distribution. Resutls showed that: 1) The matrix of longwangmiao formation is completely dolomitized and contains high content of Fe, Al and Ba elements, and high 87Sr/86Sr ratio(0.709 2—0.710 4), reflecting depositional environments were affected by terrigenous minerals. The matrix dolomite’s δ18O values(-8.60‰ to-5.54 ‰) indicated dolomite drived from marine limestone, and the characteristics of gypsum pore lithofacies can be seen in matrix. 2) During burial diagenesis process, exogenous mixed fluid dolomitization(Cd4) and deep-sourced hydrothermal fluid dolomitization(Cd5) have occurred, as well as two phases burial dolomites(Cd3 and Cd6). The Cd4 phase dolomites have bright red luminescence, high contents of Mn(the average mass fraction is 0.1%), and a wide distribution of 87Sr/86Sr values(0.709 0—0.710 4), indicating the occurrence of adjacent freshwater mixing;Cd5 phase dolomites have abnormal high Fe(the average mass fraction is 3%) and Mn(the average mass fraction is 0.3%) element contents, and extreme high values of 87Sr/86Sr ratio(approximately 0.712 7). Cd5 dolomites filling fractures, indicating the deep sourced hydrothermal activity. The carbon and strontium isotopes characteristics of Cd3 and Cd6 are similar with that of matrix dolostones, and the pore spaces are filled which indicates the burial dolomite origin. 3) Based on statistics, it is found that the level of early dolomitization degree controls the distribution of porosity. In the same depositonal environment, "overdolomitization" occurred in the eastern region that is near the evaporation lagoon and the porosity was reduced in the intra-platform. However, in the distant western region, the porosity increased because of the appropriate dolomitization. On this basis, later burial dolomites(Cd3, Cd4) and hydrocarbon filling will cause further porosity heterogeneity. During the late period, secondary dissolutions such as organic acid dissolution and thermochemical sulfate reduction(TSR) dissolution are also discovered in reservoirs.