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

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. © 2020, Editorial Board of Journal of CUMT. All right reserved.