Resumen de Study on variously dolomitized reservoir reef at platform margin of Changxing Formation in northeastern Sichuan Basin, southwestern China
Mintong Li, Xiaobing Lin, JingChun Tian, ShunFeng Peng, Liang Xu, Lin Su
Dolomitization of reservoir reefs is common at platform margins of the Changxing Formation in northeastern Sichuan Basin, southwestern China. We carried out a research on these variously dolomitized reef microfacies based on sedimentology and petrology theories and geochemical technologies. The results show that the reservoir reefs at the platform margins of the formation can be divided into several microfacies: reef flats/caps, reef cores, reef bases, etc. The reef flats contain mostly crystal grain dolomites, the reef cores are made up of largely (calcite) reef dolomites, and the reef bases are generally dolomite limestone/calcite dolomites. Samples taken from different microfacies of the reefs were measured and showed a relatively high content of Fe and Sr and low content of Mn, with Mn/Sr ranging between 0.057 and 2. Samples from the east flank of the Kaijiang–Liangping trough and the Exi–Chengkou trough have higher Fe and Mn contents and Mn/Sr ratios than those from the west flank of the Kaijiang–Liangping trough. Dolomites in reef flats have the highest Fe and Mn contents and Mn/Sr ratios, followed by dolomites from reef cores, and those from reef bases are found to have the lowest Fe and Mn contents. Isotopic analyses of the samples from different microfacies of the reefs indicate more negative δ13C and δ18O values than that of limestone in seawater during the Changxing deposition period. The δ18O values were increasingly more negative in the order of reef flat–reef core–reef base. Based on the study, we propose that the dolomitization processes the reefs at different platform margins experienced have the following features: (1) Meteoric water affected dolomites at the eastern flank of the Kaijiang–Liangping and Exi–Chengkou troughs more than those of the west flank of the Kaijiang–liangping trough. From the microfacies perspective, the influence of meteoric water decreases from reef flats to cores and then to bases. Apart from meteoric water, the dolomitization was also under control of seawater at the same period. (2) Meteoric water dolomitization is closely linked to sedimentary microfacies. Reef flats and cores with better porosity and permeability are more dolomitized than reef bases containing mostly micrite with bioclastics. (3) Dolomitization processes driven both by early mixing of water and “juiced” marine fluids during the shallow burial and compaction stage were all completed before the first stage of hydrocarbon emplacement. Hydrothermal dolomitization occurred locally and its contribution to the whole reef dolomitization of the formation may be negligible.
