Abstract:

Recent oil and gas discoveries in the Middle Permian Qixia Formation dolostone in western Sichuan Basin reveal the exploration potential of dolostone reservoirs. However， the mapping of paying dolostone reservoirs has been difficult due to their high heterogeneity and puzzling dolomitization mechanisms. In this study， the lithological and geochemical characteristics of different types of dolostone in the Qixia Formation of western Sichuan Basin are comprehensively analyzed and combined with sedimentary facies and thermal events of the Emeishan Large Igneous Province （ELIP） to summarize the diagenetic stages and genetic mechanisms of the dolostones. The results show that the Qixia Formation in western Sichuan Basin hosts mainly porphyritic or stratified dolostone that are mostly fine crystalline， sometimes medium to coarse crystalline， or micrtic to very fine crystalline， occasionally with some saddle shaped dolomite cement. The micritic to very fine crystalline dolostones， largely developed at the bottom of the Qixia Formation， consist of mainly subhedral-anhedral crystalline often accompanied by framboidal pyrite. Some micritic dolostones consist of sphaerolitic， dumbbell- or cauliflower-shaped crystalline， and have negative δ¹³C（VPDB） values and δ¹⁸O（VPDB） values in the range of that of normal seawater， all indicating possible products of microbial-mediated dolomitization process. Fine crystalline dolostones are developed both in the first and second members of the Qixia Formation， where they consist of euhedral to subhedral crystalline with mist-centered bright-edge structure and show change of color from brown-dark red inside to bright red outside and then dark again under cathode luminescence. Their obvious residual grain structure suggests a grainstone origin and their δ¹⁸O（VPDB）， δ¹³C（VPDB） as well as ⁸⁷Sr/⁸⁶Sr values all indicate seawater as the dolomitization fluid possibly formed by reflux seepage dolomitization with medium salinity during shallow burial period. The medium to coarse crystalline dolostones are generally developed in the second member of the Qixia Formation. They contain mainly anhedral nonplanar crystalline which are dark from inside and bright to the outside under cathodoluminescence. Their δ¹⁸O（VPDB） values are lower than that of the normal sea water， indicating a modification by later fluids. Some medium to coarse crystalline dolostones with ⁸⁷Sr/⁸⁶Sr values higher than that of seawater of the same period consist of porphyritic or zebra-like crystalline and often have saddle dolomites in vugs. These characteristics indicate a direct influence of thermal fluids that originated from or passed through certain clastic rocks. Those with ⁸⁷Sr/⁸⁶Sr values still within the seawater range but with no saddle-shaped dolomites being observed may be the products of recrystallization or thermal adjustment of fine crystalline dolostones. In general， the dolostones of the Qixia Formation in western Sichuan Basin are mostly likely the results of a successive syngenetic microbial-mediated dolomitization， shallow burial reflux seepage dolomitization， and late thermal fluid modification， among which shallow burial reflux seepage dolomitization could be the main genetic mechanism.

Key words: Emeishan Large Igneous Province, dolostone, Qixia Formation, Middle Permian, western Sichuan Basin