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Origin and Prediction of Dolostone Reservoirs in Tarim, Sichuan, and Ordos Basins, China


Despite the proposal of numerous dolomitization models to date, the origin of dolostone remains a controversial subject in China. In many basins in China, dolostone is a very important hydrocarbon reservoir, and for this reason we undertook this study of dolostone reservoirs formed in various environments in the Tarim, Sichuan, and Ordos Basins. Geological settings of dolostone formation include evaporitic tidal flat in the Ordos Basin, evaporitic platform in the Sichuan basin, and burial and hydrothermal diagenesis in the deep Tarim Basin. Contrary to the traditional concept that dolomitization is a significant mechanism for porosity creation, our study concludes that, in the studied basins, dolomitization has only a minor influence, or even a negative impact, on porosity creation. Two dominant resources of pore space in dolostones include the porosity inherited from limestone and the porosity resulted from post-dolomitization dissolution. These understandings on dolostone porosity should prove helpful for reservoir prediction. Sabkha dolostone reservoirs associated with evaporitic tidal flats were laterally distributed as banded or quasi-stratified units in evaporite-bearing dolostones, and were vertically presented as multi-interval patterns on tops of shallowing-upward cycles. Seepage-reflux dolostone reservoirs associated with evaporitic platforms commonly occurred along epiplatforms or beneath evaporite beds, and vertically presented as multi-interval patterns in dolostones or evaporite-bearing dolostones of reef/bank facies. Constrained by primary sedimentary facies, burial dolostone reservoirs were distributed in dolomitized, porous sediments of reef/bank facies, and occurred vertically as multi-interval patterns in crystalline dolostones on tops of shallowing-upward cycles. Hydrothermal dolomitization was apparently controlled by conduits (e.g. faults, unconformities), along which lenticular reservoirs were able to develop.