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Diagenesis and Pore Evolution of an Evaporate-Related Carbonate reservoir -- the Middle Triassic Leikoupo Formation, Sichuan Basin, China


The Middle Triassic Leikoupo Formation in the Sichuan Basin has been recently proven to potentially host about 1.1× 1012 m3 natural gases resource, with the highest daily well production of > 1× 106 m3 currently. Brecciated and fractured carbonates and grainstones are two main types of reservoir rocks. The average porosity in these carbonate reservoir is less than 5% although some cores have porosity up to 12%. Permeability data display in a range mainly between 0.01 and 1 mD, with the highest value up to 3 mD. Limestone is commonly tight, non-reservoir due to abundant calcite cementation, whereas dolostone has various types of pore spaces. Hence, dolomitization is crucial in the development of reservoirs. Pore spaces, both in brecciated and fractured carbonates and dolo-grainstone, are dominated by solution-enlarged pores/vugs and fractures. Hence, diagenesis has significantly modified the pore geometry and thus the reservoir quality. Dolomudstone associated with evaporites in evaporated lagoon facies has negative carbon isotopic values (down to < -10 ‰ VPDB), suggest that this early dolomitization stage might be related to microbe activity. Subsequent dolomitization was formed due to reflux of brine or seawater at locations near to brine source and probably during relatively early diagenetic environment. Meteoric water diagenesis occurred afterward due to uplifting, which resulted in the formation of breccia and fractures and moldic pores. The following diagenetic events were dominated by burial dolomitization, as well as some localized calcite and quartz cementation. An episode of hydrothermal event that enriched in radioactive Sr has occurred and precipitated saddle dolomite, calcite, quartz, and fluorite. This hydrothermal activity was confirmed by high homogenization temperatures (from 160 to 185 °C) obtained from the fluid inclusion assembles in above minerals, being 20 °C hotter than the deepest burial temperature of the Leikoupo Formation. Both bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR) have probably occurred and responsible for the various types of pyrite, calcite, as well as the relatively high H2S concentration (up to 3.5%) in some wells. Anhydrite-dissolution pore, solution-enlarged pores/vugs and fractures in the Leikoupo Formation are interpreted to be probably related to BSR/TSR.