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The Lower-Middle Ordovician carbonate reservoirs have been recognized as the most important strata in terms of oil and gas exploration in NW Tazhong Uplift, Tarim Basin, China. Intercrystalline porosity, vugs, and fractures were preferentially developed in the dolostone intervals, and understanding the processes of dolomitization is therefore crucial for the prediction of the connectivity and spatial distribution of reservoir quality. Two major types of replacement dolomite (RD1 and RD2) were identified based on fabrics and textures: RD1 dolomite is light gray in hand specimen with porous sucrosic texture and partially preserved precursor lithologic texture. Microscopically, it consists of fine- to medium-crystalline (50-250 μm), euhedral to subhedral dolomite crystals with planar crystalline boundaries. RD1 dolomite has δ18O values between −7.0 and −3.2‰ relative to Vienna Peedee belemnite (VPDB; mean −5.9‰ VPDB), either similar to, or slightly higher than, values estimated from Early-Middle Ordovician marine dolomite. The δ13C values (−1.9 to −0.1‰ VPDB, mean −0.6‰ VPDB) and 87Sr/86Sr ratios (0.708021-0.708526, mean 0.708351) of RD1 dolomites fall within the range of coeval seawater values. These features suggest that RD1 probably formed from the reflux of slightly evaporitic (penesaline) seawater that did not reach the salinity required for gypsum precipitation during early diagenesis. RD2 dolomite is dark gray in cores, with the precursor texture completely obliterated. RD2 is microscopically composed of medium- to coarse-crystalline (150-500 μm), subhedral to anhedral crystals with nonplanar/irregular crystalline boundaries and weak undulatory extinction. This dolomite lacks intercrystalline porosity, but vugs and fractures are locally developed and partially filled by minor amounts of saddle dolomite cements, late-stage calcite cements, and fluorite. RD2 dolomite yields depleted oxygen isotopic compositions (−10.7 to −7.0‰, mean −9.1‰ VPDB), indicating that the dolomite formed at elevated temperatures during intermediate to deep burial. Sr isotopes analyses show that the dolomitizing fluids with respect to RD2 dolomite were mainly derived from connate seawater preserved in the Lower-Middle Ordovician carbonate formations. The lower Sr isotopic values (0.707193-0.707723) of some RD2 dolomites suggest mixing of small amounts of hydrothermal fluids derived from basic magmatic fluids that had lower 87Sr/86Sr ratios, whereas more radiogenic 87Sr/86Sr ratios (0.709028-0.714287) in RD2 dolomites might have resulted from burial recrystallization and/or were associated with dolomitizing fluids passing through the deeper Precambrian clastic rocks and basement.

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