DOLOMITIZATION OF GAS RESERVOIRS: THE UPPER PERMIAN CHANGXING AND LOWER TRIASSIC FEIXIANGUAN FORMATIONS, NORTHEAST SICHUAN BASIN, CHINA

The Lower Triassic Feixianguan Formation, and to a lesser extent the Upper Permian Changxing Formation, contain more than ten giant, recently discovered dolomite gas fields on either side of a paleo-embayment (Kaijiang-Liangping Bay), Sichuan Basin, China. This study documents the origin of the dolomite using a combination of petrology, fluid-inclusion, trace-element geochemistry, and stable-isotope and radiogenic-isotope analysis of samples from outcrop and core samples from producing gas fields. Three stages of dolomitization are revealed. Dolomicrite from the lagoon facies in the Feixianguan Formation and minor fabric-retentive dolomite from reef facies in the Changxing Formation on the northeast side of Kaijiang-Liangping Bay represent the initial dolomite phase, caused by reflux of Feixianguan brine, with salinity close to gypsum saturation (similar to 18 wt % salinity), during the first stage of diagenesis at 30 degrees C. The second and dominant dolomite phase (fabric-retentive dolomite), in both the Feixianguan and Changxing formations on the both sides of Kaijiang-Liangping Bay, is the result of reflux of diagenetic fluids with salinity ranging from normal Feixianguan seawater (3.5% salinity) up to mesohaline water (6.5% salinity) during early burial diagenesis at about 35-40 degrees C, as shown by the dominance of single-phase aqueous inclusions, shallow-formed stylolites that cut the second phase of dolomite, and the Early Triassic marine carbon, oxygen, and strontium-isotope, and rare-earth-element and yttrium (REE + Y) signals. The subordinate third dolomite phase (fabric-obliterative dolomite and dolomite cement) was due to burial diagenetic recrystallization between 80 and 140 degrees C in the Feixianguan and Changxing formations on both sides of Kaijiang-Liangping Bay. The third phase of dolomitization involved water expelled from the slightly younger Lower Triassic Jialingjiang Formation marine evaporites and carbonates on the northeast side of Kaijiang-Liangping Bay, revealed by the high-salinity of the aqueous inclusions that homogenize at 80 to 140 degrees C and elevated strontium-isotope data. The carbon-isotope and REE + Y signals of the third phase of dolomite are similar to earlier dolomite cements confirming the lack of exotic input. Future exploration should be focused on dolomitized, high-energy sediments (e.g., reef and oolitic shoal and bank) in similar Triassic Tethyan-paleogeographic settings that are associated with restricted facies, but underlying dolomitized high-energy Permian strata may also be good exploration targets.