Publication Date:
2014-05-01
Description:
The dolomite-hosted, Lower Triassic Feixianguan Formation from the Northeast Sichuan Basin, China, is an economically important reservoir that contains sour natural gas. These reservoirs were initially filled with oil, later replaced by gas during burial to 7,000 m followed by uplift to about 4,000 m. We have studied the souring process (thermochemical sulfate reduction: TSR) and diagenetic evolution of the Feixianguan Formation using detailed petrology, fluid inclusion studies and stable isotope data from carbonate minerals. PreTSR diagenesis included (in time order), the eodiagenetic main stage of dolomitization by a reflux mechanism, fracture-related calcite cementation, barite, quartz, celestite, and fluorite mineralization and a dolomite recrystallization stage. TSR resulted in anhydrite replacement by calcite, petroleum destruction, formation of sulfur-rich pyrobitumen and elemental sulfur and generation of large volumes of H2S, CO2. Diagenesis during TSR can be subdivided into oil-stage TSR and gas-stage TSR with oil-stage TSR defined by the presence of primary oil and bitumen inclusions in the TSR calcite. Based on aqueous inclusion homogenization temperatures, oil-stage TSR commenced at a temperature of 116ºC with a mode between 130 and 140 ºC. Gas-stage TSR started at a temperature of 135ºC and continued to maximum burial temperatures of about 220ºC. Trace amounts of pyrite, barite, quartz and celestite grew during TSR. PostTSR diagenesis was dominated by fracture-related calcite precipitation as well as celestite and anhydrite crystallization. Formation water salinity increased from depositional values (3.5 wt%) up to 24 wt% during preTSR dolomite recrystallization, probably due to an influx of evaporite-associated water from the overlying Jialingjiang Formation, although preTSR barite, quartz, celestite, and fluorite mineralization was associated with a transient decrease in water salinity. During TSR, formation water salinity decreased from 26 wt% to as low as 4 wt% as a result of water being produced during TSR reactions.
Print ISSN:
0149-1423
Electronic ISSN:
1943-2674
Topics:
Geosciences
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