Thermochemical and bacterial sulfate reduction in the Cambrian and Lower Ordovician carbonates in the Tazhong Area, Tarim Basin, NW China: evidence from fluid inclusions, C, S, and Sr isotopic data

摘要

Petrographic features, C, O, S, and Sr isotopes were determined, and fluid inclusions (FI) were analyzed on various stages of vug- and fracture-fillings from the Cambrian and Lower Ordovician reservoirs in the Tazhong area, Tarim basin, NW China. The aim was to assess the origin of pyrite and anhydrite and the processes affecting sulfur during diagenesis of the carbonates. Pyrite from seven wells has S-34 values from -22 parts per thousand to +31 parts per thousand. The pyrites with low S-34 values from -21.8 parts per thousand to -12.3 parts per thousand were found close to fracture-filling calcites with vapor-liquid double-phase aqueous fluid inclusions homogenization temperatures (FI-Th) from 55.7 to 73.2 degrees C, salinities from 1.4wt% to 6.59wt% NaCl equiv and C-13 values from -2.3 parts per thousand to -14.2 parts per thousand, indicating an origin from bacterial sulfate reduction by organic matter. Other sulfides with heavier S-34 values may have formed by thermochemical sulfate reduction (TSR) during two episodes. The earlier TSR in the Middle and Lower Cambrian resulted in pyrites and H2S having S-34 values from 30 to 33 parts per thousand, close to those of bedded anhydrite and oilfield water (approximately 34 parts per thousand). The later TSR is represented by calcites with C-13 values as light as -17.7 parts per thousand and FI-Th of about 120-145 degrees C, and pyrite and H2S with S-34 values close to those of the Upper Cambrian burial-diagenetic anhydrite (between +14.8 parts per thousand and +22.6 parts per thousand). The values of the anhydrite are significantly lighter than contemporary seawater sulfates. This together with Sr-87/Sr-86 values of anhydrite and TSR calcites from 0.7091 to 0.7125 suggests a source from the underlying Ediacaran seawater sulfate and detrital Sr contribution.