
Thermochemical and bacterial sulfate reduction in the C ambrian and L ower O rdovician carbonates in the T azhong A rea, T arim B asin, NW C hina: evidence from fluid inclusions, C , S , and S r isotopic data
Author(s) -
Jia L.,
Cai C.,
Yang H.,
Li H.,
Wang T.,
Zhang B.,
Jiang L.,
Tao X.
Publication year - 2015
Publication title -
geofluids
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.44
H-Index - 56
eISSN - 1468-8123
pISSN - 1468-8115
DOI - 10.1111/gfl.12105
Subject(s) - pyrite , diagenesis , sulfate , anhydrite , mineralogy , geology , chemistry , sulfur , gypsum , paleontology , organic chemistry
Petrographic features, C , O , S , and S r isotopes were determined, and fluid inclusions ( FI ) were analyzed on various stages of vug‐ and fracture‐fillings from the C ambrian and L ower O rdovician reservoirs in the T azhong area, T arim basin, NW C hina. 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 δ 34 S values from −22‰ to +31‰. The pyrites with low δ 34 S values from −21.8‰ to −12.3‰ were found close to fracture‐filling calcites with vapor‐liquid double‐phase aqueous fluid inclusions homogenization temperatures ( FI ‐ T h) from 55.7 to 73.2°C, salinities from 1.4wt% to 6.59wt% N a C l equiv and δ 13 C values from −2.3‰ to −14.2‰, indicating an origin from bacterial sulfate reduction by organic matter. Other sulfides with heavier δ 34 S values may have formed by thermochemical sulfate reduction ( TSR ) during two episodes. The earlier TSR in the M iddle and L ower C ambrian resulted in pyrites and H 2 S having δ 34 S values from 30 to 33‰, close to those of bedded anhydrite and oilfield water (approximately 34‰). The later TSR is represented by calcites with δ 13 C values as light as −17.7‰ and FI ‐ T h of about 120–145°C, and pyrite and H 2 S with δ 34 S values close to those of the U pper C ambrian burial‐diagenetic anhydrite (between +14.8‰ and +22.6‰). The values of the anhydrite are significantly lighter than contemporary seawater sulfates. This together with 87 S r/ 86 S r values of anhydrite and TSR calcites from 0.7091 to 0.7125 suggests a source from the underlying E diacaran seawater sulfate and detrital S r contribution.