Sulfur isotope of pyrite response to redox chemistry in organic matter-enriched shales and implications for components of shale gas

Abstract

The Sichuan Basin has achieved breakthroughs in shale gas production from the Upper Ordovician Wufeng and Lower Silurian Longmaxi Formation black shales. Large amounts of pyrite commonly occur in the organic matter (OM)-enriched black shales, but [Formula: see text] has not been detected in the shale gas. The genetic mechanism of pyrite, its implications for redox chemistry, and the main controlling factors for the absence of [Formula: see text] are unclear. The [Formula: see text] values of the pyrite are extremely high. In particular, the nodular pyrite has [Formula: see text] values as high as 38.6‰. The high sulfur isotopic values indicate that the Wufeng-Longmaxi Formation shales were deposited in an anaerobic sulfide euxinic environment where the limited [Formula: see text] in the stagnant bottom water was completely reduced to pyrite by bacterial sulfate reduction (BSR). The heavy sulfur isotope composition of the pyrite is indicative of organic-rich intervals, which are also the high-yielding intervals for shale gas. Shale gas from the Wufeng-Longmaxi Formation is dominated by alkanes, with an average [Formula: see text] content of 97.91%. The shale gas contains a small amount of [Formula: see text], with an average of 0.34%. However, no [Formula: see text] was detected. The [Formula: see text] values have a range of 4.7‰–11.5‰, with an average of 7.8‰, which is significantly different from the [Formula: see text] related to thermochemical sulfate reduction (TSR) but similar to the [Formula: see text] from the decomposition of carbonate minerals. The black shales experienced high burial temperatures and were rich in OM, which met part of the necessary conditions for the occurrence of TSR. However, TSR did not occur. The reason for the lack of TSR process is that no sulfate mineral was available in the shales because the [Formula: see text] in the seawater was fully consumed by BSR. As a result, [Formula: see text] associated with TSR was not detectable in the shale gas.