The Origin of H2S in Lower Cambrian Longwangmiao Dolomites in the Central Sichuan Basin, SW China

Abstract

Recently, a largest sulfur-bearing gas field has been discovered in the Cambrian Longwangmiao Formation dolomite reservoirs in Sichuan Basin. The origin of H₂S is not yet known. In order to address this issue, a comprehensive integrated study based on petrological and geochemical data was carried out in this work. The following conclusions can be drawn.

(a) An episode of hydrothermal event that enriched in radioactive Sr has occurred and precipitated saddle dolomite, barite, quartz and vein calcite. This hydrothermal activity was evidenced by higher homogenization temperatures (160~270°C) of fluid inclusions in above minerals, higher bitumen bireflectances and various coke micro-textures in solid bitumens. The alteration of hydrothermal fluids probably took place during, or after, oil emplacement began at the Late Permian and might be related to the eruption of Emeishan basalts.

(b) The δ³⁴S of pyrites associated with hydrothermal minerals and solid bitumens is significantly heavier than that of biogenic pyrites, and moreover, the δ³⁴S of solid bitumens is heavier by about 12‰ than that of potential Lower Cambrian source rock kerogens, suggesting that in-situ TSR probably occurred in the Longwangmiao dolomites.

(c) The dramatic increases in S/C and δ³⁴S from hydrothermally unaltered bitumens to hydrothermally altered bitumens indicate that TSR was most likely to be induced by hydrothermal activities. The occurrences of strip-shaped isotropic zone with reflectance and S/C values generally higher than those of the bitumen mosaic suggest that incorporation of TSR-H₂S into the solid bitumens could probably enhance C-S bond formation to destroy the orientations of the aromatic layer planes, finally leading to the formation of isotropic sulfur-rich structures.

(d) The hydrothermal barites, probably derived from the Sinian Ba-rich basement rock, have been partially replaced by pyrites via incomplete TSR. Solid bitumens altered by hydrothermal fluids and TSR are generally enriched in Ba. In addition, the δ³⁴S values of H₂S, solid bitumen and pyrite are slightly lighter than those of the barite. Hence, we infer that H₂S was most likely to be derived from the in-situ TSR between barite-bearing hydrothermal fluids and hydrocarbons.

In a word, despite that rare gypsum or anhydrite was found in the Cambrian, hydrothermal fluids could also provide the sulfate for TSR, leading to higher contents of H₂S (up to 5.75%) in the Longwangmiao gas reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018