Re-Os dating of gas accumulation in Upper Ediacaran to Lower Cambrian dolostone reservoirs, Central Sichuan Basin, China

Abstract

Re-Os isotope analyses were conducted on solid bitumens from Lower Cambrian Longwangmiao and topmost Ediacaran Dengying formations in Anyue gas-field, Central Sichuan basin to date the main gas accumulation. Chemical and C-S isotopic analyses indicate that the bitumen samples have experienced thermochemical sulfate reduction (TSR), which, however, did not disrupt Re-Os isotope systematics. Re-Os isotope analyses on the five bitumen samples outside the gas field give an isochron age of 138.6 ± 2.7 Ma (MSWD = 4.0, n = 5, ±2σ) with an Osi of 2.880 ± 0.022. This new age has a smaller error than ever before. We ascribe this result to the much greater spread in our 187Re/188Os and 187Os/188Os data, with ranges reaching 126.8 to 976.6 and 3.172 to 5.142, respectively. This is the first report that Re-Os isotopes can be used to date TSR-altered petroleum and solid bitumen for their generation and migration. Other samples within the gas field have lower initial 187Os/188Os values, may have much more oils contributed from the Lower Cambrian source rock, and/or result from water-oil interactions prior to TSR. Our dating indicates that a large-scale gas generation from the cracking of paleo-oils likely occurred simultaneously with the TSR, which led to increase in δ34S value, negative shift in δ13C value of solid bitumen in the Central Sichuan Basin and MVT mineralization in southeastern margin of the basin.