Geochemistry of the Sinian–Cambrian gas system in the Sichuan Basin, China

•The Longwangmiao gas was mainly generated from the Cambrian Qiongzhusi source rock.•The Sinian Dengying gases mainly originated from the three Sinian source rocks.•The Precambrian source rock made main contribution to the Sinian–Cambrian gas system.•The results confirmed the exploration prospects of Precambrian formations of the Sichuan Basin. Based on the outcrops, core samples, drilling cuttings, seismic and geochemical data, natural gases of the Sinian–Cambrian gas system were derived from four sets of over-mature, high quality source rocks. The Cambrian Qiongzhusi source rock is sapropelic, with TOC of 1.94%, Ro eq. of 1.83–3.90% and a thickness of 140m on average. The Sinian (620–541Ma) Doushantuo source rock is sapropelic, with TOC of 1.69%, Ro eq. of 2.08–3.83% and a thickness of 11m on average. The 3rd member of the Dengying Fm. is also sapropelic, with TOC of 1.43%, Ro eq. of 3.16–3.21% and a thickness of 13m on average. The 2nd and 4th members of the Dengying Fm. are dominated by algal dolomite, with TOC of exhibiting 0.39%, Ro eq. of 1.97–3.46% and a thickness of 246m on average. Most of the δ13C2 values are lower than −28‰ and gas dryness is >99%, indicating that the natural gas is oil-derived with high maturity. The high contents of isoparaffins and cycloparaffin suggest that natural gas in the Sinian–Cambrian reservoir was originated from secondary oil cracking. The Cambrian Longwangmiao gas was mainly derived from the Cambrian Qiongzhusi source rock and the gas within the Sinian Dengying Formation mainly originated from the three Sinian source rock layers. The area with gas generation intensity superimposed the four sets of source rocks exceeding 6×109m3/km2 covered 80% of the Sichuan Basin. The proved reserves exceed 1.0×1012m3 in the Sinian–Cambrian gas system and the accumulated gas resource in the Sinian–Cambrian gas system might exceed 5×1012m3.