Geochemical characteristics of the Permian Changxing Formation reef dolomites, northeastern Sichuan Basin, China

The recent discovery of deep and ultra-deep gas reservoirs in the Permian Changxing Formation reefs, northeastern Sichuan Basin is a significant development in marine carbonate oil & gas exploration in China. Reef dolomitesand their origins have been major research topics for sedimentologists and oil & gas geologists. The petrography, trace element and isotope geochemistry of the reef dolomitesindicated that the dolomitesare characterized by low Sr and Mn contents, relatively low Fe contents, very similar δ13Cand δ18Ovalues and very different 87Sr/86Sr ratios. Although the calculated results of the fluid mixing suggested that a mixture with 85%–95% meteoric waterand 5%–15% seawater seemed to be the dolomitizingfluids of the reef dolomites, the low Mn contents, relatively low Fe contents, high δ13Cvalues and high homogenization temperatures of the dolomitesdid not support that there were large proportions of meteoric waterin the dolomitizationprocess, and the 87Sr/86Sr ratios which were close to coeval seawater also did not support the possibility of the mixture of deep-burial circulated fluids from clastic rocks. High temperature deep-burial circulated seawater with low Mn and Fe contents, high Sr content and high δ13Cvalues from the dissolution of widely distributed Triassic evaporitesduring the burial diagenetic processes (including dehydration of water-bearing evaporites) could have been the dolomitizingfluids of the reef dolomites.