The characteristics and significance of conventional and unconventional Sinian–Silurian gas systems in the Sichuan Basin, central China

Oil and gas discoveries within Proterozoic-Cambrian strata and the majority of shale gas production occurs in relatively stable tectonic regions and characterised by moderate thermal maturity (R o ° o °>°2.5%). The system comprises two components, namely a conventional Sinian–Cambrian dolomite-hosted gas resource and an unconventional Cambrian-Silurian shale gas deposit, both represent trillion cubic meter accumulations. The migration-accumulation type Sinian–Cambrian gas system, is a result of numerous processes, including (1) paleo-high trapping within a relatively stable tectonic setting, (2) appropriate source materials located in a paleo-rift trough, (3) large-scale dolomite grainstone reservoirs located near the paleo-high, (4) adequate gas produced from in-situ crude oil cracking, and, (5) gypsum-salt and shale cap rock preservation. The self-contained source-reservoir Cambrian–Silurian shale gas system, is localized by deepwater organic-rich shelf facies, characterized by high silicon and calcium contents. The recent discovery of these two massive gas fields in old and over-matured sedimentary strata, has effectively extended the upper maturity limit of R o over 3.0%, which may indicate the efficacy of broadening current natural gas exploration areas, thus potentially increasing global natural gas resources significantly. Furthermore, the discovery may spark interest in natural gas exploration in widely distributed old strata, thus encouraging co-development of conventional and unconventional petroleum accumulations.