Characteristics and accumulation mode of large-scale Sinian-Cambrian gas reservoirs in the Gaoshiti-Moxi region, Sichuan Basin

Abstract: The Sinian-Cambrian formations of the Sichuan Basin have favorable hydrocarbon accumulation conditions, but the exploration for large-scale gas fields is quite challenging due to old strata and multiple tectonic movements. Since the Weiyuan Sinian large gas field was found in 1964, the largest monoblock gas field (Anyue Gasfield) was discovered in the Cambrian Longwangmiao Formation of the Moxi region in 2013 with proven gas reserves of 440.1×109 m3. Total proven, probable and possible reserves exceed one trillion cubic meters in the Sinian Dengying Formation and the Cambrian Longwangmiao Formation of the Gaoshiti-Moxi region. The natural gas components, light hydrocarbons, reservoir bitumen abundance and other evidences prove that the dry natural gas was mainly derived from oil-cracking, with methane (a content of 82.65%-97.35%), ethane (a content of 0.01%-0.29%), nitrogen (a content of 0.44%-6.13%), helium (a content of 0.01%-0.06%), and hydrogen sulphide (0.62-61.11 g/m3). Gas reservoir pressure increases gradually from the Sinian normal pressure (a pressure coefficient of 1.07-1.13) to high pressure (a pressure coefficient of 1.53-1.70) in the Cambrian Longwangmiao Formation. The temperature of the gas reservoir is 137.5-163 °C. Gas reservoir traps are divided into three categories: tectonic type, tectonic-formation type and tectonic-lithologic type. The large-scale enrichment of the Sinian-Cambrian natural gas results from effective configuration of the large stable inherited palaeo-uplift during the Tongwan tectonic movement, wide distribution of ancient source rocks, high-quality reservoirs with vast pore-cavity, crude oil cracking of large palaeo-reservoirs and favorable preservation conditions. According to the palaeo-structure pattern prior to crude oil cracking of the palaeo-reservoirs, and bitumen abundance as well as the distribution characteristics of current gas reservoirs, the accumulation patterns of the cracking gas reservoir can be classified into three types: accumulation type, semi-accumulation and semi-dispersion type, and dispersion type. This understanding will play an important role in guiding the exploration of the Sinian-Cambrian natural gas exploration in the Sichuan Basin. Key words: palaeo-uplift, large gas field, crude oil cracking gas, accumulation mode, Gaoshiti-Moxi, Sichuan Basin