Methane-dominated gaseous inclusions in the Sinian carbonate reservoirs in central Sichuan Basin and their implications for natural gas accumulation

Abstract

The occurrence and genetic origins of gaseous inclusions in the Sinian carbonate reservoirs of Anyue gas field in central Sichuan Basin were investigated with techniques of micro-Raman spectroscopy and microthermometry to trace the oil and gas accumulation history. Three types of gaseous inclusions were identified in terms of their compositions and the homogenization temperatures (Ths) of their coeval aqueous inclusions. Both type I and type II gaseous inclusions are dominantly composed of methane with varying contents of pyrobitumen, indicating their origins of type I and II precursor oil inclusions that have been thermally converted to the present gaseous inclusions. The aqueous inclusions coeval with the present type I and type II gaseous inclusions have Th values in the range of 120.3–136.9 °C and 140.8–168.5 °C, respectively, further confirming that they are derived from precursor oil inclusions that were formed during 251–204 Ma and 202–169 Ma, respectively. For type III gaseous inclusions, their gaseous components are also dominated by methane with minor CO2 and H2S, but no detectable pyrobitumen is observed in them. This implies that they are not genetically related to any precursor oil inclusions but were formed by direct trapping of gases in reservoirs when the paleo-oil pools have been thermally converted to gas pools during the time of 153–103 Ma, which is consistent with the high Th values of 203.8–237.1 °C of their coeval aqueous inclusions.The density of methane in gaseous inclusions was also estimated from methane Raman shifts. The respective values of methane density for types I, II and III gaseous inclusions vary from 0.262 to 0.293 g/cm3 (avg. 0.272 g/cm3), from 0.231 to 0.264 cm3/g (avg. 0.247 g/cm3), and from 0.231 to 0.293 cm3/g (avg. 0.249 g/cm3). Based on the density of methane in type III gaseous inclusions and the Th values of their coeval aqueous inclusions, their trapping pressures were calculated to be in the range of 93.3–134.1 MPa with formation pressure coefficients of 1.32–1.79. This indicates that there were overpressures in the reservoirs during the oil to gas cracking and that the almost hydrostatic system of the present Sinian gas pools is mostly related to the extensive tectonic uplift and subsequent loss of natural gas since late Cretaceous.