Origin and accumulation of natural gas in the Sinian-Cambrian reservoirs of the central Sichuan Basin, southwest China: Implications from both closed and semi-open pyrolysis experiments

Large amounts of natural gas have been discovered in the Sinian-Cambrian reservoirs of the central Sichuan Basin, southwest China. However, the gas origin and accumulation patterns remain unresolved due to a poor understanding of the geochemical signatures for natural gas derived from cracking of paleo-oils. In this study, a Cambrian sourced oil sample and its SARA (saturates, aromatics, resins and asphaltenes) fractions were pyrolyzed in closed systems to investigate the effects of oil fraction composition on the molecular signature and methane carbon isotope composition of the generated gas. Meanwhile, the oil sample and its asphaltene fraction were further pyrolyzed in stepwise, semi-open system to investigate how possibly the geological openness affects the generation potential and geochemical signatures of the resulted gas from the cracking of oil and asphaltene. It is revealed that although the carbon isotopes of methane (δ¹³C₁) generated from oils change with SARA compositions in the closed system, the δ¹³C₁ values of natural gas from both asphaltene (representing kerogen) and three types of oils with different SARA compositions are lighter than −40‰, and differ significantly from the δ¹³C₁ values of the present natural gas in the Sinian-Cambrian reservoirs. In semi-open systems, the generation potential of natural gases from residual asphaltenes and oils decrease drastically. The gases are dominated by methane with δ¹³C₁ values range from −39.8‰ to −29.4‰, which could even be heavier than the δ¹³C values of original asphaltenes and oils. The geochemical signatures of the natural gases in the Sinian-Cambrian reservoirs are similar to those of gaseous pyrolyzates in the semi-open systems, suggesting that early-generated natural gases may have been partly lost and the late gas derived from residual oils and kerogen forms the present gas pools. The geochemical variation of natural gases in different Sinian-Cambrian reservoirs is primarily controlled by paleostructures. Higher positions of the paleo-uplift contain a higher proportion of early-generated gases and thus have higher gas wetness and lighter δ¹³C₁ values, such as the natural gases in the Ziyang area and the Taihe area. In contrast, the natural gases in lower positions of the paleo-uplift have a higher proportion of late gases, characterized by lower gas wetness and heavier δ¹³C₁ values, such as those in the Weiyuan gases field and the Anyue gas field.