Origin and formation of deep and superdeep strata gas from Gucheng-Shunnan block of the Tarim Basin, NW China

Abstract

Abstract A series of experiments, including natural gas molecular composition, carbon isotope analysis and fluid inclusion petrography and microthermometry were conducted to research the origin and formation of natural gases in the Middle Ordovician Yijianfang Formation and the Middle-Lower Ordovician Yingshan Formation, Gucheng-Shunnan block. Natural gases consist predominantly of methanes, with dry coefficients of 0.951–0.999 and have relatively heavy carbon isotopic signatures, with average δ13C1 and δ13C2 values of −36.4‰ and −32.5‰, respectively. The diagrams between δ13C-C1 versus C1/(C2 + C3) and methane carbon isotope versus ethane-propane isotopic separation suggest that these gases are oil-associated thermogenic gas from type Ⅱ kerogen with high thermal evolution extent of the source rocks. The empirical correlation of ln(C1/C2) and ln(C2/C3) indicates natural gases originated from crude oil cracking. Natural gases in the Gucheng block and the Shunnan block have properties similar to those of oil-cracking gases in the Eastern Lungu block and the Eastern Tazhong block. Due to reservoir temperatures being lower than formation temperatures of the gases and the potential of higher mature source rock being almost exhausted, natural gases originated predominantly from the cracking of deep paleo-oil pool rather than dispersive dissoluble organic matter in the source rocks. The comprehensive analysis of thermal evolution history, burial history, fluid inclusions and geological conditions suggest that natural gases formed during Himalayan stage. Deep oil-cracking gases accumulated in the Ordovician reservoir through migration along faults. Therefore, deep-seated natural gases have great potential for exploration.