Abstract
Natural gas of organic origin is primarily biogenic or thermogenic; however, the formation of natural gas is occasionally attributed to hydrothermal activity. The Precambrian dolomite reservoir of the Anyue gas field is divided into three stages. Dolomite-quartz veins were precipitated after two earlier stages of dolomite deposition. Fluid inclusions in the dolomite and quartz are divided into pure methane (P-type), methane-bearing (M-type), aqueous (W-type), and solid bitumen-bearing (S-type) inclusions. The W-type inclusions within the quartz and buried dolomite homogenized between 107 °C and 223 °C. Furthermore, the trapping temperatures and pressures of the fluid (249 °C to 319 °C and 1619 bar to 2300 bar, respectively) are obtained from the intersections of the isochores of the P-type and the coeval W-type inclusions in the quartz. However, the burial history of the reservoir indicates that the maximum burial temperature did not exceed 230 °C. Thus, the generation of the natural gas was not caused solely by the burial of the dolomite reservoir. The results are also supported by the presence of paragenetic pyrobitumen and MVT lead-zinc ore. A coupled system of occasional invasion by hydrothermal fluids and burial of the reservoir may represent a new genetic model for natural gas accumulation in this gas field.
Highlights
The Anyue gas field lies in the center of the Sichuan Basin in southwestern China and is one of the oldest and largest gas fields in China[1,2,3]
Pyrobitumen and quartz are observed within the pores and cracks (Fig. 3e,f)
Quartz is always associated with the III-dolomite and is sporadically mixed with pyrobitumen (Fig. 3f)
Summary
The Anyue gas field lies in the center of the Sichuan Basin in southwestern China and is one of the oldest and largest gas fields in China[1,2,3]. Inclusion petrography, the trapping temperatures and pressures of pure methane inclusions and the burial history of well GS6 indicate a new mode of gas generation that couples the effects of burial and hydrothermal fluid systems. The Anyue gas field is located on a paleo-high point of the Sichuan Basin[19] (Fig. 1a,b). The thick black shale of the Qiong Zhusi Formation was deposited within the depression between two high points and became the main resource rock for two gas fields (Fig. 1c). The geochemistry evidences of pyrobitumen indicate that the gas field was generated by an in-situ burial pyrolysis of paleo-oil[20,21]. The Z2dn[4] reservoir is composed primarily of algal bindstone and mud-sized dolostone that formed as a carbonate reef along the edge of the platform[2] (Fig. 1c). A moderately sulfur-rich natural gas that is primarily composed of methane is contained within these reservoir spaces[3]
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