Abstract

Many small and medium-sized Paleogene pull-apart basins in southern China contain an abundance of biogenic gas shows. Such shallow gas with biogenic characteristics has been thoroughly investigated only in Baise Basin. Hence, the research results from Baise Basin could serve as a model for the origin and characteristics of shallow gases in similar basins and areas in Southern China. There are ten gas fields with a total of proven reserves of 20 billion cubic meters discovered in Baise Basin. Three gas fields are located on the western and southern flank of the basin, and they are gas caps to heavy oil pools from depths between 600 m and 850 m. These accumulations contain dry biogenic gases, with C₁/C_1-5 exceeding 0.99, light δ¹³C₁ (–76 to −54‰), and heavy δD₁ (−218‰). Trace heavy gaseous hydrocarbons are strongly biodegraded (iC₄/nC₄>3). The other seven gas fields are located in the northern fault zone and the central Nakun uplift of the basin at depths between 300 m and 700 m. Gases are mainly unassociated and condensed gases. The condensed gases are depleted in ¹³C (δ¹³C₁: −67 to −53.7‰, δ¹³C₂: −52.3 to −36.1‰, δ¹³C₃: −43.3 to −33.4‰), but wet with C₁/C₂₊₃ mostly less than 20, suggesting a mainly thermal origin at low maturity. The unassociated gases are dominated by methane, with C₁/C₂₊₃ ratio above 100, variable N₂ (0 to 5.4%), and traces of CO₂. The δ¹³C value of methane in the unassociated gases is between −76 to −60‰, and δD₁ values from −248 to −213.7‰. These gases also contain isotopically light ethane with δ¹³C₂ values of −64.5 to −42‰, which we infer to have originated from deeper horizons as a result of migration or diffusion from low-maturity thermal gases with light stable carbon isotopic compositions (C₁/C₂₊₃ <20, δ¹³C₂ from −60‰ to −58‰). A similar thermal origin is inferred for the other heavy gaseous hydrocarbons. Formation water with the shallow gases of this basin is mainly NaHCO₃-type with low TDS ranging from 1000 to 4500 ppm. The (HCO₃+CO₃)/Cl ratios range from 1.5 to 100, indicating a relative open hydrodynamic condition and the possible intrusion of meteoric water. These data indicate that early biogenic gas generated syndepositionally was probably not preserved, and that the current biogenic gas accumulation formed mainly as a result post-depositional of crude oil biodegradation in the western basin and coal biodegradation in the northern fault zone. Shallow gases in Baise Basin are mainly secondary biogenic gases, with an admixture of low maturity thermogenic gases from deeper horizons.