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Nature and Origin of Dry Natural Gas in the Middle Indus Basin, Pakistan


We show how different processes impact the molecular and C isotopic composition of natural gas in the Middle Indus Basin (MIB), Pakistan where Lower Cretaceous (Sembar) shale beds in the Sulaiman Foredeep that contain Type III/II kerogen generated thermogenic gas. These very dry thermogenic gas samples were generated at high maturity, but post-generation processes also have influenced the composition of natural gas: e.g., the admixture of microbial methane; biodegradation. The C isotopic composition of methane, ethane, and propane in a dry gas sample (C1/C2 = 74) produced from a Jurassic carbonate reservoir at the Nandpur Field on the Punjab Platform indicates it was generated by isotopically-heavy kerogen (δ13C ≈ -23.5‰ PDB) in the wet gas window (VR ≈ 1.4%). A drier (C1/C2 = 279) gas sample produced from a Paleocene sandstone reservoir at the Pirkoh Field in the Sulaiman Foldbelt was generated at higher maturity (VR ≈ 1.8%) by kerogen with the same C isotopic composition. The C isotopic composition of kerogen isolated from the Sembar Formation and from shale beds in the overlying Lower Goru Formation ranges from -23.4 to -25.8‰ PDB, supporting other evidence those Lower Cretaceous source rocks generated the thermogenic gas in the MIB. The presence of isotopically-heavier propane in two other very dry gas samples produced from the Paleocene reservoir and from the underlying Cretaceous reservoir at the Pirkoh Field indicates they are slightly biodegraded. The C isotopic composition of methane in several “wetter” (C1/C2 = 82-96) gas samples produced from shallower Lower Eocene limestone reservoirs at the Sui Field in the Sulaiman Foldbelt indicates they contain a small amount (≈10%) of microbial methane. Several gas fields also are present on the Mari-Kandhkot High separating the MIB from the Lower Indus Basin. The molecular and C isotopic composition of several natural gas samples produced from Lower Eocene limestone reservoirs at the Kandhkot Field and at the Sui Field are very similar – evidence they have a similar origin. Natural gas samples produced from a shallower Upper Eocene limestone reservoir at the Mari Field are much drier (C1/C2 = 360-700). But the C iso-topic composition of methane (δ13C ≈ -48 to -52‰ PDB) indicates they contain a signi-ficant amount (≈40-50%) of microbial methane, and the presence of isotopically-heavy ethane (δ13C ≈ -14 to -21‰ PDB) demonstrates microbes have metabolized C2+ com-pounds. Thus, the very high methane/ethane ratios in the Mari gas samples are not evidence they were generated at very high maturity. Finally, most of the MIB gas sam-ples contain 3.2-8.8 mol% CO2, whose C isotopic composition (-6.8 to +0.5‰ PDB) indicates most of the CO2 formed during high-temperature diagenetic reactions that involved marine carbonate minerals: i.e., it was not generated by low-maturity kerogen.