AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

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

Abstract

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.