Stable hydrogen and carbon isotopic ratios of coal-derived and oil-derived gases: A case study in the Tarim basin, NW China

Abstract

Natural gases from the Kuqa depression, Lunnan and Tazhong low uplifts in the Tarim basin of China were analyzed for stable hydrogen and carbon isotopic characteristics, to compare the isotopic differences, particularly of hydrogen, between coal-derived and oil-derived gases. Gases from the Kuqa depression are coal-derived and sourced from Middle–Lower Jurassic coal measures, possibly with a minor contribution from Triassic lacustrine mudstones. Gases from the Lunnan and Tazhong low uplifts are thought to be sourced from Cambrian–Lower Ordovician marine sapropelic organic matter. Natural gases from the Lunnan low uplift and Ordovician strata in the Tazhong low uplift are oil-cracked, and gases from the Carboniferous and Silurian strata in the Tazhong low uplift are oil-associated. Most gases have expected hydrogen isotopic distributions among C1–C3 alkanes, i.e., δ2HCH4<δ2HC2H6<δ2HC3H8. However, nearly all samples that show carbon isotopic reversal between C1 and C2 alkanes (δ13CCH4>δ13CC2H6) in the Lunnan low uplift also have corresponding hydrogen isotopic reversal (δ2HCH4>δ2HC2H6) between these alkanes. In the Tazhong low uplift, except for the Silurian gases, nearly all the Carboniferous and Ordovician gases have regular carbon isotopic distribution pattern among C1–C2 alkanes (δ13CCH4<δ13CC2H6) but with hydrogen isotopic reversal between C1 and C2 alkanes (δ2HCH4>δ2HC2H6). This probably indicates that except the influences of mixing with gases at different maturity, hydrogen isotopes of natural gases are also sensitive to other processes such as hydrogen exchange between water and organic matter during thermal maturation. No analytical differences in δ2HCH4 have been found between coal-derived gases sourced from continental humic organic matter in the Kuqa depression and the oil-derived gases sourced from the Carboniferous and Silurian marine sapropelic organic matter in the Tazhong low uplift. All coal-derived gases in the Kuqa depression have δ2HCH4 values less than −150‰, while the oil-cracked gases in the Lunnan low uplift and the Ordovician oil-cracked gases in the Tazhong low uplift have slightly greater δ2HCH4 values (>−150‰). However, the Silurian and Carboniferous oil-associated gases in the Tazhong low uplift also have quite low δ2HCH4 values (around −160‰). The clearly different hydrogen isotopic values of gases from different strata in the Tazhong low uplift and the close hydrogen isotopic values between gases from the Kuqa depression and Silurian and Carboniferous strata in the Tazhong low uplift, all imply that hydrogen isotopes of alkane gases are influenced by factors more than thermal maturity and depositional environments. Great care needs to be paid when using the hydrogen isotopes for gas genetic study, and a combination with local geological settings and other geochemical features is suggested. This is because hydrogen isotopes of alkane gases in one basin may be not applicable in the other basins.