Hydrogen isotope composition of natural gases from the Tarim Basin and its indication of depositional environments of the source rocks

Abstract

By measuring carbon and hydrogen isotope compositions for C1, C2 and C3 of 74 gas samples, natural gases from the Tarim Basin can be divided into six groups on the basis of their origins: (1) coal-type gas derived from coal measures; (2) coal-type gas generated from the T-J lacustrine mudstones; (3) oil-type gas derived from the Cambrian and low Ordovician marine source rocks; (4) oil-type gas from the source rocks deposited in the marine-transitional facies; (5) mixing gas between gas derived from the Carboniferous transitional source rocks and the Mesozoic humic gas, and (6) mixing gases of thermal genetic gas and little deep gas in the Southwest depression of the Tarim Basin. The δD values of methane in natural gases originating from different type kerogens are affected by both palaeo-environments of the source rock formation (kerogen types) and thermal maturity, with sedimentary environment (kerogen type) as the main controlling factor. Under the similar thermal maturity, the hydrogen isotope composition of methane is more enriched in deuterium in marine environments than lacustrine one. With the increase of thermal maturity and the increase of carbon atomic numbers of gaseous alkanes, the hydrogen isotopes become enriched in deuterium. The δD values of ethane and propane (δD2, δD3) are controlled mainly by thermal maturity and to a lesser degree by sedimentary environment of the source rock formation. The partial reversal of hydrogen isotopes for gaseous alkanes would be related to the microbial oxidation, mixing of sapropelic and humic gases and / or mixing of gases from similar kerogen sources with various thermal maturities. In the oil-type gas, the sulfate reduction reaction would result in the reversed order of δD1 and δD2 (e.g. δD1>δD2).