Natural gas geochemistry and its origins in Kuqa depression

Abstract

According to gas compositional and carbon isotopic measurement of 114 gas samples from the Kuqa depression, accumulation of the natural gases in the depression is dominated by hydrocarbon gases, with high gas dryness (C1/C1–4) at the middle and northern parts of the depression and low one towards east and west sides and southern part. The carbon isotopes of methane and its homologues are relatively enriched in 13C, and the distributive range of δ 13C1, δ 13C2 and δ 13C3 is −32‰–−36‰, −22‰–−24‰ and −20‰–−22‰, respectively. In general, the carbon isotopes of gaseous alkanes become less negative with the increase of carbon numbers. The δ 13 \( C_{CO_2 } \) value is less than −10‰ in the Kuqa depression, indicating its organogenic origin. The distributive range of 3He/4He ratio is within n × 10−8 and a decrease in 3He/4He ratio from north to south in the depression is observed. Based on the geochemical parameters of natural gas above, natural gas in the Kuqa depression is of characteristics of coal-type gas origin. The possible reasons for the partial reversal of stable carbon isotopes of gaseous alkanes involve the mixing of gases from one common source rock with different thermal maturity or from two separated source rock intervals of similar kerogen type, multistages accumulation of natural gas under high-temperature and over-pressure conditions, and sufficiency and diffusion of natural gas.