Methane microseepage from different sectors of the Yakela condensed gas field in Tarim Basin, Xinjiang, China

Abstract

Abstract Methane microseepage is the result of natural gas migration from subsurface hydrocarbon accumulations to the Earth’s surface, and it is quite common in commercial petroleum fields. While the role of microseepage as a pathfinder in petroleum exploration has been known for about 80 a, its significance as an atmospheric CH 4 source has only recently been studied, and flux data are currently available only in the USA and Europe. With the aim of increasing the global data-set and better understanding flux magnitudes and variabilities, microseepage is now being extensively studied in China. A static flux chamber method was recently applied to study microseepage emissions into the atmosphere in four different sectors of the Yakela condensed gas field in Tarim Basin, Xinjiang, China, and specifically in: (a) a faulted sector, across the Luntai fault systems; (b) an oil–water interface sector, at the northern margin of the field; (c) an oil–gas interface sector, in the middle of the field; (d) an external area, outside the northern gas field boundaries. The results show that positive CH 4 fluxes are pervasive in all sectors and therefore, only part of the CH 4 migrating from the deep oil–gas reservoirs is consumed in the soil by methanotrophic oxidation. The intensity of gas seepage seems to be controlled by subsurface geologic settings and lateral variabilities of natural gas pressure in the condensed gas field. The highest CH 4 fluxes, up to ∼14 mg m −2 d −1 (mean of 7.55 mg m −2 d −1 ) with higher spatial variability (standard deviation, σ : 2.58 mg m −2 d −1 ), occur in the Luntai fault sector. Merhane flux was lower in the oil–water area (mean of 0.53 mg m −2 d −1 ) and the external area (mean of 1.55 mg m −2 d −1 ), and at the intermediate level in the gas–oil sector (mean of 2.89 mg m −2 d −1 ). These values are consistent with microseepage data reported for petroleum basins in the USA and Europe. The build-up of methane concentration in the flux chambers is always coupled with an enrichment of 13 C, from δ 13 C 1 of −46‰ to −42.5‰ (VPDB), which demonstrates that seeping methane is thermogenic, as that occurring in the deep Yakela reservoir. Daily variations of microseepage are very low, with minima in the afternoon, corresponding to higher soil temperature (and higher methanotrophic consumption), and maxima in the early morning (when soil temperatures are lowest). A preliminary and rough estimate of the total amount of CH 4 exhaled from the Yakela field is in the order of 10 2 tonnes a −1 . The present data can statistically improve the accuracy of the global microseepage flux data-set, but further surveys are needed in order to understand the frequency of occurrence and spatial variability of positive CH 4 fluxes in soils over petroleum fields.