Anomalies of natural gas compositions and carbon isotope ratios caused by gas diffusion - A case from the Donghe Sandstone reservoir in the Hadexun Oilfield, Tarim Basin, northwest China

摘要

Natural gases in the Carboniferous Donghe Sandstone reservoir within the Block HD4 of the Hadexun Oilfield, Tarim Basin are characterized by abnormally low total hydrocarbon gas contents ( 65%), low methane contents ( 10%) and low dryness coefficients ( 0.5), and a reversal of the normal trend of carbon isotope ratios, showing delta C-13 methane (C-1) delta C-13 ethane (C-2) delta C-13 propane (C-3) delta C-13 butane (C-4). Specifically, methane is enriched in C-13 with the variations in delta C-13(1) values between gases from Block HD4 and gases from its neighboring blocks reaching 10 parts per thousand. This type of abnormal gas has never been reported previously in the Tarim Basin and such large variations in delta C-13 have rarely been observed in other basins globally. Based on a comprehensive analysis of gas geochemical data and the geological setting of the Carboniferous reservoirs in the Hadexun Oilfield, we reveal that the anomalies of the gas compositions and carbon isotope ratios in the Donghe Sandstone reservoir are caused by gas diffusion through the poorly-sealed caprock rather than by pathways such as gas mixing, microorganism degradation, different kerogen types or thermal maturity degrees of source rocks. The documentation of an in-reservoir gas diffusion during the post entrapment process as a major cause for gas geochemical anomalies may offer important insight into exploring natural gas resources in deeply buried sedimentary basins.