Origin of late charged gas and its effect on property of oils in the Ordovician in Tazhong area

Natural gases in Ordovician reservoirs of the Tazhong area of the Tarim Basin appear derived by oil-cracking gas. The gases show very different geochemical characteristics. Most of the gases in the middle and eastern part of the No. 1 fault-slope zone are characterized by high gas dryness coefficients and have methane δ13C1 values heavier than primary gas. These features are different from gases in the western part of the No. 1 fault-slope zone. Thus, we propose that there existed mixing between more mature, 13C-rich methane dominated gas and less mature wet gas with 13C-depleted methane. The hypothesis is supported by (1) roughly a positive correlation between the gas dryness coefficient and methane δ13C1 value, (2) significantly heavier δ13C1 values in the eastern part of the study area than in the west, and (3) significantly heavier δ13C1 values than anticipated for unaltered gas co-generated with C2–C4. The charge and mixing of the dry gas into the Ordovician pools may have resulted in increase in wax contents of the oils, gas/oil ratios, gas dryness coefficients and methane δ13C values, leading to a positive correlation between gas/oil ratio and paraffin wax content as a result of continuous fractionation and removal of light components of an oil with progressive gas influx and thus gas washing.