Abstract
Chemical and isotopic compositions of C6-C7 low molecular weight hydrocarbons (LMWHs) were used to determine the origin of gas in the Ordovician weathered crust (O1m5) reservoir of Jingbian gas field in Ordos Basin, China. GC quantification of LMWHs for 55 gas samples [20 of them from the Xiashihezi Formation (P1x) and Shanxi Formation (P1s) reservoirs, and the rest from the O1m5 reservoir] were conducted. The compound specific stable carbon isotopes of LMWHs for 24 gas samples (five gas samples from the P1s reservoir) were analyzed with GC-C-IRMS. The results show that cycloalkanes are abundant components and their concentrations range from 23.8–52.5% with an average of 41.1% among LMWHs. Methylcyclohexane (MCH) is the most abundant one among normal heptane (n-C7), MCH and dimethylcyclopentane (DMCP), ranging from 50.1–81.4% with an average value of 71.6%. A13C enrichment for nine compounds of C6-C7 fraction was observed, and the δ13C values range from −25.4–16.3%. Based on the correlation of the Mango parameters (K1 and K2), the relationship of n-C7, MCH and DMCP and compound specific carbon isotope of C6-C7 compounds, the gas in the O1m5 reservoir shares the same origin with that in the P1x, P1s reservoirs, the natural gas in Ordovician weathered crust reservoir and Permian sandstone reservoir sourced from coal measures in Carboniferous and Permian.
