Tracing of natural gas migration by light hydrocarbons:A case study of the Dongsheng gas field in the Ordos Basin,NW China

Based on the analysis of light hydrocarbon compositions of natural gas and regional comparison in combination with the chemical components and carbon isotopic compositions of methane,the indication of geochemical characteristics of light hydrocarbons on the migration features,dissolution and escape of natural gas from the Dongsheng gas field in the Ordos Basin is revealed,and the effect of migration on specific light hydrocarbon indexes is further discussed.The study indicates that,natural gas from the Lower Shihezi Formation(Pix)in the Dongsheng gas field displays higher iso-C5-7contents than n-C5-7contents,and the C6-7light hydrocarbons are composed of paraffins with extremely low aromatic contents(<0.4%),whereas the C7light hydrocarbons are dominated by methylcyclohexane,suggesting the characteristics of coal-derived gas with the influence by secondary alterations such as dissolution.The natural gas from the Dongsheng gas field has experienced free-phase migration from south to north and different degrees of dissolution after charging,and the gas in the Shiguhao area to the north of the Borjianghaizi fault has experienced apparent diffusion loss after accumulation.Long-distance migration in free phase results in the decrease of the relative contents of the methylcyclohexane in C7 light hydrocarbons and the toluene/n-heptane ratio,as well as the increase of the n-heptane/methylcyclohexane ratio and heptane values.The dissolution causes the increase of isoheptane values of the light hydrocarbons,whereas the diffusion loss of natural gas in the Shiguhao area results in the increase of n-C5-7contents compared to the iso-C5-7contents.

The first extra-large helium-rich gas field identified in a tight sandstone of the Dongsheng Gas Field,Ordos Basin,China

Helium gas is a scarce but important strategic resource,which is usually associated with natural gas.Presently,only one extra-large helium-rich gas field has been found in China:the Hetianhe Gas Field in the Tarim Basin.This paper reports a new example,the Dongsheng Gas Field(DGF)in the Ordos Basin.In this study,92 natural gas samples from the DGF were analyzed for helium content and isotope composition using isotope mass spectrometry.The natural gas samples were found to have an average helium content of 0.133%,with 65(70.7%)of the samples having a helium content of 0.1%or more.Based on the proven natural gas reserves of the DGF,the proven geological helium reserves were calculated to be 1.96×10^(8)m^(3),suggesting that it represents the first extra-large helium-rich natural gas reservoir to be hosted in tight sandstone in China.The ^(3)He/^(4)He ratios of 5 natural gas samples from the DGF are within the range of 3.03×10^(−8)–3.44×10^(−8).Therefore,the helium in the gas field is thought to be of typical crustal origin and to have formed in the granitic basement that is rich in uranium and thorium.The accumulation of helium-rich natural gas was controlled by regional tectonic activities.Activity along the fault connecting the reservoir with the basement caused release of the helium gas,which entered the overlying strata along the fault and accumulated with conventional hydrocarbon gas.Based on the structural background and the distribution of helium source rocks in the Ordos Basin,the main helium source rocks with high exploration potential are located in deep strata within the north and middle parts of the basin.