Abstract
Based on the comprehensive analysis of the characteristics of tight sandstone gas composition, carbon isotope, light hydrocarbons and source rocks in Linxing area of Ordos Basin, the reservoir-forming model of tight sandstone gas in this area is discussed. The study shows that methane is the main component of tight sandstone gas, with low contents of heavy hydrocarbons and non-hydrocarbons, mainly belonging to dry gas in the Upper Paleozoic in Linxing area. The values of δ13C1, δ13C2 and δ13C3 of natural gas are in the ranges of -45.6‰ ~ -32.9‰, -28.9‰ ~ -22.3‰ and -26.2‰~ -19.1‰, respectively. The carbon isotopic values of alkane gas show a general trend of positive carbon sequence. δ13C1 value is less than -30‰, with typical characteristics of organic genesis. There is a certain similarity in the composition characteristics of light hydrocarbons. The C7 series show the advantage of methylhexane, while the C5-7 series mainly shows the advantage of isoalkane. The tight sandstone gas in this area is mainly composed of mature coal-derived gas, containing a small amount of coal-derived gas and oil-type gas mixture. According to the mode of hydrocarbon generation, diffusion and migration of source rocks in Linxing area, the tight sandstone gas in the study area can be divided into three types of reservoir-forming assemblages: the upper reservoir type of the far-source type (upper Shihezi formation-shiqianfeng formation sandstone reservoir-forming away from source rocks), the upper reservoir type of the near-source type ( the Lower Shihezi formation sandstone reservoir-outside the source rock), and the self-storage type of the source type (Shanxi formation-Taiyuan formation source rock internal sand reservoir).
Highlights
With the continuous breakthroughs and development of exploration and development technologies, unconventional oil and gas has become a replacement energy source for the stable crude oil production and the rapid growth of natural gas production in China [1,2,3,4]
17 gas samples from 17 wells in Linxing area were collected for analysis of natural gas composition, carbon isotope and light hydrocarbons; more than 200 samples of mudstone and coal from 35 wells were analyzed for rock pyrolysis, organic carbon content determination, and mirror analysis
The test results of tight sandstone gas components in Linxing area (Table 1) show that the content of CH4 ranges from 90.74% ~ 99.32%, with an average value of 95.29%; the content of C2H6 ranges from 0.12% ~ 6.31%, with an average value of 2.04%; the content of C3H8 ranges from 0~ 1.68%, the average value is 0.42%; the drying coefficient is between 0.924 and 0.998, and the average value is 0.973
Summary
With the continuous breakthroughs and development of exploration and development technologies, unconventional oil and gas has become a replacement energy source for the stable crude oil production and the rapid growth of natural gas production in China [1,2,3,4]. The Ordos Basin is the second largest petroliferous basin on land in China It is a large, asymmetric, north-south, superimposed craton basin with a steep and narrow west flank and a wide east flank [5,6], which contains very rich unconventional oil and gas resources[7,8]. After more than 50 years of exploration and development research, the Sulige, Daniudi, Yulin, Zizhou, Mizhi, Shenmu, Wushenqi, Dongsheng and other large tight sandstone gas fields have been discovered in the basin, which are hundreds of billions of cubic meters currently discovered in China. It is necessary to strengthen the analysis and research on the tight sandstone gas accumulation model in Linxing area to provide guidance for the exploration and development of tight gas reservoirs in this area
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