Gas-Bearing Characteristics Of The Continental Shale From Shahezi Formation In Xujiaweizi Fault,Songliao Basin

Shale gas is considered a huge resource in China.Effective exploration and development of marine shale gas have been achieved in Sichuan Basin and its surroundings,while few studies have been conducted on the continental shale.Shahezi shale in Xujiaweizi Fault Depression belongs to typical continental shale with thickness greater than 200 m.Previous studies mainly took these shale layers as the source rock instead of the reservoir,therefore,their physical properties and gas-bearing characteristics need to be evaluated.In this paper,series of experiments including Leco,Rock-Eval,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,high-pressure mercury injection（HIMP）,low-temperature nitrogen adsorption（LTNA）,and high-pressure methane adsorption were conducted on the Shahezi shales,so as to reveal the organic geochemical features,pore structures（pore types,pore size distribution）and adsorption characteristics.Three methods including USBM,ACF,and process analysis method were used to recover the gas-in-place content.Moreover,a model was established to evaluate the shale gas content and then determine its plane and vertical distributions,thus laying a foundation for the exploration and development of shale gas in the studied area.The results show that the TOC content ranges from 0.33 to 13.09 wt.%,with an average of 1.51 wt.%;the organic matter type is dominated by type III,and the T_max is in the range of486°C-535°C,indicating that the maturity of organic matter is in the high-mature and over-mature.The mineralogical compositions are mainly clay minerals,quartz and feldspar,with an average content of 46.76%,34.71%and 17.20%respectively.The SEM images show that the primary pore type of shale samples is inorganic pores such as intercrystalline pores in clay and feldspar dissolution pores,by contrast,organic pores are not well developed.Specifically,the HIMP experiment shows that shale samples are poorly sorted,and the distribution range of pore throats varies greatly among different samples.Two types of shale inferred from the LTNA experiment,one is characterized by H2 hysteresis loops with high specific surface area prone to adsorb gas,the other is characterized by H3 hysteresis loops with numerous mesopores and macropores,which is conducive to the diffusion and seepage of shale gas.The Langmuir volume ranges from 0.97 to 3.58 m³/t with an average of 1.91 m³/t,which is positively correlated with organic matter abundance,clay mineral content and micropores development degree.The gas-in-place content of shales evaluated by the USBM method is significantly lower than that by the ACF method or the process analysis method.The process analysis method can effectively describe the loss process of shale gas and obtain the adsorbed/free gas content simultaneously.The adsorbed gas content varies from 0.2 to 0.44 m³/t with an average of 0.36 m³/t,and the free gas content is between 1.07 m³/t and 2.00 m³/t,with an average of 1.7 m³/t,indicating a low adsorbed gas ratio.Based on the logging data,the gas content calculation model proposed in this study was used to predict the shale gas content,and results show that the favorable area is mainly distributed in the Anda-Songzhan area and surroundings of Well Zhaoshen 32.