The characteristics of liquid hydrocarbon in Zhangjiatan shale,Ordos Basin and its effects on the estimation of shale gas amount

Industrial shale gas has been discovered in the lacustrine organic-rich Zhangjiatan shale of the Late Triassic Yanchang Formation, Ordos Basin. However, a large amount of liquid hydrocarbon are retained in shale due to the low organic-matter maturity (Ro is 0.7%-1.3%)of Zhangjiatan shale, which exerts great influences on the occurrence state of shale gas and the assessment of origin gas in place. Using fluorescence microscope, scanning electron microscope (SEM)and energy dispersive spectrometer (EDS)in combination with Soxhlet extraction, Rock-Eval pyrolysis, methane isothermal adsorption test and other means, this paper analyzes the occurrence state and content of liquid hydrocarbon in Zhangjiatan shale, and studies the effects of liquid hydrocarbon on the occurrence state and content of shale gas. The results show that Zhangjiatan shale has a high content of liquid hydrocarbon; the content of chloroform bitumen A ranges between 0.2% and 1.2%, with a mean of 0.72%; the content of pyrolysis S1 mainly varies from 1 mg/g to 7 mg/g, with a mean of 3.87 mg/g. The microscopic observations and test results show that liquid hydrocarbon mainly exists in the pores and fractures of shale in a free state, or may be absorbed on the surface of organic matter and clay mineral. Further research has revealed that the liquid hydrocarbon can cause a significant decline in the methane absorption capability of shale; it is decreased by 35%-57%, with an average of 45%. Under the formation temperature and pressure conditions, a large amount of natural gas may be dissolved in the liquid hydrocarbon of Zhangjiatan shale; the amount of dissolved gas mainly rangs from 0.20 mL/g to 1.20 mL/g and has an average of 0.65 mL/g, accounting for about 20% of the total. If considering the loss of light hydrocarbon, the amount of dissolved gas will be higher with an average of 0.91 mL/g, accounting for about 28% of the total. In addition, the liquid hydrocarbon in Zhangjiatan shale has certain influences on the content of shale gas in various occurrence states. When making an assessment on the shale gas resource amount of Zhangjiatan shale in Yanchang Formation and other similar shale gas with low maturity, it is required to perform light hydrocarbon correction and heavy hydrocarbon compensation for the liquid hydrocarbon content, and also consider the dissolved gas content, the pore size of liquid hydrocarbon and the influences of liquid hydrocarbon on the adsorbed gas amount and free gas amount, so as to achieve a reliable objective assessment of shale gas resource. © 2016, Editorial Office of ACTA PETROLEI SINICA. All right reserved.