Abstract
Indentifying reservoir characteristics of coals and their associated shales is very important in understanding the co-exploration and co-production potential of unconventional gases in Guizhou, China. Accordingly, comprehensive experimental results of 12 core samples from well LC-1# in the northern Guizhou were used and analyzed in this paper to better understand their vertical reservoir study. Coal and coal measured shale, in Longtan Formation, are rich in organic matter, with postmature stage of approximately 3.5% and shales of type III kerogen with dry gas generation. All-scale pore size analysis indicates that the pore size distribution of coal and shale pores is mainly less than 20 nm and 100 nm, respectively. Pore volume and area of coal samples influenced total gas content as well as desorbed gas and lost gas content. Obvious relationships were observed between residual gas and BET specific surface area and BJH total pore volume (determined by nitrogen adsorption). For shale, it is especially clear that the desorbed gas content is negatively correlated with BET specific surface area, BJH total pore volume and clay minerals. However, the relationships between desorbed gas and TOC (total organic carbon) as well as siderite are all well positive. The coals and shales were shown to have similar anoxic conditions with terrestrial organic input, which is beneficial to development of potential source rocks for gas. However, it may be better to use a low gas potential assessment for shales in coal-bearing formation because of their low S1+S2 values and high thermal evolution. Nevertheless, the coalbed methane content is at least 10 times greater than the shale gas content with low desorbed gases, indicating that the main development unconventional natural gas should be coalbed methane, or mainly coalbed methane with supplemented shale gas.
Highlights
The remarkable success in the exploration and development of coalbed methane (CBM) and shale gas in the United States has stimulated CBM and shale gas exploration boom worldwide (Kenomore et al, 2018; Moore, 2012; Prpich et al, 2016; Soeder, 2018)
We utilize the analysis of canister desorption, isothermal adsorption, mercury injection, low-pressure N2 and CO2 adsorption, total organic carbon (TOC), Rock-Eval pyrolysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), elemental analysis and so on to determine reservoir characteristics, depositional environment and gas potential of coals and shales. These results provide an important basis and valuable insight for comprehensive evaluation and further study of the potential of CBM and shale gas potential in the transitional Longtan formation
To characterize coal and shale reservoirs, all experimental tests were performed on core samples in this study (Table 1), and these tests were conducted in accordance with national standards and related specifications, and no abnormal conditions were examined during the measurements, which were all performed at qualified laboratories
Summary
The remarkable success in the exploration and development of coalbed methane (CBM) and shale gas in the United States has stimulated CBM and shale gas exploration boom worldwide (Kenomore et al, 2018; Moore, 2012; Prpich et al, 2016; Soeder, 2018). The shale is the source rock of natural gas generation and the reservoir and cap of the accumulation and preservation of natural gas. The coal-bearing shale gas and CBM have been paid much attention in China (Luo et al, 2018; Yang et al, 2017) because of the coal measure gas (CBM, shale gas and sand gas) in superposed reservoirs existing a possibility of co-exploration and co-production. Studies on the continuous accumulation of coal-bearing gas is relatively scarce and concentrated in the Ordos Basin and Qinshui Basin (Li et al, 2018b, 2016)
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