Abstract
Investigating shale pore characteristics has deepened our understanding of shale reservoir, while that of postmature-overmature shales is yet to be revealed, which is especially critical for shale gas evaluation in southern China. Ten Middle-Upper Devonian organic-rich shale samples were collected from well GY-1 in the Guizhong Depression, and the paleoenvironment, geochemistry, and pore system were analyzed with a series of experiments, including trace element analysis, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), low-pressure N2 adsorption, and source rock geochemistry. Results show that the Middle-Upper Devonian shales in the Guizhong Depression are organic-rich mudstones with TOC ranging from 0.14% to 6.21%, which is highest in the Nabiao Formation ( D 2 n ) and Lower Luofu Formation ( D 2 l ) that were deposited in the anoxic and weak hydrodynamic deep-water shelf. They are thermally postmature to overmature with equivalent vitrinite reflectance ( EqV R o ) of 3.40%~3.76% and type I kerogen. The lithofacies in D 2 n and D 2 l are primarily siliceous/argillaceous mixed shale as well as a few siliceous argillaceous shales and argillaceous siliceous shales as well. Organic matter- (OM-) hosted pores within bitumen are primary storage volume, rather than inorganic pores (interparticle and intraparticle) which are rare. The total helium porosity of samples varies between 1.20% and 4.49%, while total surface area and pore volume are 2.39-14.22 m2/g and 0.0036-0.0171 ml/g, respectively. Porosity, pore surface area, and pore volume are in accordance with increasing TOC, R o , and siliceous mineral contents. Considerable OM-macropores are found in shales with R o > 3.6 % in our study which demonstrates that the porosity at postmature to overmature stage ( R o = 3.5 − 4.0 % ) does not change fundamentally. The high level of maturity is not considered the main controlling factor that affects shale gas content, and more attention should be paid to preservation conditions in this area.
Highlights
Shales have attracted significant attention in the past few years because of their emergence as unconventional hydrocarbon reservoirs [1,2,3]
We will (1) investigate geochemistry and pore system of Devonian shales in the Guizhong Depression that can be further compared with organic-rich shales from other basins in South China and other countries and (2) discuss the organic matter-hosted pores in postmature to overmature stage and the contributors to their development
Researchers have concluded that the pore structure of organic-rich shale is generally governed by diagenesis, organic matter, mineral components, thermal maturity, kerogen type, etc., depending on specific geological conditions [14, 23, 53,54,55,56,57]
Summary
Shales have attracted significant attention in the past few years because of their emergence as unconventional hydrocarbon reservoirs [1,2,3]. Characterizing the pore system of such high-mature Devonian shales in the Guizhong Depression can provide insight into the shale gas resource assessment in China. GY-1 well was drilled at the northwest of the Guizhong Depression to investigate the occurrence of the Devonian shale, understand its geochemical characteristics and storage capacity, and evaluate shale gas resource potential. We will (1) investigate geochemistry and pore system of Devonian shales in the Guizhong Depression that can be further compared with organic-rich shales from other basins in South China and other countries and (2) discuss the organic matter-hosted pores in postmature to overmature stage and the contributors to their development
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