Gas storage and controlling factors in an over-mature marine shale: A case study of the Lower Cambrian Lujiaping shale in the Dabashan arc-like thrust–fold belt, southwestern China

Abstract

Lower Cambrian units in the upper Yangtze area of China are important strata for shale gas exploration. The Lower Cambrian Lujiaping shale in the Dabashan arc-like thrust–fold belt of the upper Yangtze area is thermally over-mature and has been transformed to slate by epimetamorphism. The Lujiaping shale has also experienced strong structural deformation; its current stratigraphic dip is 35°–80°. Over-maturity and strong deformation are considered unfavorable conditions for gas storage. However, desorption tests using canisters indicate that gas contents in the Lujiaping shale range from 0.05 to 3.39 m3/t (average, 1.12 m3/t); this level of gas presents challenges for conventional theories of shale gas enrichment. To analyze the controls on gas enrichment, we performed petrological, geochemical, and petrophysical experiments on shale samples from the Lujiaping Formation. Results show that the TOC contents are between 1.17% and 6.91%. The porosities of the shales are very low, in the range of 0.68%–3.93%, possibly related to over-maturity. The correlations between petrophysical parameters and the total organic carbon (TOC) content indicate that the organic fraction contribution to micropores and mesopores is highly significant. The Langmuir volumes for the studied shale samples are in the range of 1.07–5.87 m3/t. The gas content and methane adsorption capacity are strongly dependent on the TOC content. Because the studied shales have low porosities and are from shallow depths, their capacity for free gas storage is extremely limited, and the gas is stored mainly in an adsorbed state. The studied shales occur at temperatures of 20–37 °C. The maximum sorption methane capacity occurs at depths of 500–1500 m. Well-developed micropores, high surface area, and strong gas adsorption capacity are important controls on gas enrichment in the Lujiaping shales. The three controls may be related to the inter-particle pores between the organic matter and clay minerals.