Hydrothermal activity in ultra-deep strata and its geological significance for deep earth gas exploration: Implications from pyrobitumen in the Ediacaran-lower Cambrian Strata, Sichuan Basin

Abstract

Abundant pyrobitumen related to an abnormal thermal event has been observed in the Ediacaran-lower Cambrian reservoirs of the central Sichuan Basin. Using reflected light microscopy, this study classified reservoir pyrobitumen in the Ediacaran-lower Cambrian into three groups: isotropic, mosaic, and fibrous; the mosaic pyrobitumen was further subdivided into fine-grained, medium-grained, and coarse-grained. Based on organic elemental analysis, carbon isotopic composition, nuclear magnetic resonance, X-ray diffraction, thermogravimetric-mass spectrometry, and fluid inclusion geochemistry, the relationship between the optical texture of pyrobitumen and hydrothermal temperature was determined and the influence of hydrothermal activity on crude oil cracking is discussed. With the exception of the mosaic pyrobitumen (quinoline insoluble matter-enriched tar pad precursor) found in the most porous reservoir zone, the optical texture in the Ediacaran-lower Cambrian reservoirs can be used to trace the hydrothermal activity, which occurred at the end of the Permian and may be related to the Emeishan mantle plume. The hydrothermal fluid may have invaded the strata of the second and fourth members of the Dengying Formation and the Longwangmiao Formation, along deep faults in the Gaoshiti area and migrated laterally to the northeast. The intrusion of the hydrothermal fluid directly promoted the cracking of crude oil in the reservoirs, and this behavior advances the previously accepted cracking time by approximately 60 Ma. The study found that hydrothermal activity promoted the cracking of crude oil in the Ediacaran-lower Cambrian reservoirs, which is a significant recognition for understanding the history of hydrocarbon accumulation in the Sichuan Basin.