Abstract:

The Lower Cambrian Qiongzhusi black shale of the Middle-Upper Yangtze region is considered the best candidate for shale gas exploration after the breakthrough of shale gas development in the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations because of its high total organic carbon (TOC) content and high hydrocarbon-generating potential. The high-quality argillaceous source rock is mainly distributed in the Mianyang-Changning trough in Sichuan, the Xiang’exi trough in western Hunan and western Hubei, and the deep-water shelf and slope basins in eastern Sichuan and northern Guizhou, with source-rock intervals mainly developed at the bottom of the troughs and in the middle and upper parts of the deep-water shelf and slope basins, as indicated by TOC profile studies. In this paper, fluid inclusion analysis and super-low-concentration U-Pb dating were performed on collected shale specimens, in combination with comprehensive literature research, to investigate the hydrocarbon generation history in the Lower Cambrian Qiongzhusi stage. In addition, the shale gas composition and isotopic characteristics are discussed. There are two possible modes of shale gas generation and accumulation: (1) Source rock developed in western Hunan, western Hubei and eastern Sichuan experienced a single-stage oil/gas generation process, where oil generation started in the late Caledonian (430 Ma), at temperatures of 95.7-105.2 ℃, followed by crude oil cracking in the late Hercynian (270 Ma), at ~144.7℃, wet gas secondary cracking in the early Yanshanian (270-130 Ma), at 215.3 ℃, and shale gas leakage from the late Yanshanian to present (130-0 Ma). (2) Source rock from southern Shaanxi and southern, central and southwestern Sichuan experienced a two-stage oil/gas generation process, where the 1st and 2nd stage oil generation occurred at 421.5 Ma and 262.4-256.4 Ma, at temperatures of 115.8-128.9 ℃ and 137.1-150.0 ℃, respectively; crude oil and/or kerogen cracking in the Mianyang-Changning trough started in the Late Hercynian (259.4±3.0 Ma), at ~140 ℃, on average, followed by wet gas secondary cracking in the middle-late Yanshanian (175-133 Ma), at ~220 ℃, with ongoing shale gas accumulation and reservoir adjustment due to favorable preservation conditions. The differential hydrocarbon generation is controlled by regional tectonic and depositional evolution. Shale gas in the study area is mainly oil-type gas of organic origin, generated from kerogen and crude oil cracking. Due to the development of faults in northern Guizhou and southeastern Chongqing, the early-stage shale gas was washed by surface and atmospheric water, causing changes in its elemental and isotopic compositions and resulting in nitrogen enrichment and decrease of δ¹³C₁ value.

Key words: Middle-Upper Yangtze region, Qiongzhusi stage, argillaceous source rocks, modeling hydrocarbon generation history modeling, shale gas, fluid inclusion, U-Pb dating