Geological Characteristics of Shale Gas in China

During the geological period from Proterozoic to Quaternary, China has successively formed many sets of organic-rich shale formations under various depositional environments from marine, sea-land transition phase to terrestrial phase, and many types of organic matter have been formed. However, due to the complexity of tectonic changes in the late period, the conditions of the organic matter anger, gas-bearing and preservation vary greatly, which has resulted in the diversification of China's shale gas, which is mainly manifested in the following four features.

(1) Sea-phase, sea-land transition and land-phase shale are all developed, and the shale stratigraphic assemblage has its own characteristics

China has formed sea-phase, sea-land transition and land-phase stratigraphy in different geologic eras, which contains more than ten sets of organic-rich shale formations, and the stratigraphic assemblage has different characteristics. Marine shale is mostly thick, widely distributed and stable, and can be sandstone, carbonate rock, etc.; sea-land transition phase shale is widely distributed, and the relatively stable organic-rich shale is often interbedded with sandstone, coal beds and other lithologies frequently; land-phase shale is mainly manifested as a huge thick mudstone layer system, mud shale and sandy thin rhyolite development, the cumulative thickness of the large, planar distribution limitations, and rapid lateral changes. The characteristics of stratigraphic assemblage determine the huge difference of shale gas geological conditions.

(2) Paleozoic, Mesozoic and Cenozoic multi-layered shale, with different gas-bearing characteristics

China's Lower Paleozoic Cambrian, Ordovician and Silurian, Upper Paleozoic Devonian, Carboniferous and Permian, Mesozoic Triassic, Jurassic and Cretaceous, and Cenozoic Paleozoic are all developing multiple sets of organic-rich shale layers and systems. From a national perspective, the shale facies have a wide planar distribution and many sectional layers. From the new to the old, although the maturity of organic matter increases sequentially, the rock-forming effect of shale gradually strengthens in each stratum, and the primary free gas-bearing space dies out sequentially, resulting in different gas-bearing characteristics of shale in each stratum. In terms of gas-bearing characteristics, the relative content of adsorbed gas increases sequentially. Under the same preservation conditions, the adsorbed gas content of shale in the Lower Paleozoic is relatively the most, the relative content of free gas in the Upper Paleozoic is increasing, and the dissolved gas content in the Middle Cenozoic is relatively the most due to the maturity reason of the formation of shale oil and gas***generation phenomenon.

(3) Complex tectonic changes in subsidence, stabilization and uplift zones, and very different preservation conditions for shale gas

In China, shale gas is more widely distributed than conventional oil and gas. In North China, Northeast China, Northwest China, and some of the subsidence areas in the South due to the thicker overlying strata, shale gas preservation conditions are good; in the Ordos Basin and the West Sichuan Depression and other stable areas, the burial conditions are relatively moderate, the preservation conditions are good, and it is the most promising area for the development of shale gas; in the Upper Yangzi, Lower Yangzi and other tectonics movement complex late stage of Tai Long Zone, although the shale organic geochemistry and other conditions are favorable, but strata are generally subjected to lifting and late stripping, preservation conditions are affected by the shale gas preservation conditions, and the shale gas preservation conditions are very different. Although the organic geochemical conditions of the shale are favorable, the strata generally suffer from uplift and late-stage denudation, and the preservation conditions are seriously affected, resulting in a general decrease in the total gas content of the strata.

(4) Biological, pyrolytic, and fracture genesis types are diverse, and shale gas evaluation methods and selection criteria are different

Different geotectonic backgrounds determine large changes in sedimentary phases, which result in the formation of different types of organic matter, corresponding to the creation of different shale gas generation conditions and gas characteristics, and under the conditions of the unified industrial gas content standards, shale gas of different genesis types, such as biological, pyrolytic, and fracture, need to be targeted separately. Under the condition of unified industrial gas content standard, for different types of shale gas such as biological, pyrolysis, cracking and other genesis types, it is necessary to adopt targeted resource evaluation methods and favorable selection criteria. Under the condition of marine phase, the sedimentary organic matter with the characteristic of biogenic oil requires relatively high degree of thermal evolution, and the evaluation method and selection criteria can refer to the shale gas in the eastern region of the United States. Under terrestrial conditions, all three types of caseous roots have different degrees of development, and when the degree of thermal evolution is low, the shale oil and gas **** generation is manifested, and the change of gas content is controlled by multiple factors such as the type of organic matter, the degree of thermal evolution, the depth of burial, and preservation, etc., and the evaluation methods and criteria need to be focused on the study of different depositional basins.