Dongliang Wang

New indexes and charts for genesis identification of multiple natural gases

Abstract Identification of natural gas genesis and source for high-matured multiple natural gases is a great challenge in the exploration of deep–ultra deep and unconventional natural gases. In this paper, the genesis identification method system of multiple natural gases is enriched through new experimental techniques and comprehensive analysis of geological data. New indexes and charts of genesis identification for multiple natural gases were determined to distinguish the sapropelic kerogen degraded gas and oil cracking gas, accumulated and scattered liquid hydrocarbon cracking gas in different evolution stages, nitrogen, carbon dioxide of organic and inorganic origins, inert gases of crustal and mantled origins, coal-formed gas and oil-typed gas by helium, nitrogen, carbon dioxide and mercury content in natural gas. These indexes and charts have been successfully applied in the Sichuan, Tarim and Songliao basins to identify the natural gas genesis and source for complicated gas reservoirs. The research results have provided effective supports for the natural gas exploration in the Sinian–Cambrian ancient carbonate formations in the Sichuan Basin, deep formations in the Kuqa depression of the Tarim Basin, and deep volcanic formations in the Songliao Basin.

Geochemistry Characteristics of Hydrogen Sulphide-Bearing Gas Pools in Sichuan Basin:

Sichuan Basin is the oil and gas bearing basin that is discovered in China, with the most hydrogen sulfide gas pool and the greatest reserve. In this article, gas pools with high hydrogen sulfide in Sichuan Basin were taken as the object of study; the geochemistry characteristic of natural gas was systematically studied, and the origin of natural gas with hydrogen sulfide in Sichuan Basin was discussed. The hydrogen sulfide content in the hydrogen sulfide gas pools of Sichuan Basin is 0.6%–14.5%; the sulfur isotope values of hydrogen sulfide gas have a close relationship with the sulfur isotope values of anhydrite in the same reservoir, and the sulfur isotope fractionation between the anhydrite and the H2S is 4‰–14‰, showing that the H2S in natural gas and the anhydrite in reservoir have the same sulfur source, the H2S in natural gas is the product of TSR reaction. The relationship among the hydrogen sulfide content in the gas pool, the hydrogen isotope of methane and the sulphur isotope of hydrogen sulfide is not obvious; but the high hydrogen sulfide content corresponds to the heavier ethane carbon isotope composition, and the carbon isotope of carbon dioxide is lower than −10‰, which belongs to the organic origin and has further verified the fact of TSR occurrence in the reservoir. There is the phenomenon of carbon isotopic reversal of methane and ethane in some areas of the Huanglongchang gas field, Wubaiti gas field and Puguang gas field in the Northeast Sichuan as well as Weiyuan gas field in South Sichuan, which is mainly caused by the later period of cracking-based gas-production process of the crude oil. TSR has less influence on these gas pools.