Genetic Types of Natural Gases and Conditions for Formation of Giant Gas Reservoirs in the Daniudi Gas Field of Ordos Basin, China
At the end of 2011, the Daniudi gas field had discovered gas reserves of 4168.28×108m3 mainly in Upper Carboniferous and Lower Permian. Based on source rocks, reservoir rocks, fluid passage system and hydrocarbon generation and charging histories as well as chemical compositions of natural gas, carbon and hydrogen isotopes of alkane gases and 3He/4He ratios, we discussed the genetic types, sources and reservoir formation mechanism of the natural gases in the Taiyuan, Shanxi and Xia Shihezi formations of the Daniudi gas field. The natural gases for 25 industrial gas well samples are principally composed of hydrocarbon gases with dry coefficient of 0.884-0.978. The carbon isotope composition of ethane and propane are heavier than −28‰ and −25‰, respectively. The hydrogen isotope composition of the methane is lighter than −180‰. The carbon and hydrogen isotope compositions suggest that the natural gases are typical coal-type gas, mainly generated from the Carboniferous-Permian terrestrial and terrestrial-marine humic organic matters. The positive correlation between the hydrogen isotope compositions of the methane and hydrogen indicates that they are controlled by thermal maturity. If the ethane generated from both coal and mudstone with similar kerogen type and thermal maturity was mixed in the reservoirs, the carbon isotope compositions of ethane cannot be used to indicate thermal maturity. Although the fractionation effect of hydrogen isotope of ethane is greater than that of the carbon isotope, the relationship between hydrogen isotope composition and thermal maturity is still not obviously recognized. Based on K abundance in source rocks and 40Ar/36Ar ratios in natural gases, the contribution of gases from Carboniferous Taiyuan Formation coal is about 60%; and the ratio for the Permian Taiyuan and Xia Shihezi formation Coal is about 48% and 51%, respectively. The models of gas filling for the Daniudi gasfield can be divided into two types, i.e. inner-source and near-source behaviours. The former suggests the formation of gas reservoir inner the source rock body, and the latter near the source rock. The formation process of gas reservoirs is episodic migrating and charging from source rocks to reservoirs. The formation process of the gas reservoirs includes the inner-source with episodic charging in the latter Triassic-early Jurassic, the inner-source and near-source in the latter Jurassic – early Cretaceous, and near-source from the latter Cretaceous.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014