Natural gas accumulation and models in Ordovician carbonates, Ordos Basin, NW China

According to sedimentary environment, sources, hydrocarbon accumulation characteristics and gas reservoir types of the Ordovician in the Ordos Basin, the Ordovician carbonate rocks in the basin have good gas accumulation conditions. A large number of geological and geochemical evidence shows a triple source supply. In addition to the Upper Carboniferous – Permian coal measures hydrocarbon source rocks as the main hydrocarbon source rocks, the Middle-Upper Ordovician marine hydrocarbon source rocks and Carboniferous Benxi marlstones have certain ability for hydrocarbon. Analysis of the known gas reservoir reveals that the gas in the gas fields of the central basin belongs to thermal cracking gas which is a mixed gas of coal-derived gas and crude oil cracking gas. Coal-derived gas comes from the coal measures hydrocarbon source rocks of the Carboniferous-Permian undoubtedly. Crude oil cracking gas mainly comes from the high temperature cracking gas in the Jurassic reservoir. Asphalts and hydrocarbon inclusions in the Ordovician reservoirs evidence that the oil of the Jurassic paleo-oil reservoir is mainly from the Middle-Upper Ordovician marine hydrocarbon source rocks. Gas accumulation has evolved from oil reservoir to gas reservoir: oil pool formed in the Jurassic and the oil cracked into gas in the Cretaceous. Controlled by tectonic sedimentary setting, four accumulation models of carbonate gas reservoir have been structured. Among them, weathering crust lithostratigraphic gas reservoir in the central part of the basin and composite lithology gas reservoir of structure – bedding karst type in the northwest platform marginal zone are the focus of the carbonate gas exploration.