--> Hydrocarbon Generation and Accumulation Process of Marine Carbonate in Ordovician of Ordos Basin, China

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Hydrocarbon Generation and Accumulation Process of Marine Carbonate in Ordovician of Ordos Basin, China

Abstract

The objective of this paper is to describe the main controlling factors on gas accumulation including gas source analysis, accumulating period and process in Jinxi area of Ordos basin, China. Knowledge of gas accumulation in this study area has been gathered by several methods: by the analysis of natural gas isotope and probing fluid inclusion compositions to identify gas source, by fluid inclusion homogenization temperature and geological history analysis to determine the hydrocarbon charging periods, by basin modeling to explain the gas accumulation process. About 80 core samples were collected and a large amount of analysis experiments were conducted. A variety of data show that the Ordovician gas reservoir exist the traces of petroleum migration. A large amount of fluid inclusions distribute in calcite veins. Most of them response light blue fluorescent under ultraviolet radiation, indicating light oil or gas inclusions. Laser Raman analysis results show that asphalt exists pores, fractures and fluid inclusions. These information means that asphalt distribute extensively and there was oil migration or accumulation in geological history period. A large amount of gas samples isotope analysis results show that most of the gas samples belong to the oil type gases. The carbon isotope value of methane from the overlying coal-bearing strata ranges from −34.84‰ to −31.85‰ at large while the carbon isotope value of methane from Ordovician itself ranges from −38.92‰ to −34.00‰ or even lower. Methane gases were extracted from fluid inclusions for stable carbon isotope analysis. The results show that most of the sample value are less than −35.1‰. So it's extremely likely that most of the gas is from the Ordovician formations. According to the inclusions homogenization temperature, there exist about two hydrocarbon charging stages for fluid inclusions. The earlier stage inclusions are mainly filled with thin oil, with homogenization temperature ranging from 80 to 100 celsius degree, corresponding the late period of Early Jurassic Epoch according to the burial history diagram. The later stage inclusions are mainly filled with natural gas, with homogenization temperature ranging from 130 to 170 celsius degree, corresponding the late period of Early Cretaceous Epoch. The gas in Jingxi gas field is mainly from the Ordovician itself. There are two main hydrocarbon fluid migration periods, one is mainly liquid hydrocarbon migration, another is the pyrolysis gas migration.