Geochemical Characteristics of Ordovician Subsalt Gas Reservoir and Their Significance for Reunderstanding the Gas Source of Jingbian Gasfield, East Ordos Basin
Received date: 2008-12-25
Revised date: 2009-08-03
Online published: 2009-02-10
Low yield gas was discovered in the subsalt Mawu 7 reservoir at Longtan 1 well in the east of Ordos basin. As revealed by the test data, the gas contains 96.871% of methane, only 2.396% of C2+ heavy hydrocarbon characteristic of dry gas of high maturity, and very low contents of non-hydrocarbon components, indicating that the gas is a normal type in a typical reservoir. The carbon isotope of methane is light type, with δ13C1 being -39.26‰, indicating that the gas is of primitive oilfield type at overmature stage in the Ordovician. The hydrocarbon specific carbon isotope of C2+ heavy hydrocarbon components is obviously heavy type, withδ13C1varying between -19.27‰ and -23.78‰, suggesting that the gas is similar to coal-generated gas of high maturity. Based on the spatial distribution of the gas reservoir, coal-generated gas in upper Paleozoic is less likely to be involved in the formation of the salt gas, as a result, secondary reactions like TSR reaction could be the dominating affecting element for formation of the salt gas. When Mawu 7 gas from Longtan 1 well is taken as an end member of oilfield type gas and the carbon isotope of methane as a major index, the major source for the Ordovician gas in the Jingbian gasfield can be identified, and a conclusion can be drawn that the major source for natural gas in the Ordovician weathering crust in Jingbian gasfield is the Upper Paleozoic coal measure.
YANG Hua, ZHANG Wen-Zheng, ZAN Chuan-Li, MA Jun . Geochemical Characteristics of Ordovician Subsalt Gas Reservoir and Their Significance for Reunderstanding the Gas Source of Jingbian Gasfield, East Ordos Basin[J]. Natural Gas Geoscience, 2009 , 20(1) : 8 -14 . DOI: 10.11764/j.issn.1672-1926.2009.01.8
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