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40Ar/39Ar geochronology constraints on hydrocarbon accumulation and destruction periods in the Bankeng paleo-reservoir in the southern margin of the middle Yangtze block

DOI: 10.1007/s11434-011-4625-6

Keywords: hydrocarbon geochronology,fluid inclusion,40Ar/39Ar dating method,paleo-reservoir,middle Yangtze block,Jiangnan-Xuefeng uplift

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Abstract:

The exact determination of isotopic ages of hydrocarbon accumulation, reconstruction and destruction periods using traditional isotopic dating methods is complex because of the small numbers of minerals that correlate with hydrocarbons. The quantitative and direct study of hydrocarbon geochronology is therefore an important scientific problem for isotope geochronology and petroleum geology. This study obtains two isotopic ages from a quartz vein sample associated with bitumen in a reverse fault located in the Bankeng paleo-reservoir on the southern margin of the middle Yangtze block (the northern margin of the Jiangnan-Xuefeng uplift) by inclusion 40Ar/39Ar stepwise crushing in a vacuum. Two different and good linear inverse isochrons that correspond to two age plateaus were determined. The two ages correspond to primary inclusions of about 228 Ma and secondary inclusions of about 149 Ma. These inclusion groups represent two distinct kinds of fluids. Quartz veins associated with bitumen in faulted paleo-reservoirs, which have a strict response relationship with tectonization and hydrocarbon accumulation, are the unified products of tectonic processes, hydrocarbon accumulation and reconstruction. Therefore, they can be used to constrain the hydrocarbon accumulation, reconstruction and destruction periods that are controlled by multiphase and complicated tectonic actions. The evolutionary processes of hydrocarbon accumulation can be divided into two periods consisting of a primary oil and gas reservoir formation period in the late Indosinian epoch (about 228 Ma) and a period of oil and gas reservoir reconstruction in the early Yanshan epoch (about 149 Ma). This study quantitatively reconstructs the hydrocarbon accumulation and destruction chronological framework of a giant hydrocarbon accumulation belt along the southern margin of the middle Yangtze block (the northern margin of the Jiangnan-Xuefeng uplift) controlled by multiphase and complicated tectonism. The two ages associated with hydrocarbons here correspond to the special controlling actions of continental tectonics in the Jiangnan-Xuefeng uplift that affected the timeline of reconstruction and destruction in this giant marine hydrocarbon accumulation. This study shows the feasibility and usefulness of dating inclusions with the 40Ar/39Ar technique for hydrocarbon geochronology, especially in the marine hydrocarbon accumulation region of southern China within a geological setting of old strata, high thermal evolution hydrocarbons, and complex, multiphase and multicycle tectonization.

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