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.
Highlights
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
Many methods and technologies can be used to determine the age of hydrocarbon accumulations, such as constraining the earliest time of hydrocarbon accumulation based on the age of a regional structural slope, matching the trap evolution history and hydrocarbon generation-expulsion history, determining the gas reservoir emplacement time by gas law, the petroleum reservoir saturation pressure method [1], the petroleum reservoir geochemistry method [2], the iodine isotope dating method for oil field brine [3], the water/oil/ gas interfaces retroaction method [4], the authigenic minerals dating method for a reservoir [5,6,7], paleomagnetic dating methods [8], the fluid inclusion homogenization temperature method [9,10], the fluid inclusion composition analysis method [11,12] and evaluating the hydrocarbon generation and migration time by bitumen reflectance [13]
There are disagreements regarding the ages of hydrocarbon accumulation, reconstruction and destruction in the northern margin of Jiangnan-Xuefeng uplift
Summary
The Jiangnan-Xuefeng uplift, which is located on the southeastern margin of the Yangtze block where it is clamped between the Yangtze and Cathaysian blocks, is a peculiar geotectonic unit [55] that extends from eastern Zhejiang in the east to northern Guangxi in the west. A giant hydrocarbon accumulation belt about 1400 km long and 40–80 km wide developed from west to east crossing Guizhou, Hunan, Hubei, Anhui, Jiangsu and Zhejiang provinces along the northern margin of the Jiangnan-Xuefeng uplift (the southern margin of the Yangtze block). The original oil in place of the entire hydrocarbon belt in Neoproterozoic, Mesozoic and Paleozoic marine strata, based on the field-scale sequence method, could be close to 350 million tons This amazingly huge resource of marine hydrocarbon is what remains from a history of extensive exposure and destruction of marine oil and gas reservoirs over a long period of geologic time. The source rock of bitumen lies in the Lower Cambrian and/or Lower Silurian [53,54]
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