Two phases are recognized in the maturation of organic matter and evolution of the oil-generative window (OGW): (a) a basinal subsidence phase during which potential source rocks undergo burial through a static initial OGW, and maturation is due essentially to increasing temperature with depth, and (b) a postsubsidence phase during which the OGW moves vertically upward though the static sedimentary fill, and maturation is due to time effects. Only the final shallow position of OGW can be defined by geochemical indices of maturity. The hydrocarbon generation sequence progresses from bottom to top, beginning with the lighter hydrocarbons (or even thermogenic gas) at the initial OGW at depth, followed by progressively heavier hydrocarbons as the OGW moves vertically upward. This dynamic model permits modeling of past positions of OGW relative to basin evolution through interpretation of oil-genesis nomographs. It also provides genetic explanation for the increase in API gravity with depth, areal variation of maturation facies, variation in temperature at present tops of OGW with age of source rocks, and gas/oil volume ratios of sedimentary basins.--Modified journal abstract.

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