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A geological simulation model of generation and expulsion of hydrocarbon can be a useful tool in hydrocarbon exploration. The advantage of the model is to realize different environments in which hydrocarbon accumulations form under various geologic conditions.
A geologic cross section of an area is divided into a series of vertical columns, which are sectioned into rectangular cells. The model simulates the various geologic processes during basin development: (1) burial compaction of sediments, (2) history of temperature estimated from thermal conductivity and heat flow, (3) Ro (vitrinite reflectance) value calculated by Lopatin's method, and (4) the amount of generated hydrocarbon as a function of generation potential and of transformation ratio represented by Ro. Increase of pore pressure is assumed to be caused by (1) increase of overburden, (2) increase of volume of free water resulted by clay dehydration, (3) aquathermal expansion of water, and (4) expansion of fluid phase by hydrocarbon generation. Residual pore ressure in each step of geologic time in the model is calculated by Rubey-Hubbert's equation:
PA = kT + (Po - kT) . e-tT/
where PA = residual abnormal pressure, Po = initial abnormal pressure, k = ratio of pressure increase, t = duration, and T = relaxation constant. T is a function of permeability that is derived from porosity and grain size. The amount of hydrocarbon expelled is calculated from residual abnormal pore pressure as a function of relative permeability and viscosity of fluids. Direction and time of hydrocarbon migration can be interpreted from spatial distributions of paleo-pore pressure and of hydrocarbon expelled from source rocks for each geologic time.
The model is applied to the Niigata sedimentary basin of the coastal region of the Sea of Japan. Regional differentiation of time of hydrocarbon accumulation in the basin is observed. Upward hydrocarbon migration across the strata is also implied at culminations of the trapping structures.
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