ABSTRACT

A computerized simulation model was constructed to synthesize the processes of petroleum generation, migration, and accumulation under relatively simple conditions. The model successfully simulated accumulation in an existing anticlinal gas field, and was used to estimate the possibility of fault and stratigraphic entrapment nearby.

The geologic cross section of the area is divided into a series of vertical columns, which are sectioned into rectangular cells representing successive intervals of time and the strata deposited therein. Four geologic processes are sequentially performed on each cell or on each pair of adjacent cells: (1) deposition, (2) compaction, (3) petroleum generation, and (4) petroleum migration. First, sediment is deposited in the cell, with its original thickness restored by removing the effects of compaction. Then for each time-stratigraphic unit, the system calculates the amount of compaction caused by increasing time and depth of burial, and also the amount of petroleum generated, which is assumed to be a function of temperature. Primary migration is assumed when the petroleum saturation of the shale source beds exceeds the residual amount normally in thermally mature shale. Secondary migration is assumed to result from buoyancy alone; any petroleum which exceeds the hydrostatic trapping capacity of the shale seal migrates into a cell located along some upward path or escapes to the surface.

The model was applied to the anticlinal East Niigata field, Japan, using carefully selected input parameters. Results made it possible to estimate the migration paths and the timing of entrapment in each producing zone under the assumed conditions. The model may also be applied to exploration problems. It was used to estimate the possibility of petroleum entrapment in homoclinal strata near the East Niigata field under several assumed geologic situations. Results of this experiment show that the simulation method is potentially very useful for estimating the possibility and places of entrapment, especially for stratigraphic traps.

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