Study of the Hydrodynamic Pattern in a Sedimentary Basin Subject to Subsidence

The structure of a sedimentary basin is modelized, assuming that this basin is subject to subsidence and has the following characteristics:shaped like an inverted cone with a circular base having a radius of 300 km,depth in the middle varying from 0 to 3,000 m during an evolution lasting 150 million years andduring this evolution, successive deposits of sand and clay strata occurred with gradual expulsion of water from the clay as the result of burial. This basin structure is used to study the hydrodynamic conditions that have occurred in the different sedimentary strata. The main conclusions of this study are as follows.Fluid circulations through the clay are quite complex because the direction of flow has usually reversed itself, with this reversal occurring earlier or later following the moment of deposition of clayey material. This leads to an evaluation vs time of the cumulative amounts of water, respectively, running upward (W2) and downward (W2) over a 1 sq cm horizontal surface associated with the clay deposited in a given place at a given moment.The circulation of water through clay causes fluid overpressures compared with the hydrostatic pressure. The vertical distribution and the order of magnitude of these overpressures agree with standard observations made on actual sedimentary basins.The circulation of water through sandy horizons occurs at very low speeds of between about 1 and 10 mm/year, but the volumes of water involved are large (from 10(12) to 10(13) cu m for a period of between 40 and 150 million years). These results may provide useful indications for studying some of the phenomena governing the formation of petroleum accumulations, i.e., diagnesis and hydrocarbon migration. Introduction Hydrocarbon displacement phenomena inside stratigraphic series (primary migration in the source rock and secondary migration in permeable layers where traps are located) cause permeable layers where traps are located) cause the sizable accumulations making up exploitable oil fields. The leading mechanisms assumed to be behind these displacements are displacement of hydrocarbons in a dissolved or emulsion state in water and displacement of continuous oil and gas phases (or possibly oil dissolved in gas) by gravity or aquifer movement.