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Pore water in sedimentary rocks is normally in motion. In general, gravity-induced flow driven by the elevation gradient predominates in a basin with orogenic deformation; however, in a basin with continuous deposition, compaction-induced flow driven by the excess fluid-pressure gradient predominates. Subsurface water flow is considered to have a controlling influence on the migration of widely dispersed petroleum. Therefore, the analysis of a basin-wide flow system, particularly its paleohydrogeologic conditions, is essential for understanding the history of petroleum migration and entrapment.
The nonlinear finite element method has been used to simulate coupled processes of sediment deformation and fluid flow in sedimentary
sequences. By activating and deactivating elements at various stages in the computation process, the sequential deposition and erosion during evolution of a sedimentary basin can be modeled. Simulated results indicate that excess fluid pressure occurs when a basin is progressively loaded by overlying sediments. An excess pressure gradient will cause pore fluid to flow vertically and horizontally, depending upon the regional stratigraphy and structure, toward the sediment surface. In sandstone-shale sequences, pore fluid in shales tends to flow toward adjacent sandstones, increasing the effectiveness of petroleum accumulation. The downward flow from overlying shales to sandstones, plays an important role in providing resistence to the upward migration of petroleum. The concentrated flu d flux in sandstones tends to flow parallel to the bedding plane toward highest positions of permeable strata, such as crests of anticlines, pinch-outs, or outcrops. Although the orogenic deformation further compresses sediments initially, the subsequent erosion rapidly reduces excess pressure and causes the invasion of meteoric water.
This study suggests that numerical modeling is an effective technique in evaluating histories of subsurface flow, sediment compaction, and petroleum accumulation.
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