This accelerates the natural subsidence in the marshes from the consolidation of the previously deposited sediments. Any additional subsidence that may be caused from the extraction of subsurface fluids such as oil and gas is masked by this natural subsidence until someone devises a method to delineate the subsidence caused be each process. Sharp and Hill 5 attempted to separate the subsidence contribution from OGE through a comparative study of areas along the Texas coast where different subsidence mechanisms are predominant. They investigated the Big Hill and Fannett fields of the Frio Formation in the northeastern portion of the Texas Gulf coastal plain. They stated that these reservoirs are similar in depth, amount of reservoir depressurization, and geology to many others in the Texas and Louisiana Gulf coastal region.
More study is required to make a definitive determination.
Surface subsidence results from the combined effects of the subsurface reservoir compressing or compacting a sufficient amount that it causes a displacement in geological formations above that are not strong enough to withstand the underlying structural force changes. If the changes in reservoir conditions are small enough or the upper geological structures are strong enough, surface displacement will not occur or will be only negligible. Reservoir compaction is a function of the extent of pressure reduction, the thickness of the producing zone, and the compressibility of the formation structure. Most reservoirs in which OGE has caused significant compaction consist of a sequence of unconsolidated sands and shales. Studies have shown that the amount of surface subsidence caused by reservoir compaction decreases with increasing depth of the reservoir from the surface and increases with the effective diameter of the reservoir and thickness of the reservoir.