| Description |
Subsidence caused by hydrocarbon production at shallow depths has been documented in most major producing basins of the world. Despite widespread recognition of this phenomenon, the potential for significant subsidence as a result of moderate to deep production has generally been disregarded. Previous studies in the Gulf Coast Basin demonstrate that reductions in land elevation commonly occur several kilometers away from producing wells rather than directly above the producing reservoirs. The locus of subsidence is controlled by the coupling between reservoir compaction and slip along the faults. The induced subsidence and land losses are concentrated along growth faults that become active when sufficiently large volumes of fluid (oil, gas, formation water) are removed from the subsurface. Fluid extraction can cause a decline in pore pressure within the rocks and alter the state of stress near the faults. Thus, both the pattern of hydrocarbon production and the three-dimensional geometry of faults need to be considered in predicting the location and magnitude of subsidence. The proposed study addresses a broad spectrum of questions regarding the mechanisms of recurrent fault movement, subsidence, and attendant changes in land characteristics. Specific objectives include (1) identifying areas where subsidence and fault activation may be exacerbated as a result of hydrocarbon production, (2) determining magnitudes and rates of elevation changes induced by fluid withdrawal, (3) identifying the geological factors that enhance or retard subsidence, and (4) understanding the physical processes associated with recent movement of deep-seated faults. A multidisciplinary approach involving subsurface geology, reservoir engineering, rock mechanics, geophysics, coastal and marine geology, and remote sensing will address some of the fundamental questions regarding impacts of regional depressurization, fault activation, and subsidence. An inventory and analysis of fields where subsidence and fault activation have and have not occurred would help evaluate the historical environmental impacts of oil and gas production in Louisiana and Texas and provide a basis for determining the likely cumulative impacts. This evaluation would be accomplished by investigating the areal extent and rates of subsidence around selected large fields, correlating rates of subsidence and land loss with reported reservoir parameters (cumulative production, fluid composition, pressure histories, reservoir properties), examining the influence of geologic framework and structural style on subsidence, investigating the concept of regional depressurization, exploring rock deformation theories for explanations of subsidence and recurrent fault movement, and field monitoring of subsidence using sedimentation-erosion tables (SET) and geodetic GPS receivers. Investigation of selected fields would provide magnitudes and rates of subsidence, activated fault parameters (lengths of fault traces, magnitudes and rates of offset), and geological characterization (sand/shale ratios, sediment strength) of large fault-block compartments. The field and reservoir parameters and geologic framework will also be investigated for selected large fields where subsidence and fault activation have not been observed. Geophysical techniques will be evaluated as tools for regional detection and monitoring of subsidence. The project will complement other USGS studies. The geologic framework task will be coordinated with the Gulf Coast Framework project, and the localized induced subsidence around fields will be integrated with the regional natural subsidence trends of the Louisiana Coastal Subsidence Project. |
