Subsidence in the Louisiana Coastal Zone Due to Hydrocarbon Production

Coastal wetland loss in southern Louisiana poses a great threat to the region's ecological and economic stability. Wetland loss in the Louisiana Coastal Zone is caused by the interactions of multiple natural and human induced mechanisms, and it has been suggested that compaction of sands due to subsurface oil and gas production may be a large contributing factor. We have modeled the effect of oil and gas production in Lafourche Parish, Louisiana on surface subsidence using geologic and pressure data and a first-order leveling line along highway Louisiana 1 to constrain our models. The leveling data in our study area covers two time epochs, 1965-1982 and 1982-1993 which show an increasing rate of subsidence while production rates decreased. This indicates the potential for a time-dependent mechanism for production-induced subsidence in the Louisiana Coastal Zone. Using MODFLOW-2000 we model the role that compaction of both the reservoir sands and the confining shales has on the regional subsidence signal. We find that by including the compaction of shales we are able to match the observed subsidence in both the 1965-1982 and 1982-1993 time epochs. By extending the model to 2050 and assuming production has ended we find that subsidence over the oil and gas fields will continue as the shales compact in response to previous pressure decreases in the reservoir sands. However, the subsidence signal remains localized over the producing fields indicating the production-induced compaction of the reservoir sands and reservoir bounding shales can not explain the entire observed regional subsidence and land loss in the Louisiana Coastal Zone. This work indicates that production-induced subsidence will remain an important mechanism for land loss on the Louisiana coastal zone for at least the next 50 years and must be considered in regional restoration and protection plans.