Wetland losses and their progressive conversion to open water around producing oil and gas fields in the Gulf Coast region have been attributed to a variety of natural and anthropogenic processes. Three large, mature hydrocarbon fields in coastal southeast Texas were examined to evaluate competing hypotheses of wetland losses and to characterize subaerial and submerged surfaces near reactivated faults and zones of subsidence. Topographic and bathymetric profiles at the Port Neches, Clam Lake, and Caplen Fields and shallow cores at the Port Neches and Caplen Fields provide a basis for distinguishing between (1) extensive land-surface subsidence without significant subaqueous erosion, and (2) localized minor subsidence near faults accompanied by extensive subaqueous erosion. Subaqueous erosion results from submergence of wetlands, current and wave excavation of surface sediments and organic detritus, and exportation of the eroded sediments through adjacent water bodies with swift currents such as navigation channels.
Geologic settings and responses to induced subsidence and fault reactivation are different at each field site. Detailed stratigraphic correlations of sediment cores show that at Port Neches, subsidence of 35 to 90 cm and minor marsh erosion (20 to 35 cm) created more than 15 million m3 of accommodation space in a nearly circular pattern over the field. At Caplen the marsh surface subsided only about 4 cm, but the surface eroded 30 to 40 cm vertically, creating about 3.5 million m3 of accommodation space. The breakup of wetlands and their conversion to open water appears to be in an initial stage at the Clam Lake Field. At Clam Lake, minor subsidence along a fault is submerging the marsh plants that will weaken and eventually die either as a result of water logging or salt-water intrusion.
The different surficial responses and wetland losses at each field are also related to the primary type and rate of hydrocarbon production. The greatest land-surface subsidence occurred at the Port Neches Field where a large volume of gas was produced rapidly. At the Caplen Field, both oil and gas were produced in significant quantities and there was a period of accelerated gas production. The wetland loss, which coincided with the rapid production phase, was controlled by fault reactivation and subsequent marsh erosion. Oil was the primary hydrocarbon produced at the Clam Lake Field where a reactivated fault is responsible for the observed wetland loss. Results from this study show that although the absolute magnitude of induced subsidence may be less than 1 m, even a minor reduction in land elevation is sufficient to initiate marsh degradation that quickly results in major wetland losses.
U.S. Department of the Interior, U.S. Geological Survey
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