Coastal Environmental Implications
The different surficial wetland responses and accommodation spaces created by prolonged large-volume hydrocarbon production at each field investigated is partly related to the primary fluid type produced and the rates of production. For example, the greatest wetland loss and creation of accommodation space occurred at the Port Neches Field where the largest measured vertical subsidence was associated with the largest cumulative volume of gas produced. The second greatest wetland loss and creation of accommodation space occurred by minor subsidence and major erosion at the Caplen Field. Significant volumes of both oil and gas were produced at Caplen and the wetland loss was controlled primarily by fault reactivation and subsequent wetland erosion. Wetland losses and creation of accommodation space were the least at the Clam Lake Field, which primarily produced oil with some gas. In fact, the cumulative volume of gas produced at Clam Lake was the least of the three fields studied (Table 1). The wetland losses occurring at Clam Lake are also fault controlled as at Caplen. At both fields it appears that the vertical displacement (subsidence) is less than the breakup of wetlands and conversion to open water by erosion and exportation of organic-rich sediments. Wetland losses are localized around the hydrocarbon fields examined in southeast Texas because the fields are widely spaced relative to the nearest large field. In parts of coastal Louisiana, large fields are relatively close to one another, and there is greater potential for subsidence, faulting, and wetland losses between the fields induced by cumulative production and regional depressurization.
There has been substantial speculation regarding the significance of subsidence in the Gulf Coast region induced by prolonged extraction of hydrocarbons. One-dimensional modeling of reservoir compaction around oil and gas fields in Louisiana suggested that potential land subsidence would be insignificant (Martin and Serdengecti, 1984; Suhayda, 1987). Without debating the accuracy of those conclusions, it is clear that the modeling results focused on the vertical displacement that might be expected and not the impact of that vertical displacement. Subsidence of a meter or less induced by oil and gas production is not great, especially when compared to the extreme examples of subsidence such as 9 m at Wilmington, California (Yerkes and Castle, 1976) or 4.5 m at the Ekofisk Field in the North Sea (Mes, 1990). Nevertheless, the relatively small observed reductions in elevation in the Gulf Coast region are sufficient to cause dramatic impacts on the affected wetland ecosystems. This study has demonstrated that even minor lowering of the marsh surface can translate to large wetland losses through a combination of coastal-plain submergence and marsh-sediment erosion.
U.S. Department of the Interior, U.S. Geological Survey
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