Coastal & Marine Geology Program >Subsidence and Fault Activation . . . > Open File Reoprt 01-274

Shallow Stratigraphic Evidence of Subsidence and Faulting Induced by Hydrocarbon Production in Coastal Southeast Texas

USGS Open File Report 01-274

Robert A. Morton, Noreen A. Purcell, and Russell L. Peterson

Contents: Summary Introduction Geology & Production Histories Sediment Surface Profiles Sediment Cores Results Coastal Environmental Implications References Appendix A Appendix B Project Contact: Bob Morton

Discussion of Results Subsidence versus Marsh Erosion At Port Neches, the uppermost stratigraphic unit of each core contains at least 0.32 m of organic-rich sediments. These organic-rich sediments are the same unconsolidated and rooted muds that comprise the present marsh facies recovered in BH-00-01. Considering the thickness of organic-rich sediments in the cores, less than 20 cm of marsh sediment has been eroded from the surface over the Port Neches Field. The evidence strongly indicates that the open water created as a result of wetland loss was caused largely by land-surface subsidence. Subsidence estimates on the west side of Port Neches are based on stratigraphic comparisons with BH-00-01. If the land surface at this location has experienced historical subsidence, but the marsh surface subsequently aggraded, then the estimated magnitudes of subsidence would need to be increased the same amount as the subsidence at BH-00-01. The maximum induced subsidence observed at the Port Neches Field (0.9 m at BH-00-03) is the same magnitude of subsidence reported by Ratzlaff (1982) for the nearby Port Acres Field on the basis of releveling surveys. Slightly greater water depths at Caplen Field along the marsh banks that coincide with the fault traces suggest that scour by currents is enhanced by the boundary conditions. Geologic Setting Each of the three fields is located in a different geologic setting. The Port Neches Field is within the floodplain of an alluvial valley, the Clam Lake Field is within the coastal-plain marsh between major rivers (interfluvial setting) and the Caplen Field is located in a barrier-island setting. However, the different geologic settings do not appear to have any significant influence on the subsidence and fault reactivation responses of the wetlands. Perhaps this is because the Holocene sediments at each site are relatively thin (< 6 m, Morton et al., 1999) and the underlying Pleistocene strata are over-consolidated for their depths because they were subaerially exposed when sea level was lower about 20 ka. Production Characteristics Each of the fields experiencing subsidence and fault reactivation has produced for more than 60 years from more than 50 wells that together yielded more than 20 million bbls of oil and 10 Bcf of gas (Table 1). Of the three fields investigated, the greatest subsidence occurred where gas was the primary produced fluid and the rates of production from a relatively small area were exceptionally high, albeit for a brief period. The accelerated gas production causes rapid declines in pore pressure because the withdrawal of fluid greatly exceeds the transmissivity of the formation and its ability to recharge through inflow of water from the aquifer(s) contiguous with or associated with the reservoirs. Preliminary pressure-depth plots for the Port Neches and Caplen Fields also demonstrate that hydrocarbon production eventually reduces pore pressures to the extent that the increased effective stresses around the reservoirs cause reservoir compaction, subsidence, and fault reactivation. « Sediment Cores | Environmental Implications»

Coastal & Marine Geology Program >Subsidence and Fault Activation . . . > Open File Report 01-274