LAND SUBSIDENCE ALONG THE NORTHEASTERN TEXAS GULF-COAST - EFFECTS OF DEEP HYDROCARBON PRODUCTION

The Texas Gulf of Mexico coast is experiencing high (5-11 mm/yr) rates of relative sea level (RSL) rise that are the sum of subsidence and e ustatic sea level (ESL) rise. Even higher rates are associated with ar eas of groundwater pumping from confined aquifers. We investigate the possibility of deep petroleum production as a cause for the high regio nal rates of subsidence. The northeast Texas coast was chosen for the study because it has a high rate of RSL rise, very limited groundwater production, and a long history of petroleum production. We examine in detail the Big Hill and Fannett fields, for which adequate bottom hol e pressure (BHP) and well log data are available. The hypothesis of de ep petroleum production is tested in three ways. First, industry BHP t ests show many of the fields are depressurized to far below hydrostati c pressures. Second, analysis of BHP data over time in the Big Hill an d Fannett fields indicates that some zones in these fields were below hydrostatic when production commenced. This indicates that depressuriz ation from production in neighboring fields or zones within the same f ield is not limited to the production zone. Third, three models for su bsidence (a general 1-D regional model, an intrareservoir model, and a reservoir bounding layer model), using reasonable hydrogeological par ameters, predict subsidence within the inferred range of data. The lat ter two models use data from the Big Hill and Fannett fields. Addition al verification of the hypothesis that deep petroleum production is ca using or accelerating regional subsidence will require the collection and analysis of data on the subsurface hydrogeological parameters and detailed measurements of the spatial and temporal distribution of subs idence along the Texas Coast.