Distribution and Generation of the Overpressure System, Eastern Delaware Basin, Western Texas and Southern New Mexico

Abstract

Three subsurface pressure systems have been identified in the Delaware basin: an upper normal pressure system, a middle overpressure system, and a lower normal pressure system. The overpressure system occurs in the eastern Delaware basin, covering six Texas and New Mexico counties. The depth of the overpressure system ranges from 3100 to 5400 m. The normal fluid pressure gradient is 0.0103 MPa/m in the eastern Delaware basin. The highest overpressure gradient, however, approaches 0.02 MPa/m, which is close to the lithostatic gradient of 0.0231 MPa/m. The overpressure system has a relatively flat top and a downwarped bottom. The maximum thickness of the overpressure system reaches about 2300 m at the depocenter and pinches out toward the edges. An area of excess pressure o curs within the system where the highest excess pressure reaches 60 MPa. Local underpressured areas due to production are found in the lower normal pressure system in the War-Wink field area. Overpressure in the eastern Delaware basin is mainly associated with Mississippian, Pennsylvanian, and Permian (Wolfcampian) shale sequences, which also are major source rocks in the basin. Initial sedimentation rates within the overpressure system range from 17 to 90 m/m.y. Corrected bottom-hole temperature measurements indicate that the geothermal gradient within the overpressure zone is 25.1°C/km, which is higher than the basin's average geothermal gradient of 21°C/km. Temperatures at the top and bottom of the overpressure system are about 80 and 115°C, respectively. This temperature range approximates the temperature of the average clay dehydration zone. The oil window in the War-Wink field is coincident with the overpressure system, which implies that hydrocarbon g neration and migration are active in the overpressure system. A two-stage overpressure model is proposed. Hydrocarbon maturation combined with mechanical compaction disequilibrium and clay dehydration are the initial causes for overpressure generation due to an abnormal increase of fluid volume and pore space. Subsequently, the increase in temperature due to a decrease of thermal conductivity and fluid migration within the preexisting overpressure system would reinforce further overpressuring due to the fluid thermal expansion.