Generation and development of the overpressure system in the eastern Delaware Basin, west Texas and southern New Mexico

Ming Luo, University of Texas at El Paso


An overpressure system occurs in the eastern Delaware basin, covering at least six Texas and New Mexico counties. Local underpressuring is found also in deep producing zones underlying the overpressure system. Earthquakes in the War-Wink field mainly occur in the overpressure system and the local underpressured production zone. These small magnitude earthquakes appear to be associated with a nonuniform stress field due to oilfield operation, dramatic change of pressure gradients, and minor activity on deep faults.^ Overpressuring in the Delaware basin is mainly associated with Mississippian, Pennsylvanian, and Wolfcampian aged shale sequences. Temperatures at the top and bottom of the overpressure system are about 70$\sp\circ$C and 120$\sp\circ$C, respectively. This temperature range is coincident with the average clay dehydration zone. A Time-Temperature Index (TTI) study indicates that the "oil window" in the War-Wink field is coincident with the overpressure system, where vitrinite reflectance (R$\sb{\rm o})$ ranges from 1.0 to 1.8%. This implies that the hydrocarbon generation and migration are at active stages in the overpressure system. No significant structural feature appears to influence the overpressuring in the basin.^ A two-stage overpressuring 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 of temperature due to decrease of thermal conductivity and thermal convection within the preexisting overpressure system would reinforce further overpressuring due to the fluid thermal expansion.^ Quantitative modeling of the overpressuring in the War-Wink field has been conducted using a one-dimensional diffusion equation of porous flow. This overpressure modeling integrates vertical principal stress, geothermal temperature, diagenesis, clay dehydration, and hydrocarbon generation. Results of fluid pressure simulation suggest that overpressuring in the War-Wink field did not significantly occur until the end of Permian time. Overpressure contributions from different causes can be simulated in the overpressure modeling. Permeability simulation indicates that the boundary of the overpressure system is controlled by a sharp decrease of permeability to an extremely low level. On the other hand, permeability within the overpressure system is abnormally high possibly due to an occurrence of fractures. The simulated geothermal temperature and conductivity appear to support the idea that a decrease of thermal conductivity may reinforce overpressuring. (Abstract shortened by UMI.) ^

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Recommended Citation

Luo, Ming, "Generation and development of the overpressure system in the eastern Delaware Basin, west Texas and southern New Mexico" (1992). ETD Collection for University of Texas, El Paso. AAI9303368.