Abstract

Midland Field is a major gas field developed on a faulted structure in the Anahuac-Frio producing trend of south Louisiana. Most of the gas in the field is in abnormally pressured reservoirs. The fluid environments suggested by pressure data are critical to the entrapment of petroleum in Midland. All structural and stratigraphic traps clearly indicated to be in favorable fluid environments contain petroleum. All barren traps are indicated to be in fluid environments unfavorable for entrapment. The hydrodynamic flow indicated by pressure data has caused anomalous oil-water and gas-water contacts in many reservoirs.

The location and magnitude of the abnormal pressures in the field appear to be related to variations in the gross permeability of the sedimentary section. Temperature, depositional environment, and time of fault movement may be important in the regional occurrence of abnormal pressures, but they are less clearly associated with the local occurrence of abnormal pressures at Midland.

The composition of the formation fluids is tentatively explained by thermodynamic and migration processes. Variations in the composition of formation water are related to varying amounts of membrane filtration along the indicated migration path. Differences in petroleum composition appear to be the result of petroleum maturation and the secondary migration pattern indicated by pressures and the configuration of permeable flow paths. Although other factors may also have affected fluid composition, they are not supported by the data presently available.

The results of this study should be applicable to other areas. Specific applications to geophysical interpretation, prospect evaluation, drilling, development, and production are suggested.