Abstract: JFS Field, Dimmit County, Texas--Some Unique Aspects of Edwards-McKnight Diagensis

Alonzo D. Jacka, Jack E. Stevenson

The JFS field is the first discovery of commercial gas production from the Edwards Formation in Dimmit County, Texas. The field is located within a transition zone between the typical Edwards of the Comanchean shelf and the equivalent McKnight of the Maverick basin. The transition-facies belt consists of a pod of dolostone and limestone intervals which interdigitates with dark basinal limestones. The pod is enclosed by two thick anhydrite-bearing intervals.

In the JFS field, the stratigraphic pod between anhydrite intervals is divided into three distinct zones which are easily recognized from the density log and are designated as "A," "B," and "C," in descending order. The "A" zone, which lies directly below the upper anhydrite, has several vugular zones which are clearly defined by the density log, and the hole usually remains in gauge. The "B" zone typically washes out badly and the density log gives false high-porosity readings. The "C" zone consists of thin-bedded limestones intercalated with shales, and this interval also is characterized by considerable hole enlargement and false density-porosity readings. Seemingly, nearly all production in JFS field is from the "A" zone.

Dry, high hydrogen sulfide gas is produced from the JFS field. The hydrogen sulfide content averages 2.5 mole % or 2,000 grains of sulfur, and a sulfur-stripping plant is a necessity.

The Edwards-McKnight interval exhibits well-developed depositional and diagenetic cycles consisting predominantly of intercalated limestones and dolostones. Lithofacies include wackestones, packstones, grainstones, and algal boundstones. Depositional and diagenetic cycles reflect interaction of (1) seaward progradations, (2) eustatic changes in sea level and associated climatic fluctuations, and (3) subsidence.

Nearly all primary porosity in the grainstone facies has been occluded by calcite, dolomite, and dickite (a rare clay mineral) cements and by gilsonite. Almost all secondary intercrystalline porosity, that had formed in many dolostones, has been occluded by gilsonite, calcite, and silica. Extensive secondary leached porosity and fracture porosity formed in many limestone intervals during freshwater diagenesis, and became almost completely occluded by calcite, dolomite, dickite, and gilsonite.

In the "B" zone, secondary porosity was developed extraordinarily well by leaching and fracturing, and it became filled predominantly by soft powdery dickite and gilsonite; consequently, drilling causes washouts.

In the main pay zone ("A" zone) gas production is from tertiary porosity in dolostone. Tertiary voids were formed when anhydrite, which was emplaced during dolomitization, was dissolved by fresh groundwaters after secondary intercrystalline voids had been filled by gilsonite.

Throughout the Edwards interval, except in tertiary voids, gilsonite is a major occluder of porosity and is recorded as false porosity on the density log.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas