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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Vol. 27 (1977), Pages 45-60

The JFS Field, Dimmit County, Texas: Some Unique Aspects of Edwards--McKnight Diagenesis

Alonzo D. Jackal (1), Jack C. Stevenson (2)


The JFS Field represents 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 succession of dolostone and limestone intervals which interdigitates with dark basinal limestones and is enclosed by two thick anhydrite-bearing intervals.

In the JFS Field the stratigraphic sequence between anhydrite intervals is divided into three distinct zones which are easily recognized from the density log and which are designated as "A", "B", and "C" in descending order. The "A" zone, which lies immediately 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 is also characterized by considerable hole enlargement and false density log porosity readings. Seemingly, nearly all production in JFS Field is from the "A" zone.

Dry, high H2S gas is produced from JFS Field. The H2S content averages 2.5 mole %, or 2000 grains 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.

Primary porosity in grainstone facies has nearly all been occluded by calcite, dolomite and dickite (a rare clay mineral) cements and by a solid hydrocarbon. Almost all secondary intercrystalline porosity, that had formed in many dolostones, has been occluded by solid hydrocarbon, calcite and silica. Secondary leached porosity and fracture porosity were extensively formed in many limestone intervals during freshwater diagenesis, and became almost completely occluded by calcite, dolomite, dickite, and solid hydrocarbon.

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

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 and after dolomitization, was dissolved by fresh groundwaters after secondary intercrystalline voids had been filled by solid hydrocarbon.

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

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