--> ABSTRACT: Facies and Diagenesis of Goen Limestone Cyclothem (Early Late Desmoinesian), Concho and Runnels Counties, Central Texas, by Samuel A. Marquis, Jr.; #91030 (2010)

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Facies and Diagenesis of Goen Limestone Cyclothem (Early Late Desmoinesian), Concho and Runnels Counties, Central Texas

Samuel A. Marquis, Jr.

The Goen Limestone (early late Desmoinesian) of Concho and Runnels Counties, central Texas, consists of three vertically stacked transgressive-regressive sequences, each cycle beginning with a thin clay-rich transgressive unit which passes upward into thick regressive limestones. Five distinct facies have been identified in the Goen Limestone and are associated with three major depositional environments of the low-gradient Concho platform: (1) lower ramp (outer subtidal) environment, characterized by transgressive clayshale-clayey wackestone; (2) middle ramp (phylloid algal mound and mound-associated) environment, consisting of regressive algal wackestone-boundstone, foraminiferal wackestone-packstone, and calcareous sandstone; and (3) upper ramp (backmound) environment, epresented by regressive bryozoan-rugose coral wackestone. Glacio-eustatic sea level fluctuations controlled the observed cyclicity and facies distribution, while subsidence and deltaic sedimentation possibly added local, second-order modifications.

The major porosity-forming process in the Goen Limestone was early leaching as a result of meteoric-phreatic lenses which migrated primarily in response to glacio-eustatic sea level changes. The dominant porosity-reducing processes included early cementation by equant non-ferroan spar in a meteoric-phreatic environment and late stage saddle dolomitization in a deeper burial connate environment. Bladed high-Mg calcite, silica, equant ferroan spar, and anhydrite cements resulted in minor porosity occlusion.

The hydrocarbon-producing regressive units (foraminiferal and bryozoan-rugose coral facies) underwent the most complex diagenetic histories and contain extensive secondary pores (algal molds, vugs, micropores, and open stylolitic pores), which contrasts sharply with the unleached, nonporous transgressive facies. The presence or absence of glacio-eustatically controlled, penetrating freshwater lenses, the distribution of green phylloid algae, and the extent of secondary pore-filling cementation were the major factors controlling reservoir capability of Goen facies. Mound-associated (foraminiferal) and backmound (bryozoan-rugose coral) carbonate reservoirs may be of greater importance in petroleum exploration than algal mounds, the most frequently exploited late Paleozoic carbonates of he Eastern shelf.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.