Origin and Characterization of the Dual Micropore/Macropore Network in Pennsylvanian (Desmoinesian) Caddo Algal Buildup Complexes; Stephens County, North Central Texas

Abstract

The Pennsylvanian Caddo Formation in north central Texas is an excellent example of a dual pore carbonate reservoir composed of macropores and micropores that form economic reservoirs. The reservoirs are associated with algal mound buildup complexes. The Caddo Limestone is up to 800 ft thick, where the uppermost section contains abundant algal mound complexes. These complexes were deposited on the eastern flank of the Concho Platform in the Fort Worth Basin. The best preserved macropores are interparticle, moldic, and collapsed vuggy molds of phylloid algal fragments. A variety of cements ranging from former marine aragonite cement to deeper burial coarse-crystalline calcite and saddle dolomite cements partly occlude the macropores. Micropores in the Paleozoic have not been well documented and the micropores in the Caddo Formation have been overlooked by several authors who only recognized the megapore network. A rich-array of former Mg-allochems show well-developed intraparticle micropores including Komia, fusilinids, bryozoans, ostracods, encrusting foraminifera, benthic foraminifera, and brachiopod spines. Numerous small peloids (some possibility related to cyanobacteria) are commonly microporous. The origin of the micropores in the Caddo Limestone is related to the transformation of unstable Mg-calcite allochems to stable calcite probably starting in near surface meteoric environment and continued with shallow burial in the subsurface. The micropore system in the Caddo Limestone was characterized by analyzing Ar-ion milled samples on an SEM. The micropores occur between micropolyhedral calcite crystals that predominantly range in crystal size between 2 and 3 microns. However, some crystals are as large as 5 microns and some are less than 1 micron and the crystal shape is polyhedral. Some areas that appear to be larger crystals are actually composed of coalesced microcrystals. Core plug analysis was conducted on 69 core plugs. A wide range of porosity and permeability is noted with porosity ranging between 0.8% and 25.1% and permeability ranging between 0.01 md and 370.5 md. The highest permeabilites are in the Komia packstone/grainstone facies that contain interparticle, moldic, and micropores and phylloid algal bafflestone facies that contain vuggy pores. This study documents a Paleozoic case history of economic Pennsylvanian carbonate reservoirs that contain an economic dual pore network.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016