William A. Morgan

Permo-Carboniferous carbonate platforms and reefs

The results are presented from an integrated sedimentologic, structural, stratigraphic, and diagenetic study of syndepositional faults and fractures that cut the Permian Capitan reef and equivalent platform strata within three outcrop windows in Slaughter Canyon, Guadalupe Mountains, New Mexico. The studied faults are dip slip, have displacement of up to 30 m parallel the shelf margin, and are vertically and laterally segmented. The faults dip steeply shelfward and basinward, and grew incrementally during deposition of the Yates and Tansill Formations. Steep polycyclic paleocavern systems developed along faults and fractures. These can have cumulative vertical relief of more than 270 m, extend at least 1.2 km along strike, and are typically less than 10 m wide but can attain a width of 90 m. The paleocaverns can extend more than 110 m below the top of the Capitan reef. The paleocaverns are filled mainly with sediments deposited during Capitan progradation and aggradation. Seven Permian lithologies are distinguished: (1) limestones and limestone breccias, (2) microspar-lithified breccias, (3) carbonate-rich breccias, (4) reworked and remnant breccias, (5) beige dolomitic siltstone–sandstone and associated breccias, (6) pink dolomitic siltstone–sandstone and associated breccias, and (7) spar-cemented breccias. The fills vary vertically and along strike within the paleocaverns but have a clear organization and stratigraphy. The integration of the stratigraphy of the paleocaverns with the structural and sequence stratigraphic framework developed in back-reef strata provides evidence for incremental fault growth and multiple episodes of dissolution, brecciation, collapse, deposition, cementation, and dolomitization within the paleocaverns during Capitan times. Solution-modified syndepositional faults extend at least 33 km along strike from Slaughter Canyon and are considered to be an integral component of the Capitan platform. The fault-zone paleocaverns contain a unique internal record of events and processes that have no counterpart in the shelf succession. Their study provides new insights into the internal heterogeneity and diagenesis of the Capitan platform. The results have important implications for the Capitan platform, and more generally for the heterogeneity of syndepositional fracture-controlled karst systems formed in carbonate platforms with steep unstable margins, subject to compaction-induced tilting and/ or developed in active tectonic settings. Permo-Carboniferous Carbonate Platforms and Reefs SEPM Special Publication No. 78 and AAPG Memoir 83, Copyright

Subtle Porosity and Traps Within Frisco Formation (Devonian, Hunton Group): Geologic-Seismic Waveform Approach, Example from West El Reno Field, Canadian County, Oklahoma: ABSTRACT

End_Page 960------------------------------The Frisco Formation is a middle Lower Devonian limestone within the Hunton Group (Upper Ordovician-Lower Devonian). In the Anadarko basin, the Frisco Formation consists of skeletal packstones and grainstones, whose main components are pelmatozoans, brachiopods, and, locally, corals. Depositional intergranular porosity has been mostly obliterated through syntaxial cementation on pelmatozoans, and mechanical and chemical compaction. Only minor intrabryozoan primary porosity remains. Secondary porosity, which formed during subaerial exposure of the Frisco Formation during the late Early and Middle Devonian, occurs locally at the top of the formation in the form of partly leached grains, vugs, and solution channels. This secondary porosity is best developed close to areas where the forma ion was completely eroded; these areas commonly correspond to Middle Devonian paleostructures. Hydrocarbon accumulations in the Frisco Formation are mainly in stratigraphic traps situated downdip of the areas where the formation has been severely truncated. The Woodford Shale (Upper Devonian-Lower Mississippian) unconformably overlies the Frisco Formation in the study area and provides a source, trap, and seal for Frisco Formation reservoirs. Geophysical identification of Frisco Formation porosity is possible using Relative Amplitude (RAM) processing. Mapping of porosity and truncated margins, and identification of potential hydrocarbon traps, are facilitated by using these RAM processed seismic sections. The West El Reno field, Canadian County, Oklahoma, produces gas and condensate from an outlier of the Frisco Formation, and provides a template for this technique. End_of_Article - Last_Page 961------------

ABSTRACT: Stratigraphic Framework and Exploration Potential of Sligo Formation Carbonates (Lower Cretaceous) in South Texas

The two complete ‘3rd’-order Sligo sequences and an older Hosston sequence form a transgressive-regressive ‘2nd’-order sequence. The architecture and facies mosaics of each of the ‘3rd’-order sequences were influenced by available accommodation space, driven by thermal subsidence, eustasy, and paleotopography (mostly the result of tectonism, including salt movement, and inherited paleodepositional topography). In general, the oldest sequence (Sequence 20, Hosston) largely fills in pre-existing accommodation space, most of which was tectonically controlled, with little evidence of significant progradation. The oldest Sligo sequence (Sequence 30) continued to fill preSligo topography, but its architecture is aggradational to progradational. The influence of inherited structural paleotopography is not as important for the youngest complete Sligo sequence (Sequence 40), which is strongly progradational and has a well-developed shelfmargin buildup. The onset of Sequence 50 to the top of the Sligo marks the initial flooding of the next 2nd-order sequence, and this succession is retrogradational.