MAULDIN, DARRELL L., Phillips Petroleum Company, Houston, TX

ABSTRACT: Regional Depositional Relationships and Fracturing of the Wapanucka Limestone, Frontal Ouachita Mountains

Although gas production in the Ouachita Mountains has been primarily from the Spiro (Atokan), the Wapanucka (Morrowan) is locally a gas reservoir and thus is a secondary objective. Outcrop study of imbricate thrust sheets forms a basis for predicting the character of rocks in the subsurface. Thick successions of repetitive subtidal carbonate cycles of platform, platform margin, and basinal facies characterize the Wapanucka in outcrop, except for most of the basinal outcrops, in which cyclicity is not evident. Each cyclic sequence records aggradation and southward progradation of the ramp-like shelf margin. Shoaling marine environments are represented by bryozoan crinoid shallow shelf bars capped by oolites that developed on local and regional topographic highs. These repetitive subtid l cycles are occasionally capped by localized beach deposits (limestone pebble conglomerates with abundant oolites and carbonaceous wood fragments).

Demosponges were the most abundant biota at times when the shallow shelf waters were muddied by fine clastic influx. Sponges commonly grew with the tubular algae, Donezella and were occasionally encrusted with Archaeolithophyllum a phylloid algae. These two algaes form boundstones that dominate the basinward facies with associated encrusting forams. In the eastern portion of the study area, phylloid algae is often the dominant bioclast in arenaceous grainstone shoal deposits. They are also found as the major constituent of algal-mud mounds.

The top of the Wapanucka can be correlated throughout the outcrop region as a stratigraphic sequence boundary. A fine clastic influx, probably from the terrestial Foster channels to the north, choked out the carbonate production of the Wapanucka. This shaly facies conformably overlies the Wapanucka and is either termed the middle shale or sub-Spiro shale. In this interval, cross-bedded sandy calcarenites within the shale probably represent shelf bars. These local buildups, make it difficult to separate the Wapanucka and Spiro. Several of these discontinuous bars occur in outcrop within a few miles laterally, and were also observed in subsurface core. The Spiro Formation conformably overlies the middle shale and represents a return to shallow shelf carbonate deposition similar to the W panucka. However, significant amounts of sand were transported to form sand dominated shallow marine bars, in contrast to the bioclastic and oolite shoals of the Wapanucka.

Several facies of the Wapanucka are potential hydrocarbon reservoir rocks. These include limestone pebble conglomerate, oolitic grainstone,

algal boundstone and spiculitic limestone. Spiculitic limestones (often sponge boundstones) often develop fracture porosity. Numerous fractures in the frontal Ouachitas cut through bedding, and most of these are oriented perpendicular or parallel to strike. Fracturing is believed to be of major importance to reservoir potential of the Wapanucka, and probably the Spiro as well.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.