Abstract: East Texas Update: Reef Play Enters a New Phase

MONTGOMERY, SCOTT L., Petroleum Consultant, Seattle WA

Exploration for gas-charged Upper Jurassic (Cotton Valley Lime) reefs in the East Texas Basin underwent rapid expansion in 1997, entering a new phase. This new phase is related both to developments in drilling and to improved geologic knowledge regarding the nature, distribution, and growth of productive reef features. As of October, 1997, a total of 62 wells had been completed, with 24 additional wells drilling or waiting on completion. Of the total completed, 42 wells (67.7%) encountered reefs and 31 (50%) were discoveries. Activity has expanded to the north and downdip from the previous drilling fairway. Important downdip discoveries have encountered reef features over 1100 ft thick along the lowermost portion of the paleo-ramp.

Reefs of such height confirm interpretations that these are stacked features composed of multiple nucleation/catch-up/keep-up units, each representing a 4th or 5th order sea-level cycle superimposed upon a background 3rd order transgressive-regressive cycle. Individual growth units consist of several potential facies types: coral-microbialite; coral-sponge-microbialite; spongemicrobialite; thrombolite. The presence of microbialite (microbe-produced micrite) is considered essential for stabilizing reef debris, thus allowing for rapid upward growth and repeated reef renucleation. While reefs grew in a variety of settings, it appears that fault-related highs were the best sites for porosity development Zoned porosity development was related to dissolution and recrystallization of original reef material, caused either by subaerial exposure (e.g during sea level fall subsequent to keep-up growth phase) or to post-burial fresh-water invasion. Diagenesis has been varied and complex. Porosity in productive reefs varies from 7-30% (7% cutoff) in zones ranging from 45-300 ft thick.

Reefs are detected on 3-D seismic profiles by dim-out of the upper or both of two reflectors marking the Cotton Valley Lime. Drape and onlap are occasionally seen in the Overlying Bossier Shale. Due to the small size of reef features, data migration must be performed carefully. Seismic data has been unable to consistently distinguish porous and non-porous reefs.

Production rates and reserves vary from 4-30 MMcf/d and 0.5-80 Bcf/well. A minimum of about 550-600 bcf total reserves has been discovered thus far. Based on company reports and drilling plans in progress, reef activity will remain at a high level during 1998.

AAPG Search and Discovery Article #90936©1998 AAPG Southwest Section Meeting, Wichita Falls, Texas