Exploration has exploded in the mature East Texas Basin for deep, overpressured “pinnacle” reefs in the Cotton Valley Limestone (Haynesville). Reefs have been reported to be more than 1300 ft high, up to 100 acres and contain as much as 100 BCF in a single reef. The reefs germinated during CV Lime deposition and grew into the mixed siliciclastic/carbonate environment of the Bossier Shale along compactional features, salt withdrawal induced fault scarps and turtles, salt cored and basement structures. Generally, any scenario where accommodation space was provided, a reef could have grown. Productive reefs always have abundant delicate, finger coral and faunal diversity has been seen to increase with apparent energy. Generally, wave energy was moderate and the reefs grew within wave base. All reefs seen by the authors have had coral and deep water, strictly microbial mounds have not been seen to date. Reef poisoning and growth inhibiting influences such as nearshore influxes of terrigenous sediment and fresh water have been inferred locally which resulted in slightly deeper water, tight reefs. Cumulative reef height was driven by vertical accomodation space provided by long period (3rd order) relative sea level rise. In the East Texas basin, local (salt withdrawal) and regional tectonics (post rift extension and thermal subsidence) may have been the dominant processes affecting longer period sea level. Recent studies of the Upper Jurassic eustacy curve indicate that sea level was probably static and did not provide accomodation space globally. The Upper Jurassic was a time of “green house” climate where smaller polar ice caps induced low amplitude (10-20 ft?) fourth and fifth order sea level cycles. This composite sea level curve proved the opportunity for reef communities to accumulate vertically via the 3rd order rise and provided a mechanism to create significant porosity via the higher frequency sea level fluctuations. Drops in sea level exposed reefs (islands) where rain collected at the near surface generating a flesh water lens (water table). Gross recrystallization occurred over much of the most recent reef cycle construction into micro-rhombic calcite and micro-to-free porosity. Less stable calcite components (aragonite, Mg calcite) morphed into more stable calcite and vugular porosity. These reefs were rhythmically stacked patch reefs (pancakes) with synoptic relief probably never exceeding 50 ft. Each “pancake” grew vertically 20-50 ft during each 4th or 5th order sea level rise that was then leached during the subsequent sea level drop. Exceptions to this process have been inferred locally where local flesh water influx depressed the tops of the reefs preventing the formation of islands and the subsequent exposure to flesh water leaching. The reef tracts backstep in space and time away from the locus of thermal subsidence at the center of the basin — a failed rift. There is ample evidence for these hypotheses from well logs, Sr and C/O isotopes, fluid inclusions, SEM and petrography.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah