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Exploration for Deep Miocene Reservoirs in South Louisiana: The Story of the Etouffee Discovery, Terrebonne Parish, Louisiana [Abstract]
In 1997 Geco-Prakla and Union Pacific Resources secured a seismic option covering all the unleased acreage owned by Continental Land and Fur in western Terrebonne Parish to undertake one of the first regional, truly exploratory, 3-D surveys in the onshore of south Louisiana. Earlier 3-D surveys had typically targeted producing fields, making imaging of deep targets problematic. The Etouffee Prospect was the first well drilled based on this new 3-D survey and became a significant success, with estimated reserves of 250 Bcfe. Comparing the Etouffee discovery to other activity in South Louisiana shows it to be the one of the largest discoveries made during the 1990's.
The Etouffee discovery south of Kent Bayou Field in Terrebonne Parish is a down dip extension of the prolific middle Miocene Robulus "L" producing trend. Prior to the discovery, the closest production from the Robulus "L" section was 5 miles to the northeast and structurally 4000 feet higher across a system of large growth faults of varying age. The Etouffee Sands were deposited with thinly bedded shales on the distal front of a shelf-edge delta. Slumping, faulting, and salt tectonics combined to deform the large shelf edge delta and control sand distribution. Hydrocarbon migration probably occurred early as the Etouffee Sands formed a typical rollover anticline associated with growth faulting. The overlying Cibicides opima section prograded over the Etouffee deltaics forming a southward-thickening wedge of sands and shales. This subsidence of Cibicides opima sediments south of Kent Bayou began the rotation of the Etouffee structure onto south dip. Further rotation onto south dip and complications due to Cristellaria "I" aged deposition and faulting completed the formation of the structure as it appears today. From Biginerina humblei time to present, the structure uniformly subsided to its current depth. A 700-foot gas column was preserved on a faulted, down thrown, three-way closure within the Robulus "L" aged Etouffee sands.
The Union Pacific Resources Continental Land & Fur #1 reached a total depth of 19,226 feet and logged 156 feet of pay between the depths of 18,654-18,954 feet on October 31, 1999. Since that time, five development wells have been drilled with net pay reaching 306 feet in the CLF #4. Production rates peaked in April 2002 at 95,400 Mcfd, 18,600 Bcpd, and 0 Bwpd. The current field rate (1/31/03) is 85,000 Mcfd, 19,600 Bcpd, and 3,000 Bwpd. The Etouffee Sands cumulative total production to date is 54.5 billion cubic feet of gas and 11.6 million barrels of oil and condensate. Converted to gas, the field has produced 125 Bcfe or about half the estimated reserves.
The Etouffee prospect was identified as the result of a 280 square-mile 3-D seismic survey acquired in the fall of 1997 and spring of 1998. The existing 2D seismic grid in the area neither identified nor resolved the structurally complex prospect area. The data did show an overall structural high and the large expansion fault system, but it failed to identify the detailed fault pattern and actual structural crest. The 3-D data correctly imaged Etouffee as a three-way dipping structure on the downthrown side of a large down-to-the south growth fault system. The large expanse of 3-D data also allowed for the reconstruction of the depositional history of the area through the use of multiple sets of isochrons.
All six wells in the field have increasing sonic velocities and decreasing densities through the pay zone, creating a weak amplitude event at the top of sand. The lack of strong amplitudes and the structural complexity make it difficult to draw conclusions on rock properties and provide no definitive seismic signature for the pay sands.
Three separate reservoirs have been recognized based on log correlation, pressure analysis, and fluid properties. The Etouffee 1 and Etouffee 2 sands are high-temperature and high-pressure gas condensate reservoirs. The Etouffee 3 sand is an oil reservoir. Porosities average 25 percent with permeabilities ranging from 50 millidarcies to 5 darcies. These rock qualities are conducive to excellent production rates, especially from reservoirs below 18,000 feet. Early onset of overpressure and high bottom hole pressures (17,000 psi) have helped to preserve the excellent reservoir parameters.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
Anadarko, Houston, TX
Copyright © 2003 by NOGS (The New Orleans Geological Society)