SHEW, ROGER D., Shell Development Co., Houston, TX; and P. S. GARY, D. S. PFEIFFER, and P. W. BILINSKI, Shell Offshore Inc., New Orleans, LA

ABSTRACT: Evidence and Importance of a Pliocene, Carbonate-Rich Deposit Formed in Association with a Hydrocarbon Seep at Auger Field

Numerous examples of modern hydrocarbon seeps and associated buildups have been documented on the Louisiana continental slope. At Auger Field, located in Garden Banks Block 426 in approximately 915 meters of water, a subsurface interval dominated by carbonate sediments is interpreted to be related to a Pliocene hydrocarbon seep. Thirty sidewall samples, spanning depths of 6380 to 6413 meters, are dominated by finely crystalline calcite and carbonate mud. Also present are varying amounts of cemented forams, carbonate cement, coalescing carbonate clasts, minor quartz silt, abundant pyrite, and, below 6400 meters, abundant large shell fragments. Carbon isotopes from the cements (botryoidal and blocky) are very negative (-44.3 o/oo) and indicate that the cement is associated with methane xidation near a hydrocarbon seep. The shell fragments have more typical isotope{13}C values of -9.9 to -12.1 o/oo. Oxygen isotope values range from +1.6 to +3.3 o/oo, which are normal marine values.

The presence of this thick, low porosity, high density deposit is apparent from 3-D seismic and well logs. Seismically, the carbonate deposit has a higher acoustic impedance than its bounding lithologies. This large impedance contrast results in a mappable seismic anomaly which is linear in extent and runs parallel to a large down-to-the east fault system.

On the induction log, the carbonate deposit has a very low gamma ray and high resistivity signature. In addition, well log correlations and Formation Microscanner logs indicate that the carbonate deposit locally influenced subsequent turbidite deposition, specifically the "S" sand which is the largest hydrocarbon reservoir at Auger.

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