AAPG Bulletin, V. 82 (1998), No. 5B (May 1998 Part B), P. 986-1012.

Three-Dimensional Seismic Stratigraphic Expression of Pliocene-Pleistocene Turbidite Systems, Northern Green Canyon (Offshore Louisiana), Northern Gulf Of Mexico¹

Peter Varnai²

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received September 6, 1996; revised manuscript received June 23, 1997; final acceptance November 20, 1997.
²Department of Geological Sciences, Energy and Minerals Applied Research Center, University of Colorado, Boulder, Colorado 80309-0399. Present address: Exxon Exploration Company, P.O. Box 4778, Houston, Texas 77210-4778.

This paper summarizes the results of a portion of my Ph.D. studies at the University of Colorado at Boulder. I express my most sincere gratitude to Paul Weimer, my advisor, for all his encouragement, help, advice, and support throughout my studies. I thank Mark Rowan for his help with the structural interpretation, and David Knapp for helping me negotiate the hurdles of the workstation.

I thank the following companies and their representatives for their support and input as part of the Gulf of Mexico Research Consortium at the University of Colorado: Agip, Amoco, Anadarko, BHP, BP Exploration, Burlington Resources, CXY Energy, Chevron USA, CNG, Conoco, Exxon, Marathon, Mobil, Occidental, Pan Canadian, Pennzoil, Petrobras, Phillips, Shell, Texaco, Total, Union Pacific, and Unocal. I extend special thanks to Halliburton Geophysical (now Western Geophysical), specifically to Bob Graebner, John Anderson, and Gary White, for the seismic data used in this project. I am indebted to PaleoData Inc. and Art Waterman for use of their biostratigraphic data. Software donations from GeoQuest, CogniSeis Development, Zeh Graphics, Landmark Graphics, and Platte River Associates were essential to the success of my study. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society for partial support of this work. Special thanks go to Paul Weimer, Laurie Lamar, and to the AAPG reviewers, former Elected Editor Kevin Biddle, Cindy Yeilding, and Jory Pacht, for their careful reviews. Their comments greatly improved the manuscript. 

ABSTRACT

Interpreting three-dimensional (3-D) seismic data is of great value in high-grading prospects in frontier exploration areas in the Gulf of Mexico. This technique is now routinely used to reduce the exploration and production risks associated with turbidite reservoirs. A subregional 3-D seismic data set was used in this study to describe the Pliocene- Pleistocene turbidite systems in three intraslope minibasins in the north-central Green Canyon protraction area of the Gulf of Mexico.

The stratal geometries and lithology of the different turbidite elements were interpreted based on the integrated analysis of well-log data, seismic facies observed on the vertical seismic profiles, and amplitude distribution recognized on series of horizon slices and amplitude extraction maps (horizontal seismic facies). The lithologic interpretation, however, was limited by the lack of seismic velocity data; the wells were tied to the seismic data through synthetic seismograms created by integrating the sonic and density logs.

Three turbidite elements were mapped in the selected upper Pliocene-lower Pleistocene interval of the study area: basin-floor fan (amalgamated depositional lobes), erosional and depositional channels, and overbank sediments. Lobe-shaped high-amplitude areas (onlapping high-amplitude continuous reflections on seismic profiles) indicate laterally continuous, sheetlike deposition, interpreted as turbidite lobes. Elongated sinuous high-amplitude areas (high-amplitude discontinuous reflections) correspond to depositional channel systems. Sinuous low-amplitude areas in generally high-amplitude areas (e.g., sequence boundaries) reflect the erosional channels. The depositional channels are interpreted to be filled with sands; the shale-filled erosional channels are interpreted as conduits for sediments bypassing farther down slope. The detailed 3-D seismic interpretation allowed description of the characteristics of the channel systems, analysis of their evolution through 2 m.y., and determination of the controls on the variations in turbidite deposition. Widespread low- to moderate-amplitude areas without dominant amplitude patterns are interpreted as overbank deposits.

The turbidite elements identified in this study, including basin-floor fans (depositional lobes) and channel systems, form the reservoirs in the fields of the study area and in many other fields and discoveries of the northern Gulf of Mexico. The geometries of the sand-rich turbidite elements mapped in this study are considered to be good analogs for these producing fields and some exploration targets. The results of this study also can be used as analogs for the description of reservoir architecture in the subsalt trend of the Gulf of Mexico and in other deep- water exploration areas.