Peter Varnai

Biostratigraphic Techniques for Analyzing Benthic Biofacies, Stratigraphic Condensation, and Key Surface Identification, Pliocene and Pleistocene Sediments, Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf of Mexico

The northern deep Gulf of Mexico is a geologically complex province consisting of Neogene intraslope minibasins created by sediment loading onto and evacuation of allochthonous salt. Sedimentary fill in the minibasins consists of bathyal turbidite systems with highly variable facies distribution. In this paper, we present three biostratigraphic techniques developed to increase our understanding of the geologic evolution of the northern Green Canyon and Ewing Bank lease areas. The first two techniques address the nature of stratigraphic condensation in minibasins and the relative areal extent of the condensed sections. The third technique uses benthic foraminifers to improve the stratigraphic knowledge of the area. (1) The first technique is concerned with sediment accumulation plots from wells helping to identify possible areas of condensation based on variations in the rate of accumulation. By displaying several wells together, the areal extent of the condensed section can be identified, and the causes for the condensation can be evaluated. (2) The second technique addresses condensation within one minibasin during a 2.5-m.y. interval. Relative abundance values are plotted from a series of wells at selected time surfaces and tied to seismic horizons that are correlated throughout the basin. This allows plotting a series of maps that show how stratigraphic condensation changes throughout a basin. (3) The third technique develops benthic biofacies maps that more accurately reflect the highly irregular paleobathymetry of the slope at the time of deposition. These maps were created by plotting the benthic biofacies of the sediments recovered from a well at the stratigraphic extinction of 29 selected planktonic foraminifer and calcareous nannoplankton species. The maps are integrated with lithostratigraphic and structural palinspastic reconstructions. The resulting maps are interpreted to more accurately reflect benthic biofacies of the continental slope at specific geological times during the Pliocene and Pleistocene.

Atlas of Petroleum Fields and Discoveries, Northern Green Canyon, Ewing Bank, and Southern Ship Shoal and South Timbalier Areas (Offshore Louisiana), Northern Gulf of Mexico

Thirty-five fields and discoveries from the northern Green Canyon, Ewing Bank, and southern Ship Shoal and South Timbalier protraction areas (offshore Louisiana) are characterized by a series of seismic profiles, wireline logs, and summaries of their production characteristics. All fields or discoveries consist of combined structural/stratigraphic traps. Reservoirs occur in various elements of turbidite systems-primarily in sheet sands, channel fill, and thin beds. Best production rates occur in sheet sand facies. Development facilities vary from fixed platforms to subsea tiebacks to existing fields.

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

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.

Sequence Stratigraphic Analysis of Plio-Pleistocene Turbidite Systems, Northern Green Canyon and Central Ewing Bank Areas, Northern Gulf of Mexico

ABSTRACT Plio-Pleistocene turbidite systems were mapped in the northern Green Canyon and central Ewing Bank protraction areas of the Gulf of Mexico. Ten depositional sequences were identified in the 5.5 Ma and younger intervals using 2560 km 2-D seismic profiles, biostratigraphic data from 34 wells and well logs from 39 wells. The Plio-Pleistocene geologic evolution of the area is divided into four geologic intervals. Each phase differs in its seismic and geologic facies, depositional rates, nature of turbidite systems, sand content, amount and type of slides and presence of erosional features. Three turbidite elements, basin-floor fans, channel-levee systems and overbank sediments, constitute most of the Plio-Pleistocene succession. Large unchannelized flows became volumetrically significant in the younger sequences. Slides represent only a small portion of the sequences. Basin-floor fans (sheet sands) were deposited overlying sequence boundaries. Salt tectonics greatly influenced the development and the location of these fans. These sheet sands are relatively thin (100-150 m), and consist of 15-50 m thick sand layers. The areal extent of the basin-floor fans varies between 20 and 210 km2. The size and thickness of the fans gradually decrease in the younger sequences. The basin-floor fans are reservoirs in some fields in the area, and they also have the highest reservoir potential. The channel-levee systems are composed of up to 25 m thick channel-fill sands, which may coalesce into 60-90 m thick intervals forming stacked channel-levee systems. The number and sand content of the channel systems gradually decrease upward through the interpreted sequences. Coalesced channel-fill sands form the reservoirs in several fields of the area.

The use of sequence stratigraphy in evaluating the prospectivity of Neogene turbidite systems, northern Green Canyon and Ewing Bank, northern Gulf of Mexico

The Neogene turbidite systems of northern Green Canyon and Ewing Bank lease areas in the northern Gulf of Mexico are a major exploration play. The regional sequence stratigraphic framework for this area has been interpreted to help define potential areas for future exploration. The study area includes 17 fields/discoveries primarily in turbidite-related reservoirs. Once the sequence stratigraphic framework was established, the existing fields and potential prospects can be quickly evaluated in terms of sequence stratigraphy. Good potential for future exploration exists both in sub-salt prospects and in the deeper water portions of the area. For example, two discoveries (GC 200/244, 506) have been made during the past year. Two main potential reservoir intervals have been identified, which reflect different Winds of turbidite systems based upon lithologies and seismic facies. Lower Pliocene sediments (5.5 to 3.0 Ma) consist of sand-rich turbidite systems (up to 50 % sand in some sequences). Sand bodies are interpreted to be a really widespread sheet and amalgamated sheet sands. The lower Pleistocene interval (1.9 to 0.7 Ma) is predominantly shale-rich with localized thick sands developing in channel-levee systems and unchannelized sands (basin-floor fans). Most of the reservoirs in the area occur within this interval.