Intraslope basins reveal US Gulf of Mexico secrets

The northern Gulf of Mexico is one of the best-studied hydrocarbon provinces in the world. But despite the ever-increasing exploration in the deepwater intra-slope-basin province, relatively few studies have focused on the latest Quaternary depositional processes and stratigraphy of the intraslope basins.

"Modern" depositional analogs derived from studies of present intraslope basins provide excellent exploration models for predicting sediment facies and sand-body geometries of more deeply buried productive deposits. A better understanding of the depositional processes and lithology of the very recent seafloor (upper 10-50 meters) promotes better risk assessment in selecting the safest locations for the placement of seabed structures for hydrocarbon exploitation, including templates, piles, and pipelines.

The Gulf of Mexico Intraslope Basins (GIB) project is a detailed, comprehensive, and integrated synthesis of the Late Quaternary depositional processes and facies in intraslope basins. Recent seafloor depositional processes and lithologies are being assessed, as well as the influence of sea-level changes on controlling these facies and processes.

A wide variety of very high-resolution data sets, including seismic, side-scan sonar, bathymetric swath maps, and shallow cores, exist for the intraslope basin area. These data have been collected during the past 30 years by the Institute for Geophysics, The University of Texas at Austin (UTIG), the US Geological Survey, and NOAA. They are being studied and synthesized as part of the GIB project to effectively examine the details of latest Quaternary intraslope basin deposits. This research project integrates seismic interpretation, sedimentology, and biostratigraphy.

During the first phase of the GIB project (GIB I, 1998-2000), 70,000 km of high-resolution seismic was interpreted, approximately 300 piston cores (up to 11 meters long) were described with depositional processes interpreted. Biostratigraphy from the most stratigraphically coherent cores was also determined. A map of the high-resolution (3.5 kHz) seismic facies was constructed for the intraslope-basin province of the northern Gulf of Mexico. It displays the aerial distributions of a variety of depositional facies. Integrated analyses of core and seismic data reveals the details:

• Depositional features: Submarine fans, mass-transport deposits, and contourite deposits
• Architecture: Channels, overbank deposits, lobes, slumps, debris flows, and sediment drifts
• Depositional processes: Turbidity currents, bottom currents, plastic flows, etc, and sediment facies distributions - sand versus shale.

Sediment cores, seismic facies

Cores were utilized to "ground truth" the 3.5 kHz seismic facies types mapped within the intraslope basins. Two intervals of UTIG piston core IG 41-24 are shown. This core contains thick beds of medium sand with numerous large mud and rock clasts of various shapes and sizes. The deposit is interpreted as a sandy mass-transport deposit such as a debris flow. The accompanying UTIG 3.5 kHz profile across the core site displays a semi-transparent wedge-shaped deposit that returns prolonged, fuzzy echoes. This type of seismic facies is typical of a mass-transport deposit, such as a debris flow or slump. The piston core supports this interpretation of the seismic facies, and visa versa.

The contrasting seismic facies on the left half of the basin floor, which displays parallel, flat-lying reflections, probably represents ponded turbidity-current deposits. By calibrating the mapped high-resolution seismic facies types with the piston cores wherever possible, a higher-confidence predictive tool is developed for understanding intraslope-basin lithologies and depositional processes. Understanding the modern depositional processes and facies distributions within intraslope basins can therefore help explorationists better predict and interpret the reservoir geometries of more deeply buried, productive intraslope-basin reservoirs.

Biostratigraphic studies

Biostratigraphic zonation and biofacies analysis of piston cores were undertaken to study the relationship of depositional processes to high-order (4th-6th order cycles) glacio-eustatic climatic and sea-level cycles. Detailed, quantitative studies of work on planktonic foraminifera (specifically Globorotalia menardii) have enabled correlation of events, such as the Younger Dryas cooling interval, between intraslope basins as far as 250 km apart. Such correlations are being used to compare depositional features, processes, and facies, as well as faunal patterns, with existing sequence-stratigraphic and sea level models.

For example, biostratigraphic zonation of cores is being used to date the timing of influxes of significant sand by turbidity currents and related gravity-controlled deposits along the Bryant Canyon/fan turbidite system pathway. These existing models can then be "ground-truthed" and calibrated so that they can be more effectively utilized to predict the depositional settings and facies of more deeply buried, prospective intraslope basin deposits.

Future work

A second three-year phase of the GIB Project (GIB II) currently is being planned to start in early 2001. This phase will augment Phase 1 and will focus on more detailed calibration of the seismic facies map to further improve understanding of intraslope-basin sedimentary processes. Phase II will be a collaborative effort with Dr. W.R. Bryant and Dr. H.C. Nelson of the Oceanography Department at Texas A&M University (TAMU) and will involve both collection of new data and analysis of existing TAMU data.

The TAMU Deep Tow system enables collection of 3.5 or 7 kHz very high-resolution seismic data, as well as 100 kHz side-scan sonar data. The positively buoyant "fish" is towed at a constant height (30 meters) above the seafloor, resulting in sub-bottom profiles and side-scan sonographs with outstanding clarity, resolution and consistency in any water depth.

We plan to analyze Deep Tow data that currently exists at TAMU for the Alaminos Canyon and Bryant Canyon systems. We also plan to survey additional areas of interest with the Deep Tow such as the East Breaks slide/channel system, Rudder-Magellan sandy turbidite system and Rio Grande sandy channel system. Previously collected cores (a subset of Western Gulf SGE Program cores donated by TDI-Brooks International to TAMU) will be analyzed in the manner of those during GIB Phase I and new cores will be collected as needed to calibrate seismic and side-scan data.

Summary

Current research as part of the GIB project provides a better understanding of latest Quaternary depositional processes in one of the most active regions of the Gulf of Mexico, the intraslope basin region. The integration of high-resolution seismic data and core data to study these "modern analogues" promotes a higher-confidence predictive tool for understanding seafloor and very recent intraslope basin lithologies and depositional processes, which can then be applied to the exploration and production of more deeply buried intraslope basin deposits.

Interpretations and analyses from Phase 1 and Phase 2 will be compiled in a global information system (GIS) environment to create a database with a wide range of applications for users. Included will be analogs for exploration at depth, lithology of the upper 50 meters of seafloor for seafloor stability at potential platform sites, and sedimentology and depositional processes of the upper 10 meters of seafloor in the vicinity of pipelines.

The Gulf Intraslope Basins project is an industry-supported consortium at the Institute for Geophysics of The University of Texas at Austin and at the Earth Resource and Environment Center of The University of Texas at Arlington. Contributing members are AGIP, Anadarko, BPAmoco, Conoco, Elf, Marathon, Spirit Energy, Texaco, and Vastar. Further information can be obtained from [email protected] or [email protected].