Geologic Considerations on Successful Deepwater Exploration and Field Development

Michael H. Gardner
Montana State University, Bozeman, Montana ([email protected])

Abstract

All sedimentary basins share a common fossil fuel extraction history characterized by the development of progressively deeper and/or more complicated hydrocarbon reservoirs over time. Economic thresholds based on high production rates and large reservoir volumes suggest that the maturation of offshore deepwater basins may follow a somewhat different path. High production rates are required to overcome the increased time between an offshore discovery and first production. High operating costs require field developments with fewer wells and emphasize seismic-based determination of reservoir volume. These economic requirements are fundamentally geologic in origin, but they also reflect the additional consideration of ocean bathymetry and subsurface drilling depth as key drivers on high extraction costs.

Reservoir distributions in onshore structural basins reflect the incomplete and fragmented record of depositional systems amalgamated from many sedimentary basins, which are often unrelated to the present basin configuration. Onshore reservoir quality generally decreases with increased age and depth. By contrast, deepwater reservoir distributions and types record contemporaneous deposition and ocean basin evolution. This produces a family of reservoir types, genetically related by a common depositional system and basin history. Because the genetic thread is preserved in the more complete deepwater record, predictive geologic rules may be successfully applied.

The need for high production rates and reservoir volumes drives the economics of deepwater field developments in offshore basins. This elevates the importance of the reservoir as a key geologic factor. Reservoir quality reflects the sedimentary architecture of the depositional system. Important controls include the distributions and types of deepwater reservoirs often reflect the present ocean basin bathymetry of the offshore basin. Depositional outbuilding of the continental margin generates predictable patterns of syn-sedimentary deformation and sedimentation within offshore-thinning sediment prisms. Submarine channel fairways represent long-lived sediment transmission sites and form sand-prone regions within these sediment wedges. The close linkages between basin evolution, sedimentation, and fluid migration require multidisciplinary teams to successfully integrate information currently partitioned into different geoscience sub-disciplines. Successful integration should aid the development of predictive geologic rules that exploit the more complete stratigraphic record and close linkage between structural deformation and sedimentation. The paradigm shift required to successfully adapt to these geologic conditions remains a significant challenge.

AAPG Search and Discovery Article #90086 © 2008 AAPG Foundation Distinguished Lecturer Series 2008-2009