Exploration in deepwater basins….where next?

Worrall, D. M., Shell International E&P Inc., M. W. Bourque and D. R. Steele, Shell Deepwater Services, Houston, TX

Abstract

Exploration success in the first two decades of deepwater exploration has been highly focused on a particular basin type: Atlantic rift margin basins draped by thick deepwater clastic strata, which are deposited downdip of large Tertiary drainage systems and above a mobile substrate of shale or salt. Most are downdip extensions of working petroleum systems on the adjacent shelf and onshore. Reasons for the preponderance of discovery volume to date in this basin type include: (1) a very high density of structural and stratigraphic/structural leads and prospects, (2) the ready availability of high quality reservoir in deepwater channels and fans, and (3) focused maturation of oil-prone source rocks caused by the Tertiary depocenters. The source rocks are varied and have been found in synrift, post-rift and deltaic settings. Trap types have included channels over deep-seated salt-or-shale structural highs, flanks and faulted flanks of salt or shale minibasins, and submarine fans draped over underlying structural highs. Exploration in plays in the upper slope portions of some of these basins is beginning to show signs of creaming, as new play development continues to move outboard and deeper. In the latter part of the last decade, exploration plays have been made in the distal-most structured portions of some of these basins.

Where then might the next prolific frontier basins and/or plays in the deepwater theatre be found? Many ideas that are being pursued in our industry, or are still on the drawing boards, include

1. Moving still farther downdip of these focus basins into the largely unstructured basin floor fans that lie on the toe of slope and abyssal plain. Although analogs exist for successful traps in this setting given at least low angle structural inclination, clear difficulties include the occurrence of thermal maturity and the inferred low density of traps.
2. Moving deeper within these focus basins, especially to non-amplitude supported and/or presalt objectives.
3. Moving laterally from the mobile substrate basins into less structured provinces nearby, that still have sufficient sedimentary section to provide reservoir and mature source rocks.
4. Exploring other, similar mobile substrate basins.
5. Exploring basins with other structural origins, for example transform margins, or fold and thrust belts in active margin settings.
6. Exploring non-turbidite (e.g., carbonate) reservoirs in deepwater settings.

These and other more novel play concepts will be required to continue the rapid growth seen in the past two decades of the deepwater play.