A "healed slope" model for the deposition of turbidite reservoirs is applied to Shell’s Zia and Oregano discoveries in the deep water Gulf of Mexico

Mitchell, B. T., and M. C. Dean, Shell Offshore, New Orleans, LA

Abstract

In recent years, Shell Offshore, Inc. has made several smaller volume discoveries in the deepwater Gulf of Mexico including Zia (Mississippi Canyon 496) in 1998 and Oregano (Garden Banks 559) in 1999. Prospects such as these are not large enough to justify their own fixed production facility, but rather are profitable only if they can be tied back to an existing host platform via a subsea development. For the primary pay sands at Zia and Oregano, an understanding of the depositional environment and reservoir architecture has been an important component of the development planning.

The primary oil-bearing reservoirs at both Oregano and Zia are examples of turbidite sands that are deposited in what has been referred to as a "healed slope" depositional setting (Booth et al., 2000). The stratigraphic interval in which these reservoirs occur is generally a bypass setting; most of the sand in the vicinity is diverted out into the basins in deeper water. However, in the case of Oregano and Zia, a local topographic indentation or change in slope has formed along the seafloor, possibly in response to salt movement nearby. Turbidite sands transported down from the shelf encounter the break in slope at the edge of the indentation and drop out of suspension as a local area of amalgamated turbidite fan complexes (sheet sands). The area of topographic relief is quickly filled or "healed over" by these initial sands being deposited and the system transitions back into the bypass mode. Subsequent turbidite sands may be deposited on top of the "healing phase" sheets as amalgamated channel complexes. In the case of both Oregano and Zia the channel sands cut into the underlying sheet sands and may improve reservoir connectivity. Finally, as the last slight topographic variations are smoothed out, the system returns to being a bypass system again and younger shale-filled channels are observed on seismic 3D data that often partially cut into the underlying reservoir and help to trap the accumulation.

In turbidite reservoirs formed by this type of process, the proximal end of the amalgamated fan complex is a higher energy depositional setting where there is often a predominance of sand-on-sand contacts. This is also the place where the overlying amalgamated channel complexes may cut into the sheet sands beneath, providing many different mechanisms for interconnected sand bodies. The distal end, by contrast, will have a lower overall net-to-gross and interbedded shales that can isolate portions of the reservoir. Understanding these components of the depositional model and their areal relationship to the depth structure of the accumulation is important in planning a successful development.