Announced discoveries at Great White, Trident, Tobago, Silver Tip and Tiger have established the Perdido fold belt, Gulf of Mexico, as a significant new ultra-deepwater petroleum province in the initial stages of evaluation. Great White Prospect was drilled to 19,907 ft and is beneath approximately 8,000 ft of water. The play has now moved beneath salt with the recent Diamondback prospect test. Three additional subsalt exploration tests are planned for 2005 and 2006 at prospects called Leopard, Whale and Ontario.

Prospect Baha (Alaminos Canyon Block 600), a high-relief four-way closure drilled in 2001, detected residual oil in multiple Oligocene and Paleogene turbidite sands and established the presence of an active petroleum system. Prospect Trident, the first Perdido discovery drilled in 2002, encountered multiple pay-bearing Paleogene sands trapped in a low-relief four-way closure. Great White (Alaminos Canyon 857), also drilled in 2002, found oil in three different Oligocene and Paleocene to Eocene turbidite sand packages that are trapped in within a moderate- relief, four-way closure. Tobago prospect (AC 859) encountered hydrocarbons in a Paleocene age stratigraphic trap.

Perdido folds trend northeast-southwest. These folds are segmented along strike by low-relief saddle, and appear to have autochthonous salt cores. In a dip direction, folds deepen from west to east and are separated by deep synclines. Folds diminish eastward as autochthonous salt becomes thin. Much of the play is covered by tabular allochthonous salt. Shell geologists can identify three distinct play segments— “Eastern Subsalt,” “Western Subsalt” and “Outboard” (no allochthonous salt). The Perdido area has a high geothermal gradient causing rapid degradation of porosity and permeability with depth, the prediction of which is a key to risking and ranking Perdido prospects.

Unnumbered Figure.

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The Great White Prospect

Great White is a large, doubly-plunging, thrust-propagation fold with numerous crestal collapse normal faults. Three main pay intervals exist, including a stacked sand series in the Frio, a single pay sand in the Eocene Upper Wilcox and a thick sand sequence, partially pay bearing, in the Paleocene Lower Wilcox. All the sands comprise low- and high-density turbidites with minor debris flow components, but the composition, texture and diagenesis vary markedly. Reservoir porosities range from near 40% shallow to less than 10% near TD.

Oil properties also vary significantly among the reservoirs at Great White ranging from low-API crude shallow to high-API crude deep. The interval is largely hydropressured, with an interpreted pressure leak point on a large structure to the west, providing a protected trap. Key appraisal challenges include understanding variable reservoir quality as well as hydrocarbon distribution and connectivity in the various fault blocks.

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