One Sea Level Fall and Four Different Gas Plays: the Gulf of Cádiz Basin, SW Spain

Mojonero, C.G.,and Wenceslao Martínez del Olmo, Repsol-YPF E&P, Madrid, Spain

Abstract

The Messinian sea level fall generated three different and synchronous turbiditic systems in the Gulf of Cádiz. Close to the passive margin of the basin (Iberian craton), sediment was transported down a valley forming an excellent example of a lowstand wedge having a normal backstepping facies assemblages: basin floor fan, slope fan complex and prograding complex, rapidly waning in directions both parallel and perpendicular to flow. Sediment bypassing can be estimated to have occurred for over 70-90 km. Nine biogenic gas fields have been found in this system.

On the active margin side, (Betics Ridge), two distinct turbidite systems are present:

• The first consists of sheet turbidites carried off the shelf edge and deposited in the gradient change between basin floor and slope. They are in front of a river paleo-mouth that eroded its own shoreface deposits. Classical lowstand facies are not fully developed and quickly deteriorate. There is almost no sediment bypassing.
• The second corresponds to turbidite systems of multi-storied, confined channel complex type. They begin in the upper part of the slope. Channels are of medium to low sinuosity and their dimensions are comparable to the ones discovered in West Africa.

In addition to turbidite processes, strong near-bottom currents, created by the interaction between the hypersaline Mediterranean waters with the less saline Atlantic waters along the Strait of Gibraltar, lead to the deposition of sandy contourites along the deep water Gulf of Cádiz continental slope. This play consists of more than 800m of upper Pliocene-Quaternary sand-shale section with exceptional petrophysical characteristics.