Serrablo Gas Field: An Example of a Trap in Syntectonic Brecciated Reservoirs, Pyrenees (Spain)

Pedro Camara, Jose Rodriguez de Pedro, Jose Ma Moreno

The Serrablo field constitutes a peculiar case of gas trap in tectonic-related carbonate megabeds. These reservoirs are formed by a conglomeratic and brecciated basal zone (debris flow), where gas is produced mainly from fractures. In the upper part, a sandy turbiditic sequence culminates this kind of deposit. There are several gas-bearing megabeds, each with an independent gas-water contact.

The carbonate materials composing the megabeds are the result of the partial destruction of ramp anticlines formed in the Jaca basin by a piggy-back thrust sequence, developed during the earlier stages of deformation in the early Eocene.

The megabeds have an extension of tens of kilometers in an east-west direction (parallel to tip-lines' thrust) and a few kilometers in a north-south orientation. Thickness varies from a few to 100 m, occasionally reaching 300 m.

The thrust-top basins created between thrusts were infilled by different turbiditic systems and intercalated with several units of megabeds. These assemblages migrated southward while each megabed within every assemblage shifted eastward. This arrangement thus reflects the sense of movement of the earliest Jaca basin thrust sheets.

Slope deposits were formed on the rear limb of each ramp anticline. These deposits (Burgui marls) and several other levels on the flysch megasequence are the source rock for hydrocarbons generated probably during the late Tertiary. The migrated hydrocarbons were trapped in structures created by the latest stages of thrusting (late Eocene and Oligocene) when the Jaca basin was sliding over the Ebro basin and when the main uplift of the Axial Pyrenean zone was taking place.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.