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Abstract

Casablanca Field, offshore Spain, produces oil from karsted Jurassic - Cretaceous carbonates. Subaerial exposure that produced the paleokarst was significant and affected up to 386 meters of section. Locally, karst dissolution was extensive enough to form large, solution-enhanced fractures or small, probably horizontal, caves. Multiple phreatic zones that developed during regional uplift probably produced the various cave levels recognized in cores.

Cores contain representative and distinctive attributes of paleokarst including breccias, cave-fill sediment, and fractures. Fitted, mosaic, and rubble breccias which are distributed throughout the cored interval formed in part during cave-roof collapse and compaction of cave-fill sediments. The cave-fill is principally dolomitized carbonate mud or clast-supported sediment that is red in the upper portions of the cored interval and green in the lower portions. Fractures, in which a significant volume of the reservoir pore volume is contained, formed during both karst-collapse and tectonism.

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