Abstract: Casablanca Field, Offshore Spain: A Karsted Carbonate Reservoir

Anthony J. Lomando, Paul M. Harris

Casablanca field, offshore Spain, produces oil from karsted Jurassic-Cretaceous carbonates. The structure of the field, which measures 1 × 11 km in map view, portrays the karsted and eroded remnant of a paleotopographic, fault-bounded ridge. Locally, karst dissolution was extensive enough to form large solution-enhanced fractures or small, probably horizontal, caves. The karsted and eroded Mesozoic sequence is overlain unconformably by the onlapping sediments of the middle Miocene Alcanar Group.

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 parts of the cored interval and green in the lower parts. Fractures, in which a significant volume of the reservoir pore volume is contained, formed during both karst collapse and tectonism.

Multiple phreatic zones and cave levels likely formed the various "caves" recognized in the field. The paleophreatic zone was inferred to be "multilevel" from examination of downhole logs in deep wells in the field, as well as from published outcrop studies in the coastal Catalan Ranges. While the area was being regionally uplifted during Tertiary orogenesis, karstification would have proceeded progressively downward with local base level. At Casablanca, this is interpreted to have formed an overall karsted profile up to approximately 386 mm in thickness.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994