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ABSTRACT: Casablanca Oilfield, Spain Karsted Carbonate Trap

D. E. Orlopp

The Casablanca field is Spain's largest oil field with a reserve exceeding 110 million bbl. Ultimate primary recovery will probably range between 45-50%. Peak production was in 1983 at a rate of 44,000 BOPD.

Cumulative oil production at year-end 1989 was about 95.4 million bbl. The oil is a high-quality paraffin crude with a low sulfur content of 0.2% and a low gas/oil ratio of 70 standard ft3 bbl. Oil gravity is 34° API.

The Casablanca reservoir and trap were created initially as a result of interplate movement between Africa and Europe during the early Tertiary time. Paleogene uplift raised and fractured the Mesozoic carbonate platform of the Gulf of Valencia and thereby allowed hydrologic conditions to create an extensive karst system.

Crustal thinning in the late Oligocene-early Miocene allowed the subsidence and burial of the karsted carbonate plateau. The initial Miocene beds are facies variants of the Alcanar group which represents an onlapping sequence extending from the Aquitanian to Serravalian.

The Alcanar ranges vertically and laterally from a carbonate conglomerate to chalky micrite to moderately kerogenous marlstones. The latter facies is subregional in extent and serves as the only significant hydrocarbon source rock in the Gulf of Valencia. It also serves as the cap rock for the oil fields of the region.

The Alcanar source rock is not particularly rich. Ttoal organic carbon values are generally below 2% and rarely reach 4%. The hydrocarbon yield is modest at 5 L/MT. The source rock generally is oil prone, less commonly mixed, and rarely gas prone. It is considered as a degraded type II kerogen deposited under dysaerobic conditions.

The source rock reached peak maturity in the early Pliocene (4.0 m.y.), but only in the Tarragona depocenter west of the Casablanca trend.

In a structural sense, the field is a corroded carbonate ridge with five separate paleohills or erosional culminations. Flank boundaries represent back-eroded fault traces. Generally, the structure in plan is rugose and in profile is irregular.

Reservoir character is strongly variant both laterally and vertically. The reservoir closure is about 270 m and was initially filled nearly to the spillpoint. The reservoir limestones and dolomites generally tend to be dense with only fair matrix porosities. Average porosities are 8-11% and matrix permeabilities are on the order of 10 md. Karst enhancement of the reservoir character is significant and has increased overall effective reservoir permeability to 2 d.

Reservoir pressure performance is supported by an active water drive. Pressure decline has virtually stabilized as the oil/water column has risen into the zones of better porosity and permeability. Water cuts were very low until 1989.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990