The Casablanca field, which is the largest of four producing fields in the Spanish Mediterranean, is located about 30 mi (48 km) south of Tarragona. Using the Halbouty classification in AAPG Memoir 16, it is a "paleogeomorphic trap."
The field is producing about 18,000 b/d of oil from two wells in an "early production phase." The crude has a density of 33.7° API, sulfur content of 0.2% and a gas/oil ratio (GOR) of 155 cu ft/bbl. Full daily field production of about 35,000 to 40,000 b/d of oil will start in 1982 after a platform is installed.
The reservoir is a weathered and fractured Upper Jurassic carbonate rock. The primary matrix porosity averages about 3%. However, weathering during the early Tertiary has enlarged fractures and previously existing porous zones to a depth of 300 to 500 ft (91 to 152 m) below the top of the carbonate rocks. This secondary porosity in combination with an extensive fracture network has converted the otherwise dense carbonate into a commercially exploitable reservoir.
A contour map on the top of the eroded Mesozoic carbonate reservoir defines a closure 6.8 by 1.5 mi (11 by 2.5 km). The field is elongate parallel with Miocene faults with an overall configuration of a rounded limestone ridge. The ridge is covered by middle Miocene organic-rich marls which were the source of the oil. These and younger shales are a cap for the accumulation.
The use of the paleogeomorphic model aids the interpreter in mapping data in the Casablanca area which otherwise would seem uninterpretable, or at least, difficult to interpret. The model may be of use in other carbonate areas.
Figures & Tables
The papers included in this volume reflect both the geological and geophysical rationale used in searching for the subtle trap. The scope of the papers ranges from the general appraisal papers of new concepts and methods, to those relating to specific fields in the United States, Nigeria, China, Australia, Canada, Oman, offshore Spain, and the North Sea. The 21 chapters are sourced from a 1981 AAPG Annual Meeting session, dedicated to providing explorationists with the information needed to search for and discover the stratigraphic, paleogeomorphic, and unconformity-oriented subtle-trap accumulations of oil and gas.