The Cumarebo oil field, situated near the coast in northeast Falcon, is operated, and now entirely owned, by the Creole Petroleum Corporation. It was discovered in 1931 and development was largely completed by 1940, but intermittent drilling was continued until 1947. The total number of wells in and around the field is 133. The proved area, 550 acres, is about 2 miles long with an average width of ½ mile. The gravity of the oil produced ranges from 47.0°-50.5° API.

The structure is an elongate, sharply folded, severely faulted, asymmetric dome on the northwest flank of a faulted regional fold, both features trending northeastward. The regional and associated branch faults intersecting the dome preceded its folding and thus controlled the distribution of the hydrocarbons which migrated into it. Migration was mostly from the northwest. Of the six segments into which these older, major faults divide the field, the four inner segments, which were not cut off by faults from the source area, are commercially oil-productive; the two outer segments, which were cut off, are oil-poor. A younger system of epianticlinal faults also intersects the structure. All the faults are normal faults; the apparent downthrow is commonly northeast or east, with displace ents ranging between 25 and 1,350 feet.

The strata cropping out in the field belong to the middle Miocene Caujarao formation. Production is obtained from the ten oldest sands of this formation and from the two sands in the uppermost 850 feet of the lower Miocene-upper Oligocene Mosquito formation (a new name: other new formation and member names are also published in this paper). These sands, ranging in net sand thickness from 16 to 178 feet, have not been found productive elsewhere in the area. Production from older beds has also been sought, unsuccessfully, both on the Cumarebo structure and elsewhere in the area.

Because of the step faulting and multiple sands, producing depths in the field vary widely, ranging between 600 and 2,893 feet. All the reservoirs now produce by gas-cap drive and there is also a water drive on one flank of the biggest reservoir. Pressure maintenance by gas injection was started in 1933; the project was tried in all the more important reservoirs but is now confined to the two largest with highly beneficial results. The maximum rate of oil production was attained in 1933, with a daily average of 13,511 barrels. At the end of 1950, the potential of the field was 4,000 barrels of oil per day. The cumulative production at December 31, 1950, was 42,365,200 barrels of oil.