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Assessing hydrocarbon futures in unexplored basins involves geology by analogy. These assessments are needed to help quantify the amount of oil and gas postulated. It is important for the geology and geologic history of known producing basins to be defined in some systematic way, so that their favorable or unfavorable attributes may be recognized, and subsequently looked for in untested basins. The California and Arabian analogs are important.
Through 1978, approximately 265 fields were discovered in California, containing 22 billion bbl of oil, 53% being in the 10 largest fields, ranging in size from 0.6 to 2.4 billion bbl. These fields occur in several different sedimentary basins. Through 1978, about 50 fields were found in Saudi Arabia containing 206 billion bbl of oil, 78% in the 10 largest fields, ranging in size from 7 to 83 billion bbl. All these fields occur in one part of a single very large basin. The contrasts in field size distribution and in the total amount of oil present are explained by the dramatically different geology and geologic histories.
California's surface geology is characterized by rare Precambrian, isolated Paleozoic, and widespread Mesozoic accreted terranes and intrusions, and by highly uplifted and depressed Tertiary sedimentary prisms bounded by widespread high-angle thrusting and strike-slip and normal faulting. Numerous families of medium to small anticlines and fault traps, commonly involving moderately dipping to overturned beds, have resulted from Tertiary tectonism, which segmented California dramatically. The sediments associated with the oil and gas are largely local fine to coarse-grained clastics, shed from nearby highlands, but with one important regional chert-limestone-dolomite sequence.
Saudi Arabia is characterized by a broad Precambrian shield area, flanked on the east by very long, gently dipping cuestas of Paleozoic and Mesozoic sediments, with an upper thin veneer of nearly flat Tertiary strata. Most structures involving the Mesozoic and Cenozoic are large, but gentle and unfaulted, representing a passive reaction of the sediments to underlying mild basement distortion and/or movement of Cambrian salt, all occurring while the Arabian plate continued to subside and "tip" to the northeast. The sediments associated with the oil are largely widespread carbonates of uniform thickness, with one interbedded sandstone wedge, and some shale.
The contrasts between California and Saudi Arabia oil fields and geology result from contrasting plate-tectonic settings and history.
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