Current hydrocarbon production in Saudi Arabia is from reservoirs of Cretaceous and Jurassic age. Geochemical studies of the sediments and oils suggest that the hydrocarbons were derived from two separate source-rock provinces. Oil production from the large fields in the southern part of the area is from Jurassic carbonate reservoirs. Most of these oils were derived from thermally mature, thinly laminated, organic-rich carbonate rocks of Jurassic age (Callovian-Kimmeridgian). These source rocks were deposited in an intra-shelf basin which is limited to the southern part of the main producing areas. Extensive vertical migration of oils in these sediments is prevented by superjacent evaporite seals deposited during Late Jurassic.

Fields in the westernmost and northern producing areas appear to have derived their hydrocarbons from a source-rock province to the north. Production from Cretaceous clastic and carbonate reservoirs is limited to the northeastern part of the producing areas. This distribution may be explained by limitation of thermally mature Cretaceous source rocks to the northeastern areas or by the lack of subjacent evaporite seals to separate these reservoirs from Jurassic source rocks.

Thermal maturation studies indicate that the hydrocarbons in Mesozoic reservoirs migrated into the present traps during the early Tertiary.

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