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ABSTRACT: Natural Gas Distribution, Entrapment, and Production Phenomenon within the Upper Cretaceous Forbes Formation Depositional System, Northern Sacramento Basin, CA

E. P. Horan, III

A comprehensive geologic review and computer database of 1500+ wells, integrating geologic, test, production and performance data, provided for a quantitative, detailed study of gas entrapment and production phenomenon within the complex Forbes depositional system. Stratigraphic trapping is provided by thin, lenticular, discontinuous reservoir sands. More than 2 tcf of gas reserves from over 850 gas wells has been discovered, defining hundreds of small traps. Maximum deliverability is from completions in thick-bedded (> 15 ft) sands. High cumulative production results from areally extensive reservoir systems, up to 20 bcf/well for wells with multiple gas sands. Sequences of thin-bedded (< 5 ft) sands are typically capable of only marginal deliverability unless compl ted over 100 ft or more of interval. Structural position of the completed zone within the pool is a minor factor in improving production rates or ultimate recovery due to near-universal pressure-depletion reservoir drives. Reservoirs are generally overpressured with no significant recovery of gas obtained from reservoirs with pressure gradients higher than 0.71 psi/ft. Correlating regional geology with production and performance trend maps, demonstrates, that the greatest concentration of traps and gas reserves occur in regions of structural anomaly: along basement features, faults, or volcanic uplifts. Slight reversals of regional dip dramatically enhance stratigraphic trapping as opposed to areas of regional homoclinal dip with comparable available reservoir. Given locally immature sou ce rocks, gas migration over long distances is inferred. Understanding migration timing and pathways will play an increasingly important role in future exploration.

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