Quantifying Secondary Gas Resources in Fluvial/Deltaic Reservoirs: A Case History from Stratton Field, South Texas

Geologic and engineering studies were conducted within fluvial/deltaic strata of a mature South Texas field to determine the impact of reservoir compartmentalization on secondary gas recovery. Stratton is one of several mature South Texas fields containing fluvial and deltaic gas reservoirs. This study documents the effect of depositionally controlled reservoir heterogeneity in a field with fluvially dominated reservoir architecture. Stratigraphic and production analysis of natural gas reservoirs within the fluvial facies tract of the Oligocene-age middle Frio Formation demonstrates the importance of evaluating gas-well completion spacing in areas containing multiple stacked reservoirs. Historical analyses of a densely drilled, contiguous 10-mi2 area in this field demonstrate that production decline and reserves depletion were slowed by an aggressive infield reexploration program. A quantitative case history developed during this study evaluates infield drilling and recompletions and documents the greater than 90-percent reserve replacement that was achieved after 42 years of development between 1937 and 1979.Depositionally heterogeneous fluvial reservoirs are prime candidates for the development of incremental hydrocarbon resources. An integrated geologic and engineering analysis of successful reserve growth at less than 320-acre completion spacing within heterogeneous gas reservoirs provided an infield reservoir compartment nomenclature that can be used to evaluate gas reservoirs in other fields. Three reservoir case studies outlined in this report show that reserve additions come from new infield reservoirs, untapped reservoir compartnzents, and incompletely drained reservoir compartments. Secondary (or incremental) gas reserves in the study area are greater than 65 Bcf. Economic analysis of a major infield development program indicates a finding cost of less than $0.70 per Mcf (in 1990 dollars). Documenting the occurrence of natural gas reserve growth and the steps leading to the geologic and engineering identification of secondary gas resources will help operators improve recovery from these fluvial heterogeneous reservoirs.