ABSTRACT
The Frio Formation (Oligocene-Miocene) is one of the major Tertiary progradational wedges of the Texas Gulf coastal plain and has yielded nearly 6 billion bbl of oil and 60 tcf of gas. The Frio, and its updip equivalent, the Catahoula Formation, consists of deposits of two large fluvial and associated deltaic systems, centered in the Houston and Rio Grande embayments. Structural history in the Houston embayment is dominated by syndepositional deformation of underlying Jurassic salt; mobilization of thick, undercompacted prodelta and slope muds characterized the tectonic evolution of the deltaic sequence in the Rio Grande embayment. These two major deltaic depocenters are separated by a vertically stacked; strike-parallel coastal barrier and strand-plain system. Underlying, interbedded, and transgressive shelf, prodelta, and continental slope mudstone sequences provide principal source and sealing facies. Sparse organic geochemical data, regional thermal gradients, and distribution patterns of hydrocarbons show that large volumes of oil and gas have likely been generated within and effectively expulsed upward and landward from thermally mature, normally to moderately undercompacted sequences of bounding mudstone facies.
All Frio depositional systems contain major, geologically defined, hydrocarbon-producing plays. Oil and gas field productivity data show a log normal frequency distribution for all but the largest fields. Per volume productivity, production styles, and types of hydrocarbons within each of the ten recognized plays reflect available source rock types, differing thermal and compaction histories, and variable reservoir and trap configurations that characterize each depositional system. Systematic distribution patterns of both physical and chemical properties of produced hydrocarbons can be related to source facies, regional thermal regime, and post generation modification by (1) continued maturation, thermal cracking, and deasphalting, (2) migration and attendant chromatographic separation, and (3) bacterial alteration.
