The Hibernia oil field is the largest discovery off the east coast of North America. The most important reservoir unit in the field is the Upper Jurassic-Lower Cretaceous Hibernia formation, which averages 200 m in thickness and occurs at depths between 3,475 and 4,200 m. On the basis of sedimentological descriptions of cores and downhole log responses, five lithofacies have been defined, and the formation has been subdivided into two lithostratigraphic units. The "Main Hibernia zone" is dominated by thick medium to very coarse-grained quartzarenites, interpreted as distributary channel deposits of a deltaic plain environment. The "Upper Hibernia zone" consists of relatively thin very fine to medium-grained quartzarenites interbedded with mudstones and siltstones. This zo e is interpreted as deposits of a shallow marine delta-front environment.

Porosities observed in thin section and provided by core analysis in the Hibernia sandstones range from 1 to 22%. Many sandstones with high porosities show evidence of dissolution of carbonate cement and some framework grains. This late enhancement of porosity by decarbonatization preceded main hydrocarbon migration and is a major factor in both the accumulation of large reserves and the potential producibility of the field. Lower porosities are associated mainly with well-compacted sandstones or those retaining abundant unleached carbonate cements. Time-temperature index modeling constrains the interpreted time and depth of hydrocarbon generation and accumulation in the sandstone reservoirs.