The Forties field is located nearly 180 km east-northeast of Aberdeen mainly in UK Licence Block 21/10, and was discovered in October 1970 in Paleocene sandstones of the Forties Formation. Four appraisal wells drilled during 1971-1972 proved the existence of a giant oil field with an area of some 90 sq km and (on current evidence) an estimated oil-in-place reserve of 4 billion barrels.

Additional geologic data from the 50 development wells drilled to date show rapid facies variations over the field with some sand bodies having a cross section less than the prime well spacing of 700 m. A large, partly isolated sand body, the Charlie Sand, is recognized in the upper, western part of the reservoir.

The sandstones and shales of the Forties Formation are considered to have been deposited in a middle and lower submarine fan environment. A mixture of sedimentary processes including grain flow, debris flow, and turbidity currents are identified and four broad facies were defined from cores. The facies types indicate significant vertical associations that are related to specific depositional environments and recognized by petrophysical log patterns.

Detailed lithofacies studies, together with pressure-decline data and log-pattern analyses, provide a practical means of correlating and mapping the complex sand geometry. These techniques provide a method for good well location and a better understanding of the reservoir performance.