ABSTRACT

North Choctaw Ridge (NCR) Field is in a structural trap that produces from the Jurassic Smackover Formation in the Gilbertown graben system, near the updip limit of the Louann Salt. The field is in a footwall uplift below the southernmost of the three faults that form the northern boundary of the graben system. The top seal is anhydrite in the overlying Buckner Member of the Haynesville Formation, and the boundary fault provides a lateral seal. The geometry and evolution of the structure has been determined from a three-dimensional interpretation of the graben system and from a balanced and sequentially restorable cross section. The cross section is both area and length balanced with a lower detachment in the Louann Salt. Significant structural growth began in Early Cretaceous time when three major south-dipping, down-to-the-Gulf faults formed, creating an imbricate system of half grabens (the Melvin fault system). Regional subsidence in the hangingwall of the Melvin fault system indicates contemporaneous salt deflation, with the NCR Field subsiding the least. The north-dipping boundary fault of the Gilbertown graben system (East Gilbertown fault) developed in the Late Cretaceous. Late Cretaceous and Tertiary extension produced a full graben between the East Gilbertown and Melvin C faults.