Norphlet Formation (Upper Jurassic) of Southwestern and Offshore Alabama: Environments of Deposition and Petroleum Geology

Abstract

Upper Jurassic Norphlet sediments in southwestern and offshore Alabama accumulated under arid climatic conditions. The Appalachian Mountains of the eastern United States extended into southwestern Alabama to provide a barrier for air and water circulation during the deposition of the Norphlet Formation. These mountains produced topographic conditions that contributed to the arid climate, and they affected sedimentation. Norphlet paleogeography in southwestern Alabama was dominated by a broad desert plain, rimmed to the north and east by the Appalachians and to the south by a developing shallow sea. The desert plain extended westward into eastern and central Mississippi. Norphlet sedimentation initiated as a result of basin subsidence accompanied by erosion of the southern Appalachians. Norphlet conglomerates were deposited in coalescing alluvial fans in proximity to an Appalachian source. The conglomeratic sandstones grade downdip into red beds that accumulated in distal portions of alluvial fan and wadi systems. Quartz-rich sandstones were deposited as dune and interdune sediments on a broad desert plain. The principal source of the sand was updip alluvial fan and plain and wadi deposits. Wadi and playa lake sediments also accumulated in the interdune areas. A marine transgression was initiated during the late phase of deposition of the Norphlet Formation, resulting in the reworking of previously deposited Norphlet sediments. Norphlet hydrocarbon potential in southwestern and offshore Alabama is excellent; six oil and gas fields already have been established. Petroleum traps discovered to date are primarily structural traps involving salt anticlines, faulted salt anticlines, and extensional fault traps associated with salt movement. Reservoir rocks consist primarily of quartz-rich sandstones that are eolian, wadi, and marine in origin. Porosity is principally secondary (dissolution) with some intergranular porosity. Smackover algal carbonate mudstones were probably the source for the Norphlet hydrocarbons. Jurassic oil generation and migration probably were initiated in the Early Cretaceous.