Depositional Environments and Petroleum Geology of Jurassic Eolian Deposits (Norphlet Formation), Eastern Gulf of Mexico Area: ABSTRACT

Abstract

Jurassic Norphlet sediments in the eastern Gulf of Mexico area accumulated under arid climatic conditions. The accumulation of thick Jurassic salt deposits, anhydrites, and red beds in association with Norphlet sandstones indicates that arid climatic conditions were prevalent during Norphlet deposition. Usually an association of salt deposits, anhydrites, and red beds is characteristic of arid climatic conditions, and eolian sands can be expected to accumulate under such depositional conditions. The Appalachian Mountains of the eastern United States extended into southwestern Alabama and provided a barrier for air and water circulation during the deposition of the Norphlet Formation. These mountains produced the topographic conditions that contributed to the arid climate. In the region, Appalachian structural features are recognized as basement ridges and arches, such as the Conecuh and Pensacola ridges and associated Wiggins arch. These paleohighs affected Norphlet sedimentation and acted as local sediment sources. Norphlet paleogeography in the eastern Gulf coastal plain 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 began as a result of basin subsidence accompanied by erosion of the southern Appalachians. Norphlet conglomerates were deposited in coalescing alluvial fans near 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 during the late phase of deposition of the Norphlet Formation resulted in the reworking of previously deposited Norphlet sediments. To date, 35 Norphlet oil and gas fields have been established in the region. Petroleum traps discovered are principally structural traps involving salt anticlines, faulted salt anticlines, and extensional fault traps associated with salt movement. Although basement highs also have potential as petroleum traps in the area, salt movement is the critical factor in forming a petroleum trap. Numerous Norphlet fields are located along the regional peripheral fault trend, particularly in association with the Pollard-Foshee fault system in southern Alabama and the Florida panhandle. Other onshore Norphlet petroleum traps include salt anticlines, such as Copeland, and salt grabens, such as the Mobile graben. In Mississippi, several Norphlet fields are located near the Jackson dome, a Cretaceou igneous intrusion. The Norphlet fields discovered in offshore Alabama are along the Lower Mobile Bay fault trend. The petroleum traps in the offshore area include a series of generally east-west-trending salt anticlines. Reservoir rocks consist primarily of quartz-rich sandstones of eolian, wadi, and marine origin. The average composition of these quartz-rich sandstones is 72.5% quartz, 15.0% feldspar (plagioclase, microcline, and orthoclase), 4.4% rock fragments (chert, shale, phyllite, schist, and quartzite), 3.8% cement (carbonate, quartz, and anhydrite), 3.2% authigenic clay, and 1.1% accessory minerals. Porosity includes primary intergranular and secondary intergranular, and intragranular developed as a result of decementation and grain dissolution. Porosity in Norphlet reservoirs can exceed 25%. The primary source of hydrocarbons in the Norphlet reservoirs is Smackover carbonate mudstones. Norphlet shale samples analyzed were found to be low in total organic carbon (0.1-0.2%). Smackover carbonate mudstones are locally rich in algal and amorphous kerogen. The geochemical and carbon isotopic composition of Norphlet crude oils compares favorably with the composition of Smackover crude oils and Smackover carbonate mudstones. End_of_Article - Last_Page 1441------------