Stratigraphy and Mineralogy of the Oxfordian Lower Smackover Formation in the Eastern Gulf of Mexico

Abstract

Abstract The Oxfordian Smackover Formation is generally acknowledged to be a hydrocarbon source for numerous reservoirs in the Gulf of Mexico, both onshore and offshore. More than 25 wells in the eastern Gulf of Mexico have penetrated the Smackover since 2003. Offshore, the Smackover consists predominantly of limestone and shale containing thin organic layers. Immediately above the lower Smackover is a widespread shale marker. This thin shale is correlated as the base of the upper Smackover Formation, which consists of interbedded shale and limestone. This study will demonstrate that the lower Smackover Formation in the eastern Gulf of Mexico (Mississippi Canyon and De Soto Canyon offshore areas) is composed of a series of seven units that occur in the same sequence in virtually every well in which the lower Smackover has been encountered. Although the seven individual units can be resolved readily with the proper wireline suite, each has a sub-seismic thickness. The overall thickness of the lower Smackover is about 300 +/-100 feet. Unlike the lower Smackover, the surrounding Mesozoic formations, from Cotton Valley to Norphlet, vary greatly in thickness in the eastern Gulf. The initial correlations of the units in the lower Smackover were made by comparing the gamma ray, resistivity, and density log patterns with the computed mineralogy of Elemental Capture Spectroscopy (ECS) wireline logs. It was immediately obvious that the same sequence of beds/units was present in the lower Smackover in well after well. Within the lower Smackover Formation is a conspicuous zone characterized by iron-bearing minerals having a matrix density in excess of 3.0 g/cm3 throughout. However, X-Ray Diffraction (XRD) data from rotary sidewall cores was necessary to validate the mineralogy. Because the mineralogy of the ECS log is a model-based calculation from the elemental concentrations of iron, calcium, aluminum, etc,. rather than a direct measurement, the modeled mineralogy can be inaccurate as was the case in the bottom two units. Mineralogy of the seven units has been verified by XRD analyses, albeit from a limited number of rotary sidewall cores obtained in only five wells. The top three units are limestones which vary in carbonate, clay, and pyrite content. The fourth and fifth units contain significant amounts of high density minerals, particularly siderite and pyrite. The sixth zone is dominated by anhydrite. The seventh unit is a hematite-rich shale and its base is an unconformity. Although wireline data are plentiful, analysis of the seven units within the lower Smackover is hampered by the limited amount of rock data and the complete lack of whole core. Many depositional and geochemical questions suggested by the unusual mineralogy and sequence of beds remain unanswered.