Multiproxy Analysis of Paleoenvironments in the Norphlet Formation: Conecuh Sub-Basin, Escambia County, Alabama

Abstract

Reservoirs in the Jurassic Norphlet Formation have been major hydrocarbon targets for decades in the Gulf Coastal Plain, and recent discoveries offshore in the eastern Gulf of Mexico have spurred renewed interest in this extensive aeolian unit. Focusing on the Flomaton Field within the Conecuh Sub-basin of southern Alabama, core lithofacies and well petrofacies are characterized to investigate dryland-fluvial environments from sedimentary facies in eight cores. Sedimentary facies are related to multi-proxy datasets, including well logs, pseudo-gamma logs from scintillometer core measurements, elemental composition from handheld X-ray Fluorescence and Laser-Induction Breakdown Spectroscopy devices, particle size, microfabrics from thin sections, and porosity and permeability from core plugs. The lithofacies-petrofacies relationships developed in this study allows for sedimentary facies to be interpreted from petrophysical data in wells without cores, which has implications for reservoir quality. The sedimentary facies of the Conecuh Sub-basin include alluvial, aeolian sheetsand (avalanche and wind ripple facies), fluvial/wadi environments, and reworked sand that only occurs in the top of the Norphlet Formation lithostratographic unit. In the Conecuh Sub-basin, alluvial deposits are prevalent updip with greater proximity to the Appalachians. Deposits from aeolian and reworked aeolian sands, which are favorable to higher reservoir quality (up to 110 mD in permeability and 15-20% porosity in aeolian facies and >25mD in permeability and >15% porosity in reworked sand facies), are found throughout the study area, but are thickest (up to 52 ft for reworked sand and up to 84 feet for aeolian deposits) and most prevalent downdip. Reworked sand is prevalent in the top of the Norphlet Formation and thickens (from < 1 ft up to 52 ft) to the southeast and southwest of the study area. Aeolian petrofacies can be found where there are moderate gamma ray (~25 to 75 API), low neutron porosity (~0 to 25), slightly negative to low density porosity (~-10 to 25), spontaneous potential concentrated around ~45 to 20, and variable from~-100 to150 mV, and moderate resistivity (~100 or range from ~10 to ~1000). Elemental composition analysis may aid in the identification of aeolian facies, as aeolian facies can be found where both marine (Ca, and Mn, and Mg) and terrestrial (K, Cr, Rb, Ti, V, and Y) elemental concentrations are relatively low. This study determines the dryland-fluvial facies and reservoir quality distribution in an onshore Norphlet play, which should improve predictive modeling of reservoir distribution in areas of frontier exploration, including deepwater plays of the Gulf of Mexico and greenfield areas of the recently opened Campeche margin.