Influence of Water Chemistry on Carbonate Factories and Reservoir Quality Distribution in Carbonate Ramp Systems: Comparison of Jurassic Smackover and Cambrian Wilberns Formations

Abstract

The Jurassic Smackover Formation (SMKV) is a prolific yet heterogeneous carbonate reservoir along the US and Mexico Gulf Coast. Current models based on well and sparse seismic data suggest that the SMKV formed a broad carbonate ramp with carbonate factory types varying laterally, along the depositional profile, and through time. Oolite shoals deposited during regression, dominate the upper distally steepened ramp along the central Gulf Coast, from Texas to Mississippi, whereas mid-ramp microbial mounds form the main reservoir during transgression along the NE section, in Alabama and Florida. Lower ramp and basinal laminated or bioturbated lime mudstones form an important regional source rock in both transgressive and regressive units. Stratigraphic forward modeling of regional transects of the SMKV indicate that a robust microbial factory is required in lower to middle ramp settings to account for the observed extensive progradation rate, suggesting a strong water chemistry control on facies distribution.

A regional elemental geochemistry evaluation of the SMKV and analogue systems may therefore hold the key to unlocking our understanding of water chemistry and its effect on facies distribution in carbonate ramp systems. Redox sensitive elements (U, Mo, V, Cu) are used to evaluate vertical and lateral variations in oxic conditions during deposition of the SMKV. A wealth of core data from regional E-W and N-S transects allows the analysis of regional-scale variations across the Gulf Coast. Elemental composition values are measured using handheld elemental analyzers (XRF and LIBS) from each facies of the SMKV and analogue facies of the Upper Cambrian Wilberns Formation exposed in central Texas. This approach ensures consistency of data acquisition and allows a quantitative or semiquantitative comparative analysis of the impact of changing water chemistry. We will evaluate multiple hypotheses for how seawater redox affects lateral and temporal carbonate factory distribution in ramp systems such as direct impact on skeletal biota, and impact through carbonate saturation state on microbial and abiotic carbonate factories.

Integration of geochemical and petrographic data within a regional, detailed sequence stratigraphic frame will further help our understanding of the dynamic role of water chemistry in carbonate ramp systems. Understanding this relationship will enhance prediction of facies distribution for reservoir models at both the prospect and basin scale.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018