Assessing Novel Chemostratigraphic Correlation Tools in Carbonate Reservoir Rocks: A Mississippian Limestone Case Study

Abstract

Carbonate mudrocks in the Midcontinent, United States, can be prolific unconventional hydrocarbon reservoirs and have been intensely studied with regard to lithology, sequence stratigraphy, and facies distribution. Discontinuous lithology and heterogeneous distribution of fine-grained carbonate systems, however, makes conventional characterization and correlation methods such as litho-stratigraphy difficult and unreliable. A potential solution to this correlation-challenge lies in novel chemostratigraphy tools: The isotope composition of carbonate associated sulfate (CAS), a proxy for the isotope composition of seawater sulfate at the time of carbonate formation, which has previously been successfully applied to reconstruct changes in biogeochemical sulfur cycling on a global scale; and trace metals (TM) which have been successfully applied in shales as proxies for paleoseawater redox conditions. We propose that CAS isotopes and TM abundances can be used on a basin-wide scale as new chemostratigraphic correlation tools, allowing us to define chrono-stratigraphic horizons in unconventional reservoirs where conventional litho-stratigraphic correlation yields inconclusive results. Utilizing a framework of sequence stratigraphy for the Mississippian limestone in southwest Missouri, we tested the application of CAS isotopes and TM abundances as correlating tools for limestone on samples taken along vertical and horizontal transects from the St. Joe Group. The sulfur concentration and isotope composition of CAS were determined using a sequential extraction procedure; CAS was extracted and analyzed for sulfur isotope compositions using isotope ratio mass spectrometry. TM content was determined for the carbonate fraction and the bulk rock fraction including the siliciclastic phases. The TM content was analyzed by inductively coupled plasma mass spectrometry. Our initial data set shows a good correlation of sulfur isotope values along a lateral transect over hundreds of meters and higher than 3rd order sequence cycle temporal variations. With additional data better defining the lateral and temporal variability, trends in sulfur isotope compositions and trace metal abundances may be used for stratigraphic correlation across a basin. A correlation tool unaffected by lithological variation may be effective in correlating units and help better understand unconventional hydrocarbon reservoirs in heterogeneous carbonate systems such as the Mississippian limestone.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017