Primary Depositional Facies Controls on Diagenetic Susceptibility and Porosity Evolution in Mixed Carbonate and Chert Intervals, Northwest Arkansas and Southwest Missouri, U.S.A.

Abstract

Chert porosity is abundant in Mississippian carbonates in the Mid-Continent. In outcrop, porous chert is typically housed in Osagean-age strata. Depositional trends are inferred to impart a strong facies control on alteration potential of cherts. Overall shallowing of seas during the middle to upper Osagean is indicated by a large-scale shoaling upward sequence from a mudstone-dominated to grainstone-dominated system. Multiple shoaling upward packages 4-8 meters thick are identified in each formation. In the Reeds Spring Formation, high-frequency cycles are noted by cyclic changes from carbonate mudstone to silt-size skeletal packstones as well as vertical increases in burrowing intensity within chert facies. Within the Pineville Tripolite, successions of pure tripolite coarsen upward into interbedded tripolite and skeletal grainstone, while the Burlington/Keokuk Formation expresses a shallowing upward succession from carbonate-rich chert/siliceous limestone overlain by coarsening and thickening upward crinoidal grainstone. Porosity development is strongly facies dependent, with the highest porosity values found in well-mixed carbonate and siliceous sediments. Porosity is preferentially developed at the tops of parasequences in the Reeds Spring, and base of parasequences in the Burlington/Keokuk, while cyclic variations in porosity are observed in the Pineville Tripolite due to variable facies stacking. These trends create a hierarchy of reservoir quality and compartmentalization throughout the Osagean.

Isotopic values indicate cyclic changes in O¹⁸ in relation to high frequency stacking. Samples show depletion toward more porous chert intervals, suggesting higher degrees of alteration within specific facies zones within individual cycles. These patterns indicate preferred flow pathways of diagenetic fluids. Early pore networks were likely in place to accommodate later fluid flow through the system. Mixed carbonate and spiculitic sediment shows evidence of inhibited calcite cementation due to early silicification and coating of carbonate grains by microcrystalline quartz. This process, along with dewatering of biogenic opal-A, potentially preserved and created pore networks that channeled later meteoric or hydrothermal fluids responsible for further porosity enhancement.

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