Significance of Microbialites in Reservoir Development and Evolution in Silurian Reef Slope Deposits at Pipe Creek Jr. Quarry, Indiana, and the Michigan Basin

Abstract

Microbial binding has been identified as a significant process, along with syndepositional marine cementation, that results in the early stabilization of steep carbonate slopes in the Holocene and in numerous ancient carbonate slope examples. This allows for the development and preservation of steep carbonate slopes and contributes to the overall framework and rigidity of the reef framework, and may lead to an early reduction in primary porosity and permeability which may adversely affect subsequent reservoir development and preservation during burial.

Silurian reefs in and around the Michigan Basin have been extensively studied because of their significant production of hydrocarbons (>500 MMBOE). The Silurian-age reef flanking beds exposed at the Pipe Creek Jr. Quarry in Indiana are distinguished by steeply dipping upper slopes approaching 45⁰. These cyclically bedded units are characterized by coarse grainstone and boundstone facies with syndepositional marine cement. The abundance of marine cements likely aids in the early stabilization and rigidity of the slope flanks. However, in many of these beds there is clear evidence of microbial cements that may have aided in the stabilization. The steep flanking beds of the Silurian in both the quarry and subsurface of the Michigan Basin are characterized by in situ microbial and cement boundstones with characteristic microbial microfabrics.

Comparison of interpreted depositional processes and early diagenetic modification in the Silurian example, including an abundance of in situ microbially mediated cementation, to those described in numerous subsurface examples including the supergiant Tengiz field in Kazakhstan and in the Holocene of the Bahamas, shows a remarkable similarity in depositional texture and fabric, potential reservoir geometry, and effects of early diagenesis on reservoir properties. The aim of this study is to evaluate the possible contribution of microbial binding and microbially mediated cementation to the stabilization and potential reservoir modification in the Silurian upper slope deposits exposed at Pipe Creek Jr. Quarry and in the subsurface Silurian reefs of the Michigan Basin. Thin section petrography, SEM analysis, and confocal microscopy are being used, along with detailed mapping of the slope deposits in the quarry, to provide insight into possible reduction of initial porosity and permeability, influences on later diagenetic processes and final reservoir flow properties.

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