Diagenesis and Isotopic Evidence of Porosity Evolution in Reef Reservoir-Analog Facies in outcrops of the St. Joe Group (Kinderhookian to Basal Osagean) in SW Missouri and NW Arkansas

Beau Morris and Sal Mazzullo

Fenestrate bryozoan-crinoid bafflestone, mud-dominated reefs and associated crinoid grainstones are present in the Kinderhookian Compton and basal Osagean Pierson formations (St. Joe Group) in SW Missouri and NW Arkansas. These are analogs of actual and potential petroleum reservoirs in subsurface Kansas and northern Oklahoma. The reefs likely were deposited in low-energy environments at or below wave-base, and commonly overlain by shallow-water crinoidal sands. Early diagenesis in the reef deposits primarily involved occlusion of the limited primary porosity present by marine cements, notably by former high-magnesium calcite, radiaxial-fibrous cement. The oxygen-carbon isotopic composition of this cement (means: δ¹⁸O -2.5 ^o/oo, δ¹³C +4.7^o/oo) is the proxy for seawater isotopic composition at that time. Despite the muddy nature of the sediments and their marine cementation, post-depositional subaerial exposure resulted in significant secondary porosity and the formation of vugs. In outcrops, most of these vugs were occluded by coarse calcite cement and internal vadose sediment or presumed meteoric origin. This interpretation is supported by the depleted composition of the coarse calcite cements relative to the marine seawater proxy. These reefs maintain enough porosity to be stained by oil. In contrast, the indications of high-porosity reefs in the subsurface suggest that, unlike the outcrops, they likely have preserved high secondary porosity and can be potential petroleum reservoirs. 

AAPG Search and Discovery Article #90176©AAPG Mid-Continent Meeting, Wichita, Kansas, October 12-15, 2013