A PROPOSED MODEL OF SILURIAN REEF DEVELOPMENT WITH IMPLICATIONS OF EARLY CEMENTATION AND SUBSEQUENT DIAGENESIS AT PIPE CREEK JUNIOR QUARRY

Pipe Creek Junior is a reefal limestone quarry in Grant County, Indiana with spectacular exposures of steeply dipping Silurian flank beds. Most Silurian rocks in the region were strongly affected by hydrothermal alteration and dolomitization which destroyed characteristic fossils and fabrics. Pipe Creek Junior was apparently not affected by this process, so original fabrics and cements are preserved, allowing for study which may shed light on Silurian reef development as well as hydrocarbon reservoirs in that reef system. The presence of fluid oil in these shallowly buried (<50m) rocks adds to the interest.

The petrographic analysis is a component study of a more complete study of the quarry exposures including gigapan photography, unit tracing and sampling. In this particular study we focus on petrographic analysis of the samples collected. We identify the fossil components, reef and slope fabrics, and early and late carbonate cements.

One interesting problem is that flank beds in the quarry exceed the normal angle of repose of 45 degrees. Preliminary analyses show extensive early marine cementation in some microfacies, and hints of microbially bound micrite textures in others. Evidence of binding mud includes gravity-defying mud structures and clotted fabrics. These may have contributed to the high slope angles.

The occurrence of oil and lack of hydrothermal alteration suggest the possibility that an early charge of oil acted as a barrier against hydrothermal invasion and subsequent dolomitization. One possible test, in process, of this hypothesis is too look for evidence that oil saturated samples lack late cements, or contain different styles of porosity as found in “dry” samples. Preliminary observations suggest that some oil filled corals, for example, are relatively uncemented.

By integrating petrographic data with larger area mapping we will be able to generate a 3 dimensional model of the reef flank beds, showing cycles of reef grown through time. The structural information alone could serve as a model to help interpolate between wells. This 3-D map may also be used as a basis for finite element models to help predict porosity and permeability distributions in Silurian Reef reservoir systems.