DIAGENETIC HISTORY OF CAMBRIAN SANDSTONE UNITS IN WESTERN WISCONSIN: IMPLICATIONS FOR RESOURCE EXTRACTION

The Wonewoc Fm. and Jordan Fm. in the upper Midwest were deposited as cratonic sheet sandstones during Late Cambrian eustatic sea level changes. These formations consist of cross-stratified, medium- to coarse-grained quartz arenite deposited in shallow marine conditions. The quartz arenite is highly valued by the petroleum industry for hydraulic fracturing due to its ultra-pure composition; round, high-strength grains; and weak cementation.

The diagenetic history of these formations is poorly understood, and is important for both economic and environmental reasons. Petrographic analysis of the Wonewoc and Jordan formations quantifies the composition of framework detrital grains and interstitial cement. Framework grains are primarily monocrystalline quartz (95%); polycrystalline quartz and microcline are rare. Interstitial spaces are occupied by voids, calcite, sericite, authigenic orthoclase feldspar, and hematite. Interstices of the Jordan Fm. (n=30) contain void space (63%), calcite (17%), hematite (8%), authigenic quartz (7%), authigenic orthoclase feldspar (4%), and sericite (<1%). Interstices of the Wonewoc Fm. (n=19) contain void space (70%), hematite (17%), authigenic orthoclase feldspar (9%), sericite (4%), and authigenic quartz (<1%). Void space increases and hematite and authigenic orthoclase feldspar cements decrease from northwest to southeast in the Jordan Fm. In the Wonewoc Fm., void space and hematite cement increase from northwest to southeast, but authigenic orthoclase feldspar and sericite cements decrease along that same compass trend.

One of the primary environmental concerns with frac sand mining is the potential for the generation of crystalline silica particulate matter. This fine-grained material would most likely be derived from cements during processing. However, silica cement is very rare, is restricted to the upper Jordan contact, and renders the material non-economic because it inhibits disaggregation.

The variation in the composition and quantity of interstitial material within and between formations suggests a complex multistage diagenetic history that consists of orthoclase feldspar cementation, dissolution, multiple calcite cementation events, hematite cementation, and a final dissolution event.