RI-12 Deep Stratigraphic Test Well Near Hollandale, Minnesota

Abstract

A deep stratigraphic test well, the Hollandale No.1, in Freeborn County, Minnesota was drilled in rocks that range in age from Devonian to Precambrian in search for reservoirs suitable for the underground storage of natural gas. The porous St. Peter Sandstone of Ordovician age and the Mt. Simon Sandstone of Cambrian age are potentially good reservoir rocks. The Ordovician Decorah Shale overlies the St. Peter Sandstone and probably is the most suitable cap rock in southeastern Minnesota. However, the Decorah is restricted geographically and generally lies relatively near the surface. The Eau Claire Formation, above the Mt. Simon Sandstone, also has been considered as a possible cap rock, but it is composed of shales that are interbedded with sandstones and is vertically permeable. As this was the first cored well in this part of southeastern Minnesota, a study was undertaken of the lithic units penetrated during drilling, and interpretations were inferred for the depositional environment of each unit. The rocks of Paleozoic age were deposited in a shallow-water environment in cyclic fashion. Four recurrent lithotopes that characterize the rocks are (1) quartzarenite, (2) poorly sorted lithotope with strata composed of clastic particles ranging in size from silt to granule or with arenaceous carbonate strata, (3) shale or argillaceous sandstone, and (4) carbonate rock. The recurrence of these lithotopes in the Paleozoic column in southeastern Minnesota has led to the identification of nine sedimentary rock cycles. Several of the cycles are bounded on their upper surfaces by unconformities; in others, rocks occur that appear to have been deposited by both transgressional and regressional seas. The general trend in the depositional cycles of Cambro-Ordovician rocks of southeastern Minnesota is from predominant sandstone and subordinate carbonate in the older cycles to subordinate sandstone and predominant carbonate in the younger cycles. This progression reflects the gradual degradation of the Precambrian surface and the covering of these basement rocks and older Paleozoic rocks by younger sediments.