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
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.
RI-12 Deep Stratigraphic Test Well Near Hollandale, Minnesota.
Minnesota Geological Survey.
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