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For the past 10 months the writer has been engaged in making a detailed study of the physical properties of the Bethel sandstone as they are revealed in cores from wells drilled in south-central Illinois. The investigation has been undertaken in cooperation with the Illinois State Geological Survey.
The Bethel sandstone is found to be very uniform in all of its physical properties both vertically and laterally although some gradations exist. The study has revealed that most of the sand has come from older sediments. Most of it has undergone at least one earlier period of deposition under conditions in which the cement was silica and some has undergone at least three cycles of erosion. Some of the sand has been derived from redbeds and the grains are frosted. The original source of the sand was in part from dynamically metamorphosed rocks and in part from regionally or thermally metamorphosed rocks. A large part originally came from igneous rocks, probably granites since the associated feldspar is acidic.
Heavy minerals are very rare but those found constitute about thirty species. A number of varieties of tourmaline and zircon are distinguished. Ten varieties of quartz are found which are readily distinguishable and it is proposed that more data can be derived concerning the origin and history of a given deposit by a detailed study of quartz and its varieties and inclusions, than from heavy-mineral studies and without the laborious procedure of making heavy-mineral concentrates.
A detailed discussion of the inter-relations of the physical properties of the sand, together with an analysis of the effect of these upon the porosity and permeability of the Bethel, is given. This together with certain detailed information concerning the pores and pore pattern, the relationship of the silica and carbonate periods of cementation, and soluble minerals are discussed in the relationship they have upon securing
greater primary and secondary recovery of oil. The information cementation yields concerning the periods of folding and oil accumulation is brought out.
In conclusion, the physical properties of the Bethel sandstone are compared with other sands of the Chester series, and insofar as possible the detailed data from the Bethel is applied to the other sand zones. The paleogeography and source of sediments are likewise discussed.
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