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<br />300 •
<br />consists of one or more strata with an aggregate
<br />thickness of ei¢hty to one hundred feet in the
<br />Sandt+ich area. Together these beds constitute the
<br />"Upper Salt." At a depth of about 1100 feet, the
<br />base of the Upper Salt rests on rock called lime-
<br />stone by the drillers.
<br />At depths between 1200 and 1250 feet, one or
<br />more strata of salt were encountered in many of
<br />the wells. Unnamed in brine field practice, they
<br />tell herein be called "\liddle Salt." These thin, dis-
<br />continuous strata cannot have made a notable con-
<br />tribution to the total salt production, but rockfalls
<br />associated t+ith them seem to have caused a dispro-
<br />portionate fraction of the tubing and casing breaks
<br />experienced during the operation of the brine field.
<br />Failure to distinguish between rockfalls in the 1`[id-
<br />dle Sal[ and those in the Upper Salt has apparently
<br />led some observers to overestimate the relative
<br />amounts of solution in the Upper Salt. In order to
<br />avoid such an overestimate, i[ appears useful to
<br />recognize the separate existence of the \liddle Salt.
<br />Daring drilling of some of the tells, cavities
<br />with a maximum height of 20 Eeet were encoun-
<br />tered in both the Upper and the \liddle Salts.
<br />Some, possibly all, of these were of natural origin-
<br />The Lower Salt consists of two beds separated,
<br />according to the well logs, by a few feet of lime-
<br />stone. Occurring between depths of about 1400
<br />and 1600 feet, they Rave an aggregate thickness
<br />close to 200 feet.
<br />Formations penetrated by' brine wells oC the area
<br />have undergone little or no deformation. Accord-
<br />ing to Bays (1954), regional dips vary from slightly
<br />west of north to northwest. They average 60 to 70
<br />feet per mile and rarely exceed 100 feet to the
<br />mile.
<br />BRINE EXTRACTION
<br />The First brine well t+•as drilled in the Sandwich
<br />field in 1902, but relatively intensive development
<br />did not begin until 1922. Between that year and
<br />1953, 25 wells were drilled to a depth of about
<br />1600 feet, to the base of the Lower Salt. The cas-
<br />ing appears generally to have extended to a depth
<br />between 1237 and 1290 feet, that is, below the
<br />\liddle Salt, and the tubing, to the bottom of the
<br />well. All of the wells drilled before 19°8, and most
<br />of those drilled later, were operated initially as
<br />water-forcing wells. Sooner or later, most oC the
<br />tvclls became connected through solution channels
<br />to one or more of the others, and finally to a "gen-
<br />eral cavity," which, according to Dr. Bays, teas in
<br />the Lower Salt.
<br />r,nel~eld Subsidence ar Windror, Ontario
<br />The available data concerning the design, con-
<br />struction, and operation of the wells indicates that
<br />brine was drawn from all three salt horizons. \Yith
<br />the exception of three tvclls drilled prior to 1918,
<br />all were designed to extract brine from the Lower
<br />Salt. The high incidence of recorded rock Falls at or
<br />below the level of the top of this horizon {30 out
<br />of a total of 50) indicates that the 200-Foot thick
<br />Lower Salt w•as, as intended, a major source of
<br />brir-e. In addition, an unknown but doubtless large
<br />quantit}' of brine must have been derived from the
<br />Middle and Upper Salts, owing to the introduction
<br />of water into these beds through defective well cas-
<br />ings and around inadequate packers.
<br />In view of the Eact that the total thickness of the
<br />source beds underlying the brine field is about 300
<br />feet, i[ is evident that the total height of cavities
<br />underling the Field may have been unusually large.
<br />However, because of impurities in the salt and the
<br />irregularities of the solution process, the total
<br />height of cavities beneath an} one point on the
<br />surface ryas doubtless mach less than [he possible
<br />maximum of 300 Eeet.
<br />The total volume of the cavities due to brine
<br />production at the Sandwich field can be roughly
<br />estimated from information concerning produc-
<br />tion. According to Ba}'s, the total production of
<br />the Sandwich field teas about 'r l million cubic feet.
<br />He concluded that the quantitq of injection water
<br />greatly exceeded that of the brine recovered, and
<br />he estimated that more than 150 million cubic feet
<br />of salt had been removed Erom the deposits under-
<br />lying the plant area when production ceased in
<br />February, 1954.
<br />IF salt extraction had produced a single large cav-
<br />ity in the form of an inverted cone with a diameter
<br />of 2000 feet, instead of a considerable number of
<br />originally separate but ultimately connected
<br />morning-glory-shaped cavities, the cavity t+-ould
<br />have a maximum height of 150 feet.
<br />The tell records given in Bays' report provide
<br />scant information regarding the real shape, size,
<br />and location of individual cavities or of the "gen-
<br />eral cavity" to which most of the wells ultimately
<br />became connected. The earliest wells so connected
<br />were in a centrally located zone extending from
<br />the northerly side of the brine field to the south-
<br />erly side (wells no. 7, 6, 5, and 9). It would not be
<br />surprising if maximum solution had taken place in
<br />those parts of the salt beds that underlie this zone.
<br />The area of maximum surface subsidence is located
<br />in the southerl}' part of this zone.
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