I 



A Note on Subsidence and the Exhaustion of Water-Bearing 

 and Oil-Bearing Formations 



Virtually all rocks near the earth's 

 surface are to some degree porous, 

 and if water is available it fills the 

 pores. In some rocks the pores are 

 large enough and well enough inter- 

 connected so that water can readily 

 flow from volumes of higher pres- 

 sure to volumes of lower; such rocks 

 are called aquifers — water bearers. 

 Other rocks have pores so fine and 

 so poorly interconnected that water 

 passes through them only slowly, 

 even under high pressure-gradients; 

 these are aquitards — water-retarders. 

 Among the common rocks, sand- 

 stones, conglomerates, cavernous 

 limestone, and scoriaceous lavas are 

 the chief aquifers; shales are the 

 principal aquitards. 



Subsidence 



Where water has access to an inter- 

 bedded series of aquitards and aqui- 

 fers both are commonly saturated, 

 but the aquitards are sufficiently im- 

 permeable as to permit considerable 

 pressure differences to exist between 

 the several aquifers. When a well is 

 drilled to any particular confined 

 aquifer and water is withdrawn from 

 it, the water pressure in the aquifer 

 is decreased and the aquifer shrinks 

 in thickness. The weight of the 

 rocks overlying the aquifer, which 

 had formerly been in part sustained 

 by the pressure of the contained 

 water on the base of the overlying 

 aquitard, has become effectively 

 greater because of the decrease in 

 hydrostatic pressure; under the ef- 

 fectively greater load, the aquifer 

 yields elastically and the volume of 

 its pores diminishes. 



Though Young's modulus for most 

 sandstones is between 140,000 and 

 500,000 pounds per square inch, a 

 significant pressure reduction in an 

 aquifer several hundred feet thick 

 can readily cause a subsidence of 

 several feet at the surface of the 



ground. Such a subsidence may cre- 

 ate serious problems in drainage, sew- 

 age disposal, and utility maintenance. 

 More important than simple elastic 

 compression of the aquifers, how- 

 ever, is the fact that the lowered 

 pressure in the aquifers permits slow 

 drainage into them from adjoining 

 or interbedded aquitards. This per- 

 mits the aquitards also to be com- 

 pressed by shrinking their pore 

 spaces. 



Thus, at the Wilmington oil field, 

 in California, the loss of pressure 

 in the oil sands after 1936, when 

 production on a large scale began, 

 led to a surface subsidence of more 

 than 32 feet (see Figure VII-3) before 

 recharging of the oil sands with sea 

 water under pressure finally stabilized 

 the surface. Of this subsidence, only 

 about 10 feet could be attributed to 



elastic compression of the oil sands; 

 the remaining 22 feet was almost cer- 

 tainly due to de-watering of the asso- 

 ciated shales. The cost of this sub- 

 sidence was many millions of dollars, 

 since the railroad terminals, docks, 

 shipyards, drydocks, and power 

 plants had all to be rebuilt, together 

 with the streets, water, and sewer 

 systems of a large part of the city 

 of Wilmington. 



Similar subsidence caused by with- 

 drawal of fluids under pressure has 

 been noted at many other seaside 

 localities: Lake Maracaibo, Vene- 

 zuela; Goose Creek, Texas; Hunting- 

 ton Beach, California; Redondo Beach, 

 California. None caused as great a 

 loss as that at Wilmington. 



It is possible for similar subsidence 

 to pass unnoticed at areas inland be- 



Figure VII-3 — SUBSIDENCE IN LONG BEACH, CALIFORNIA 



(Illustration Courtesy of the Geological Society of America ) 

 Superimposed on the photograph of the port area of Long Beach, California are 

 contours of equal subsidence in feet as they existed in 1962. The subsidence in 

 the upper right resulted from withdrawal of fluid from the Signal Hill oil field be- 

 tween 1928 and 1962. The major subsidence in the foreground was due to with- 

 drawal from the Wilmington oil field. 



203 



