Waste Gate Formation
1982, Hansen, H. J., and Doyle, J. A.
Open File Report 82-02-1
PART ONE - Hydrogeologic framework and potential utilization of the brine aquifers of the Waste Gate Formation, a new unit of the Potomac Group underlying the Delmarva Peninsula
by Harry J. Hansen
The Lower Cretaceous Waste Gate Formation is a new name proposed for a subsurface unit of the Potomac Group occurring beneath the eastern Delmarva Peninsula; the top of the unit occurs at depths ranging from about 3,500 feet at its up-dip edge to 5,670 feet at the Coast. It consists chiefly of unconsolidated to moderately lithified, light gray to whitish, arkosic to subarkosic sandstones and drab, often finely laminated silty shales (or clays). It ranges up to 1,500 feet thick at the Coast with sand percentages characteristically 50 to 70 percent. The unit is tentatively dated Neocomian [Berriasian to Hauterivian (?)] on the basis of limited palynological data. An apparent absence of Clavatipollenites and other Zone I (mid-Barremian to early Albian) palynomorphs implies the Waste Gate Formation is older than the Patuxent Formation, which comprises the basal beds of the outcropping Potomac Group.
Sandstone porosities of the Waste Gate Formation, estimated from geophysical logs, range between 19 and 27 percent. Analysis of production test data for a Waste Gate sandstone in the DOE Crisfield Airport No.1 well yielded permeabilities in the 75 to 118 millidarcy range. The formation waters of the Waste Gate are brines of the sodium-calcium-chloride type. The Crisfield Airport well produced brine with a chloride concentration of 42,000 mg/L (71,000 mg/L total dissolved solids). Elsewhere on the Peninsula, geophysical log estimates of NaCl salinity range from 53,700 mg/L to 97,000 mg/L . Below 3,000 feet Waste Gate fluid pressures increase under a hydrostatic pressure gradient of about 0.44 to 0.46 psi/foot. Temperature gradients appear to fall between 1.3° and 1.75° F/100 ft. Waste Gate formation temperatures range from about 124° F in up-dip areas to about 172° F near the base of the formation at the Coast; temperatures as high as 190° F are possible, but are less likely.
Possible utilization of the Waste Gate aquifer includes extraction of chemical commodities from brines, geothermal heat production, and disposal of hazardous wastes. The former is least likely, because commerical sodium-calcium chloride type brines have salinities three or four times greater than Waste Gate water. Because of the relatively low formation temperatures associated with the Waste Gate aquifers, large volumes of water are required to satisfy the energy requirements for geothermal heating. For example, about 300 gpm of 175° F water would be required to provide a 5 x 106 BTU/hr . peak requirement.
Although this yield from one well may be possible, the relatively low aquifer permeabilities would necessitate opening to production a number of sands totaling hundreds of feet of section (e.g., about 370 feet for an 85 millidarcy sand). Conversely, only modest injection rates would be possible, if it is required to confine the waste stream within one aquifer of the formation. It is estimated that injection rates in the 30 to 115 gpm range may be possible without risk of fracturing the confining beds.
PART TWO - Palynology of continental Cretaceous sediments, Crisfield geothermal test well, eastern Maryland
by James A. Doyle
Palynological study indicates that core samples from 4148', 4138', and 4121' in the Crisfield Geothermal Test Well of eastern Maryland, near the base of the Waste Gate Formation of the Potomac Group of Hansen, are mid-Berriasian in age, much older than the basal Potomac Group at the outcrop belt west of Chesapeake Bay (Barremian-Aptian). A variety of spores allow correlation with Suite C of Norris in the Purbeck Beds of England, while detailed comparisons of striate Schizaeaceae (Cicatricosisporites) and verrucate spores (Concavissimisporites, Trilobosporites, etc.) indicate. the samples are more precisely equivalent to the lower part of Suite C and the Hils 1 assemblage of northwestern Germany, and near but slightly older than the "Wealden A" of northeastern Germany. The typical Southern Laurasian aspect of the flora suggests a humid subtropical climate; the lack of marine plankton and the variable quantitative composition of the three samples are consistent with the inferred fluvial environment of deposition of the Waste Gate Formation. Ditch and core samples from higher in the Potomac Group correlate as follows: 3430', Zone I (Barremian-Aptian); 2550' and 2200', lower and upper Subzone II-B, respectively (middle-late Albian?); 1143', 1127', and 1113', middle Zone III (early Cenomanian?); environments are continental (floodplain) throughout.