OPTIMUM WELL-SITE LOCATION IN THE TABLAZO AQUIFER AT THE CHAPUCAL WELL FIELD, ECUADOR, S.A.  

Dr. William M. Turner
 

INTRODUCTION  

The Chapucal Well Field is situated near the community of Recinto Rio Verde in the Santa Elena Peninsula of Ecuador, nearly midway between Guayaquil and the resort community of Salinas.  The Chapucal Well Field supplied all of the domestic and industrial water requirements for CEPECA and its Tigre and Cautivo refineries as well as for the Gulf Refinery at La Libertad to the west.  Insufficient supplies of water were responsible for closure of these refineries in the past, causing a national emergency. 

Wells on the western side of the Rio Verde belong to CEPECA. The municipality of Salinas owns the wells on the eastern side of the Rio Verde. 

The purpose of the work carried out by AGW scientists was to locate higher capacity well in the well field. 

Figure 1 shows the location of the Chapucal Well Field in the Santa Elena Peninsula. 

HYDROGEOLOGIC SETTING  

Wells in the Chapucal Well Field produce water from the Tablazo Formation.  It is a mixed continental, fluviatile, near-shore Pleistocene deposit of loose sand, shelly sandstone, and occasionally, well-cemented limestone.  The Tablazo was deposited over extensive areas of the Santa Elena Peninsula of Ecuador. Remnants of Tablazo limestone are seen at San Juan in the Progresso Basin.  The area of major Tablazo development is in the Engunga -Engabao and Chanduy -San Rafael - Juan Montalvo area west of the Chanduy Hills to beyond Atahualpa and Ancon. 

The Chapucal Well Field is centered within the Chanduy - San Rafael - Juan Montalvo - Atahualpa area. Here the Tablazo occurs as a pinkish to pale-brown, hard, calcareous sandstone and coarse conglomerate with very shelly horizons.  Between the Rio Verde and Zapotal, thick, coarse, gravel beds are exposed in a quarry along the Guayaquil - Salinas road. At Julio Moreno and Atahualpa the Tablazo appears to be comprised of buff, friable, sandy beds.  At Atahualpa numerous hand-dug wells are constructed in brown to yellowish, soft Tablazo sands. 

The Tablazo was deposited on an old erosional surface.  This surface truncates many older structural features.  The base of the Tablazo in the Chanduy - Juan Montalvo area rests in angular unconformity above beds of San Jose Shale, Atlanta (Azucar) Sandstone, Socorro Formation, and Seca Shale. Petroleum company geologists compiled the distribution of sediment at the base of the Tablazo in the Chanduy - Juan Montalvo area from test-well data. 

The erosional surface on which the Tablazo was deposited was irregular and as a result the Tablazo thickness varies extensively. 

PRIOR WORK  

Busk (1941), of British Controlled Oilfields, Ltd., studied the geology of the Ancon oilfield and nearby areas.  Busk included information on the water-bearing characteristics of the Tablazo deposits in the Chapucal area. 

Water in the Tablazo generally occurs under unconfined conditions.  However, notes made by drillers and geologists suggest that there is a basal conglomerate of the Tablazo and that some water is confined beneath it. In a letter dated March 4, 1943, the to General Manager of Ecuador Oilfields, Ltd., Busk mentions that a hand-dug well at Chapucal "struck water at 38 feet (11.6 m) in sandstone of Oligocene age underlying a very hard shell bed at the base of the Tablazo." 

In June 1947, L.A. Spens, Resident Geologist for Ecuador Oilfields, Ltd. conducted a number of aquifer performance tests (APTs) of the Chapucal water wells.  Of the 12 wells in existence at that time, water levels rose in all wells above the depth at which water was first encountered. 

There are presently 39 wells in the well field. Only five were still in production when this study was carried out.
 

PROCEDURE

AGW hydrogeologists obtained drillers logs for all wells in the Chapucal Well Field including stratigraphic information from nearby oil wells.  We also measured water levels and temperature profiles in all wells within the well field. 

We prepared subcrop topographic maps, Tablazo isopach maps, ground-water-level contour maps, and a Thermonic contour map. 

Conventional wisdom would suggest that the best well sites would be in the thickest saturated zone within the Tablazo Formation.  This zone should show up as a subcrop paleo-stream channel or as the thickest section of Tablazo Formation on the isopach map. Wells drilled in the paleo-stream channel should encounter the coarsest and best-sorted clastic material. 

The best well locations, however, do not necessarily coincide with the greatest Tablazo thickness or the subcrop axis of the paleo-stream channel. The best well locations will overlie the part of the aquifer that has the greatest permeability throughout its saturated zone.  That location may or may not overlie the thickest sequence of Tablazo Formation. 

Thermonic measurements from all wells were plotted using our proprietary "valley-mapping function" in conjunction with ground-water-level data. 
 

RESULTS

Our interpretation of all data indicates that the zones of  highest transmissivity at the Chapucal Well Field only coincides with the thickest Tablazo Formation section in a few areas.  In general, there is poor agreement between the thickest Tablazo Formation section and the axial trace of the subcrop paleo-stream channel.  There is certainly no agreement between the course of the present Rio Verde channel and the zone of maximum transmissivity and optimum well locations. 

Of the 39 wells in the well field, the only wells that were still producing at the time AGW scientists carried out the study were three wells close to the zone of maximum transmissivity and two wells directly in the zone. 

The result of the Thermonic study was to delineate the axis of maximum transmissivity in the area where additional high capacity wells could be drilled and constructed. 

CONCLUSIONS  

We conclude that the spatial distribution of high aquifer transmissivity within the Chapucal Well Field is confined to a narrow zone about 75 feet (23 m) wide and extending from north to south through the well field. In some places, the Tablazo is thick and in other places it is thinner.  There is no apparent relationship between Tablazo thickness, the axis of the paleo-stream channel and aquifer transmissivity. 

REFERENCES  

Busk, H.G., 1941, The Geology of the Ancon Oil Field and its Perimeter, with Notes 
     on Water Supply, unpublished TENEC Report HG B-16 

Hydrotechnics, 1974, Groundwater resources of the Santa Elena Peninsula, Ecuador, 
     Albuquerque, New Mexico. unpublished consultants report to the Empressa 
     Municipal de Agua Potable de Guayaquil under funding from the World Bank. 

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