UNDERGROUND STRUCTURE AND HOW DETERMINED For the purpose of correlating formations penetrated, various ideas were at first used, such as base of conglomerate, top of brown shale as logged, and first oil showings as reported. Cor relation based on any one of the above methods was found to be roughly satisfactory, but was not in many instances accurate enough to determine the location of water sands, which later appeared in the oil measures themselves and which began to give trouble.
As a result of a further study by the writer, therefore, a new basis of correlation was established, which correlation seems to give better satisfaction than any method previously used. The indicator used in this method is the first main oil sand. With the exception of a shallower lenticular sand in the western portion of the field and a still higher (stratigraphically) lenticular sand near the apex of the fold the oil sand used for an indicator is the first oil sand below the shoe of the water string.
The method of drilling in this field previous to the time this correlation was established had been to proceed with drilling until the first showing of oil was noted in the rotary mud, when the well was bridged and the water string cemented. This first showing, with exception of lenticular sands mentioned above, was invariably in shale or sandy shale, so that the first main oil sand, being the first sand to be encountered, was more accurately logged than any of the succeeding sand strata penetrated. For this reason, when once the idea was discovered, the correlation of underground measures in the field progressed rapidly.
On the basis of this correlation, contour maps and sections have been prepared as shown on the following pages. These diagrams show in detail the underground structure of the bello field.
Reference to diagram of ideal graphic log and ideal cross sec tions indicates the following grouping of formations into zones as now used.
First or Upper Oil Zone.—This designation has been given to the upper producing series of strata in this field limited by the basal conglomerate zone above and by the intermediate edge water beneath. In the central dome is also included with this
zone, for convenience in reference, the portion of the intermediate edgewater sand which carries oil.
This zone is composed of 1000 ft. of grey or brown oil-bearing shales and sandy shales lying just beneath the base of the con glomerate and containing several lenticular oil sands near the top and middle and also the first main oil sand near the base. This formation is capable of producing from 20 to 1000 bbl. cf oil per day, depending upon its proximity to the apex of the fold.
One point of particular interest in relation to the productivity of this zone, from a study of wells producing from it, is that. regardless of the nature of the driller's log as to oil showings, no increased production is obtained by drilling the well below the base of the first main oil sand. It is, therefore, unnecessary and not economical to finish a well in this zone deeper than the base of the first main oil sand. Deeper drilling involves the danger of getting too close to the intermediate edgewater sand and endangering the well from water infiltration.
Intermediate Edgewater.—This name is given to the first persistent water bearing formation encountered below the first main oil sand in this field. The type name is derived from the fact that the lower limit or edge of the oil deposit in an oil sand is generally marked by water in situ. The intermediate edge water sand in this field, except in the central dome, contains only water. The interval between the top of this edgewater sand and the top of the first main oil sand of the first oil zone is about 300 feet. The line of edgewater exists in this stratum at a depth of about 2250 ft. below sea level. The portion of this edgewater sand which contains oil in the central dome is grouped with the first oil zone.