THE EFFECT OF STRUCTURE UPON ACCUMULATIONS value of a structure for commercial accumulations depends upon the effective porosity and the extent of a sand body, in which gravitational separation may take place. It is a well known fact that gas, oil and water if placed in a closed vessel, will arrange themselves in order of their specific gravities; thus water will occupy the lowest parts of the vessel, the oil above it and the gas occupy the upper portion. This same gravita tional separation will take place in a porous sand body. If the pores are intercommunicating and the reservoir is continuous to any extent, large accumulations become possible under favorable structure, the controlling effect and importance of which will be evident.
Dome. It will be apparent that the gases will occupy the highest portion of domes, with oil under them, resting upon the water that will be accumulated in the lower portion of the structure (or in the syncline). The degree of saturation of a dome depends upon the amount of water in the syncline and the extent of the oil above it. If the amount of gas and oil are greater than could find lodgment in the dome they will flow or "spill" into adjoining structures, finding their way into them along the spilling lines. (Fig. IS.) The spilling point of a dome is immediately below the lowest closed contour line. The general nature of a dome stamps it as a very desirable structure for prospecting, especially suitable for gas accumulations, and for oil as well in the absence of a large amount of gas.
Level Axis Anticline. A level axis anticline is the next favorable structure to a dome; they are more frequently found than are the domes, and have proven to be of great value in accumulation of oil and gas in different parts of the country.
The spilling point for a structure of this kind will be at the point wherithe major or minor axis crosses the lowest closed contour line.
Plunging Anticline. The value of an anticline is greatly diminished if its axis is a plunging one, as the plunge might bring the most favorable portions of the structure to a point where the reservoir rock has been exposed and eroded. The size of the plunge may have varying effect; the structure may be of value if the height of the water level in the sand is known; but, in general, it is of no greater value than are plane dipping homoclines.
Nose. Noses are most commonly found along the axes of plunging anticlines, and as such are important in increasing the value of such a plunging anticline. They also occur along homoclines, where they are equally important.
Embayment. These may become important in connection with the spilling line, in the absence of water at the height (or above) this structure.
Homocline. Homoclines, in general, are in great disfavor among, prospectors, as accumulations along such structures are due mainly to the lensing or differential cementation of the sand. As these conditions cannot be known from the surface a homo cline offers less chance for prediction; and it is risky to condemn a property merely for the lack of better structure, but of course, on the other hand, they are not as favorable prospects as are the more suitable structures. Homoclinal production, if exam
ined, will show that it is in accord with the structural theory, in so far as the gas, oil and water is concerned, for that particular sand. In the "Clinton" sand homoclinal accumulations of Ohio it will be noticed that the gas and oil will be found in their relative position along the dip of the rocks, and the main course of production follows very closely the strike of the rocks. A knowl edge of the lateral variation of the strata is very important in homoclinal fields.
Terrace. In places where complete saturation exists a terrace may be considered as worth while for prospecting for oil. Small terraces in surface structure cannot be depended upon as well as larger and well defined ones. A consistent and well-marked lateral variation of the strata may easily obliterate surface structural evidences and especially in case of terrace structure it may not show in the sub-surface structure at all.
Fault. The presence of faults should be carefully studied as they may seriously effect accumulations. The faulting action may form a gouge which may serve as a seal, permitting accumu lations in the stratum lying on the down-dip side, or again, the throw may bring an impervious stratum in juxtaposition with the oil bearing one, and permit accumulations in it. (Fig. 12.) Faults may permit the migration of oil and gas to a higher sand, and again may permit their escape and loss at the surface; thus, seepage along a fault may indicate that the reservoir has been broken into and its contents lost. Large faults are in most cases dangerous.
Intrusives and Extrusives. The effect of intrusives and extrusives may be similar to the action of faults; there is also a possibility of accumulation in the anticlinal ring formed during the shrinkage of the foreign material. They are to be carefully guarded against, as in most cases their presence is unfavorable.
Unconformities. In a few instances and gas have been found in connection with unconformities. A case of such accumu lation is known in the Potsdam sandstone; the Potsdam having been eroded into hills and valleys and subsequent deposition formed impervious cap-rock under which the gas has accumu lated; especially in those parts which the erosion left as hills. A similar case is known to exist in the Berea sand in Ohio, where the strata have been eroded by glacial action and parts of the Berea being eroded, the subsequent deposition of glacial drift has obliterated the former valleys formed by the glaciers, and formed a barrier permitting accumulations in the Berea, where it has not been eroded. Unconformities may be so located that they act as barriers similarly to faults.