or axis and the dip the sloping away on either side from the axis. The dome is, as its name implies, a domelike uplift in the stratum, standing alone, with the dip sloping away on all sides from the crest. Anticlinal domes sometimes occur at intervals along the top of a main anticlinal fold. Such domes are common in Oklahoma, Wyoming and California. Syncline. The syncline is the reverse of the anticline and, while usually unfavorable for the accumulation of oil, yet oil has been found in them. Synclines, that are productive of oil, usually are not water bear ing and, due to the absence of water pressure, the direction of the oil is reversed from that in the anticline and by gravity it has drained into the lowest point or trough of the structure. Oil has been found on the flanks of a syncline where the basin is filled with water. Oil in commercial quantity has beeen found in synclines in shale formations above the regular oil bearing formation, probably forced there through fissures in the rocks. This condition has been developed in the syncline outside the structure of the Salt Creek field of Wyoming. In the Coalinga, California, oil field, oil occurs in both the syncline and the anticline. Monocline. The monocline is a structure whose dip is in one direction and where the oil bearing formation may rise to the surface. Oil occurring in commercial quantity in monoclines where the oil sands crop out is usually heavy and forms asphalt beds that seal the outcrop, confining the remaining oil. Light paraffine oil would in most cases escape where the oil bearing sand was exposed, thus draining the sand for a considerable area in proximity to the outcrop. The well location should, therefore, be at a distance from the outcrop. Near Barranquilla, Colombia, there are numerous seepages of oil and natural gas. One of these seepages has formed a large mound of asphalt, locally named the "big Volcan," yet several wells drilled within a few miles of this surface showing failed to find oil in paying quantity. Terrace. Commercially profitable oil pools are sometimes found on terraces. The terrace may be a horizontal bench, ex * Illustration after Dorsey Hager. tending along a gentle slope, or a locality where the dip of an anticline becomes more nearly flat. In addition to the structures above described, oil is often found around volcanic necks or chimneys and in saline domes. The top, or on the flank near the top, of an anticline or a dome, however, is the best location for a test well. The anticline and the dome are sometimes found in close proximity to each other. The accompanying plate* illustrates this condition where the Lamb anticline and the Torchlight dome occur in the Big Horn Basin of Wyoming. The anticline in this instance is a small one, extending over only a few sections. There are many long anticlines; for example, the Preston anticline along the Red River, crossing Grayson and Fannin Counties, Texas, and Bryan and Marshall Counties, Oklahoma, and extending for a distance of over forty miles. The dome usually is a small round structure, as shown on the chart. The sub-surface contour lines on this chart show numbers indicating the distance at that point to the top of the Greybull sand, above or below sea level. In other words, if it were possible for one to follow any one of these contour lines on the ground, he would always be at exactly the same elevation. Referring again to the chart, it will be observed that the elevations reach from O to 2,800 feet above sea level and from 0 to 600 feet below sea • Footnote: Reproduction of map in the U. S. Geological Survey Bulletin No. 656, by Charles T. Lupton. level. Putting it another way, the total elevation as shown on this chart would be the sum of the distances below and above sea level or 3,400 feet. Faults. A fault is a displacement or a slip in the strata, the result of which may be the breaking off of an oil bearing formation and abutting of its face against an impervious bed. This may either cause the oil to escape to the surface, or if the contact between the broken off oil bearing bed and the impervious rock face is sufficiently close, it may seal up the oil. Thus, on one side of the fault line good wells would be secured, while on the other side, the sand would be barren. (See Fig. No. 5.) The oil field of the Puente Hills district of southern California is a good example of oil accumulation along faults. Although the structures here described are favorable for the accumulation of oil, yet it does not follow that all such structures may prove to be productive. One or more necessary elements may be lacking. The sand may be too hard or close, or it may be water bearing. Also the oil or gas present in past ages may have long since escaped for want of an impervious shale or other confining "cap rock," so called. The procedure followed by geologists in seeking for and in locating favorable structure for the accumulation of oil and gas, and in locating well sites will not here be discussed. The author is not a professional geologist, and for such geological information |