of dip, and that these minerals have great difficulty in moving past points where the rocks are approximately level. If, then, the axis of an anticline does not plunge steeply, there is a good chance that the fold will retain some oil, even though there is one direction toward which the beds in it do not dip.

A third factor is the capacity of the anticline. The writer believes that in a small minor fold on a monocline the chances of oil accumulation are also small, although the probability of gas being present is good. This may be explained by the fact that gas is more volatile than oil and will therefore pass more rapidly through the rocks. When the oil and gas begin to migrate, the gas will precede the oil and will fill to their capacity any minor domes and wrinkles. When the slower-moving oil reaches these small folds there is no room for it in the folds, and it accordingly crowds around their edges and passes on.

A fourth factor is the shape of the anticline. A very large flat anticline with a large gathering ground might supply many small wells. A similar anticline with a relatively small gathering ground might give only "shows" of oil, because the oil might be disseminated through the flat beds that form the crest. Oil occupying an anticline rising sharply to a crest or to a central point would tend to be concentrated at that crest or central point, and wells on such an anticline should have a larger daily yield and would yield more oil during their lifetime.

A fifth factor is the development work which has been done. Proof of the presence of oil is always better than theoretical arguments. A small anticline with an oil well on it is a better property than a much more promising-looking fold that has not been tested. There are so many factors any one of which may make structural work on the surface of no avail that there can be no sure thing" in untested territory, and even beds showing the most evident anticlinal structure may prove dry of oil.

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The relation of the structure shown at the surface to that in depth is a factor that is known for only a few of the oil fields of Oklahoma. This relation is difficult even to guess, but it seems probable that certain general rules may be laid down for some sections of Oklahoma when a little more work has been done on the surface and on the structure as shown by the well records. It is important to know the general degree of similarity of the folds shown at the surface and those in the oil sands, both as regards size and outline, also whether the deep-lying anticlines tend to be vertically below those at the surface, and if not, whether or not any broad general rule may be applied concerning the direction in which the deep folds tend to lie from those above them.

There are not enough well records available to make possible any such study of that portion of the Pawhuska quadrangle which is considered in this paper. One interesting divergence was noted, however. In the old shallow gas field in the southwest corner of sec. 16, T. 29 N., R. 9 E., the only two available records indicate that the gas-bearing stratum dips deeply to the southeast. This is almost at right angles to the dip which appears on the surface.

ORDER OF IMPORTANCE OF THE ANTICLINES.

By far the most promising fold in the area under consideration is the Pearsons Switch anticline. It has a broader gathering area than any of the other minor structural features described in this paper; it is a closed fold with a minimum reversed dip of 30 feet down which the oil would have to push to escape; together with the minor domes to the south it is the largest anticline in the area; and the fact that it contains at least gas had been demonstrated in January, 1917, by six shallow gas wells.

Second in importance is the Benchmark anticline. This fold has a broad gathering ground and ample capacity to store a large quantity of oil or gas. The reversal of the regional dip is slight, but even without such a reversal the broad terrace would probably serve to arrest the movement of the oil and cause it to gather in a pool of commercial size. This anticline, like the Pearsons Switch anticline, has already been tested. Several oil and gas wells and two dry holes have been drilled on its flanks, and one oil well, which was not seen by the writer, is shown on the township plats at the Indian office at Pawhuska to be near the anticlinal axis.

The order of importance of the other anticlines in this area is believed by the writer to be as follows: Triangle dome, Drennan anticline, Round Top anticline, Upper Dog Creek anticline, Lower Dog Creek anticline, Ricerock anticline with Limestone Flat terrace. western Lower Buck Creek anticline, Upper Pond Creek dome, Stone House anticline, and the eastern Lower Buck Creek anticline.

LOCALITIES FOR TESTING.

In choosing a locality for drilling on an untested anticline, the shape of the fold is the first consideration. If the fold is a dome the very top is the most favorable point for testing; if it is an elongated anticline some point along or near the axis should be chosen; if it is a terrace, the well should be drilled near the point of greatest change in dip, where the steep dip below the "step" of the terrace meets the flat upper surface.

The shape of the fold is not the only factor to be considered. If there is an appreciable amount of oil or gas in the fold, it will underlie at least several acres, and the well may be drilled a short distance from the point which is structurally the most favorable without affecting the adequacy of the test. An important factor is the accessibility of the locality. If possible, it should be either close to a road or in such a place that a good road may be made to it with little difficulty.

Another important consideration is the relation of the locality to water supply. If possible it should be chosen close to a good supply such as a creek, a good well, or a reservoir, or, if no supply is available when the locality is selected, near a point where a dam may be constructed to impound a sufficient amount of rain water for use in drilling. It should be borne in mind that the distance which the water must be lifted is of more consequence than the distance which it must be carried horizontally, unless the latter is very great, and therefore that it is easier and less expensive to get water to a locality. at a low level some distance from the source than to raise it to a locality which is not far removed but which is high above the source. Finally, unnecessary drilling should be avoided. There is seldom an adequate reason for drilling a well on the exact summit of an anticline if transferring the site a short distance will decrease by 100 or 200 feet the amount of drilling that must be done.

The writer's conception of the best localities for testing the anticlines described in this paper and the probable depth to the Fort Scott limestone at each locality are given below:

Pearsons Switch anticline: Deepen well No. 1 in sec. 18 and No. 1 in sec. 20, T. 27 N., R. 8 E. Fort Scott limestone about 1,100 feet below the lowest horizon which is at present producing gas in these wells.

Round Top anticline: 1,500 feet east of west line and 2,000 feet south of north line of sec. 32, T. 28 N., R. 8 E., in the bottom of small valley tributary to Sand Creek. Depth to Fort Scott limestone about 2,000 feet.

Triangle dome: 2,000 feet west of east line and 200 feet south of north line of sec. 28, T. 28 N., R. 8 E., on hilltop. Depth to Fort Scott limestone about 2,100 feet.

Upper Dog Creek anticline: 2,500 feet west of east line and 600 feet south of north line of sec. 20, T. 28 N., R. 8 E., in bottom of valley of Dog Creek. Depth to Fort Scott limestone about 1,900 feet.

Ricerock anticline: 1,000 feet east of southwest corner of sec. 7, T. 28 N., R. 9 E., on top of low ridge. Depth to Fort Scott limestone about 1,700 feet. Upper Pond Creek dome: 2,000 feet east of west line and 2,000 feet north of south line of sec. 19, T. 28 N., R. 9 E., in valley. Depth to Fort Scott limestone about 1,600 feet.

Drennan anticline: 1,500 feet south of northwest corner of sec. 32, T. 29 N., R. 8 E., on flat. Depth to Fort Scott limestone about 1,900 feet.

Stone House anticline: 1,000 feet north of southwest corner of sec. 15, T. 29 N., R. 8 E., in valley. Depth to Fort Scott limestone about 1,900 feet.

Lower Dog Creek dome: 2,300 feet south of north line and 900 feet east of west line of sec. 30, T. 29 N., R. 9 E., in bottom of valley of Buck Creek. Depth to Fort Scott limestone about 1,600 feet.

Western Lower Buck Creek anticline: 600 feet west of northeast corner of sec. 32, T. 29 N., R. 9 E., in bottom of valley, on section line. Depth to Fort Scott limestone about 1,400 feet.

Eastern Lower Buck Creek anticline: 1,000 feet west of east line and 200 feet south of north line of sec. 33, T. 29 N., R. 9 E., in bottom of valley. Depth to Fort Scott limestone about 1,400 feet.

Benchmark anticline: 1,500 feet east of west line and 1,500 feet south of north line of sec. 16, T. 29 N., R. 9 E., on long spur. Depth to Fort Scott limestone about 1,500 feet.

The estimated depths to the Fort Scott limestone are based on the assumption that the stratigraphic conditions throughout the district are practically the same as those observed in wells drilled in the northeast corner of the area.

GEOLOGY AND OIL AND GAS PROSPECTS OF THE LAKE

BASIN FIELD, MONTANA.

By E. T. HANCOCK.

INTRODUCTION.

LOCATION AND EXTENT OF THE FIELD.

The Lake Basin field is situated in south-central Montana and includes portions of Sweet Grass, Stillwater, Musselshell, and Yellowstone counties. It embraces an area of about 1,000 square miles between the Chicago, Milwaukee & St. Paul Railway and the main line of the Northern Pacific Railway. A branch of the Great Northern Railway running southeast from Great Falls crosses the eastern part of this field and furnishes shipping facilities at Broadview, Comanche, and Acton. The Billings & Central Montana Railway with its beet elevators furnishes a ready means of transporting the large quantity of sugar beets raised on the irrigated lands along Yellowstone River.

ACKNOWLEDGMENTS.

In presenting this report the writer desires to express his thanks to David White and M. R. Campbell for valuable suggestions and criticisms, and to W. T. Thom, jr., and J. D. Sears for assistance in the detailed mapping. He also wishes to call attention to the public service rendered by the oil and gas operators who have furnished records of deep borings and by individuals who have contributed in no small way to the success of the investigation.

EARLIER INVESTIGATIONS.

The history of geologic investigation of the region including the Lake Basin field begins with the Northern Transcontinental Survey of 1882. Prior to that time geologists had described certain structural features and the stratigraphic succession at points closely adjacent, such as Judith Gap, the canyon of the north fork of the Musselshell, and the Bridger Range, but almost nothing had been written concerning the geology of the area herein described. One of the topographic maps made by the Northern Transcontinental Survey in 1882-1884 (the Crazy Mountain sheet) includes a small area in the northwest corner of the Lake Basin field. These maps