SIMILARITY OF STRATIGRAPHY TO THAT OF NEIGHBORING PRODUCTIVE FIELDS. Plate XVII (in pocket) was prepared for the purpose of showing the stratigraphic position and relative thickness of sands in the Colorado shale in south-central Montana and north-central Wyoming. The relation of these sands is discussed under the heading "Colorado shale" (pp. 111-113). Some of the stratigraphic sections were carefully measured where the strata are especially well exposed. Others are based principally upon drill records in fields where oil and gas were found to exist in the sands. Throughout the Western States the principal concentrations of oil and gas occur in moderately coarse sands which are included as lenses and well-defined beds in great bodies of shale. These sandy layers are commonly referred to as "oil sands." From the accompanying sections (Pl. XVII) it is obvious that the principal concentrations of oil and gas in north-central Wyoming occur in sands interbedded in the lower part of the Colorado deposits. There are, however, certain concentrations of oil and gas in sands below the Colorado, as for example in the Greybull sand, at the top of the Cloverly formation, at Basin, Wyo., and in the Cloverly sandstone and in sandstones in the Sundance formation in the Powder River field, Wyo. In certain localities sandstones many hundred feet above the sands of the Frontier formation, of the Colorado group, are known to be productive, as for example the Shannon sandstone of the Powder River and Salt Creek fields, Wyo. The lack of continuity of the Frontier sands is mentioned under the heading "Colorado shale" (pp. 111-113). In certain localities-for example, at Salt Creek-there is but one important oil-producing sand (Wall Creek sandstone) in the Frontier formation, and apparently there are no other sandstones of much value as oil reservoirs between that sand and the base of the Colorado, whereas at Basin, Wyo., on the east side of the Big Horn Basin, the Colorado group contains three well-defined sands in the Frontier formation, two in the Mowry shale, and one near the middle of the Thermopolis shale, each containing a certain amount of oil and gas. Finally, at Elk Basin, on the Wyoming-Montana State line, the Frontier formation includes two sandstones each of which is productive of oil. There is little doubt that the formations present in north-central Wyoming extend northward and underlie the Lake basin field. It is also reasonably certain that the sandstones in the Colorado and Kootenai formations are sufficiently near the surface to be tested by the drill under the most favorable structural features in the Lake Basin field. The principal element of uncertainty for the oil prospector is the nature and extent of these sands. From the small FAULTED ANTICLINE OF EAGLE SANDSTONE, LOOKING SOUTHEAST FROM A POINT ON THE SANDSTONE IN THE BEARPAW SHALE IN THE NORTHWEST CORNER OF SEC. 16, T. 2 N., R. 25 E., MONT. number of well logs available it appears that well-defined sandstones such as those present in most of the productive Wyoming fields are lacking in the Lake Basin field. It may be, however, that the available drill records fail to represent the true nature of the Colorado sands and that future drilling will establish the existence of sandstones under parts of the Lake Basin field similar to those underlying certain portions of the Musselshell Valley, farther north. The following are the logs of wells drilled in and near the Lake Basin field: Log of Monarch Oil & Gas Co.'s well at Billings, Mont., near west line of sec. 34, T. 1 N., R. 26 E. Sand. Salt water came in at 1,840 feet and filled the hole about 1,200 feet. Lime shells. Shale.. Lime shells. Shale. Lime shells. Shale.. (Formation not given). 125 2,425 Some gas comes in, apparently at a depth of 2,425 feet, and finds its way to the surface. Log of well near top of Broadview dome, in the SE. sec. 13, T. 3 N., R. 22 E. Shale, black and brown. Shale, light.. Sandstone, soft and brown, containing brackish water which flowed over top of casing. 1,400 1,550 Shell limestone containing some gas. Shale, containing shells and bentonite. Sandstone, soft, brown, containing fresh water and a little gas. Shale, dark and light colored, containing bentonite and lime shell. Shale, variable in color, including lime shells. Shale, sandy. Shale, light colored (bailer and tools lost in hole at 2,680 feet). Log of 79 Oil Co.'s, No. 1 drill hole, in sec. 35, T. 5 N., R. 19 E. Dark-green to black soft fissile shale (flow of water and some gas at 130 feet; small pocket of gas at 745 feet).. Gray to black hard, dense sandy shale to shaly sandstone, thin bedded (show of gas at 1,040 feet).. Greenish-black soft fissile shale (sandy streak with small show of oil at 1,135 feet).. Soft white shale... Sandstone.. Soft gray-maroon shale (thickness estimated).. Log of well in Hailstone Basin, sec. 17, T. 3 N., R. 21 E. The four well logs given above ought to represent fairly well the nature of the Colorado shale in the Lake Basin field, as the wells are rather uniformly distributed from northwest to southeast. At the 79 Oil Co.'s well in the northwestern part of the field, the first sandy beds recorded occur about 1,400 feet below the top of the Colorado shale, but at Billings, in the southeastern part of the field, sands were first encountered about 300 feet lower down. The first three of the wells probably penetrated the main sandstones of the Kootenai as well as all those in the lower part of the Colorado shale. From the records it appears that no sandstones were encountered at all comparable with those at Elk Basin or Basin, Wyo., and apparently only meager showings of oil and gas were obtained. SURFACE INDICATIONS OF OIL AND GAS. There are very few surface indications of oil and gas in the Lake Basin field. The field notes record several reported occurrences of films of oil on springs and wells, but few of these reports were verified. At the fault near the west quarter corner of sec. 30, T. 4 N., R. 18 E., there are several strong springs, and according to reports oil has been seen floating on the water. The water was examined by one of the members of the field party, but no films of oil were detected. A well was drilled 86 feet into the Bearpaw shale near the west quarter corner of sec. 25, T. 2 N., R. 19 E., and a black film on the water was believed at the time to be oil. Other wells in the vicinity |