The commercial oil and gas pools are on crests of closed anticlines and domes except in the General Petroleum field, where oil is found in variable shaly sandstone beds on a pitching anticline without structural closure. The oil sand of the G. P. field (G. P. sand) is not productive elsewhere even on closed structures.

Commercial oil is found in the Mowry shale on the crests of the three sharpest folds--Little Lost Soldier, Wertz, and Ferris. It is not found in the Mowry shale on the gentler folds or elsewhere. Seemingly the maximum fracturing and other mechanical effects peculiar to the crests of sharp flexures are necessary to the formation of the shale oil pools or pockets.

The productivity of the structures, other factors being equal, is in close relation to the extent to which they are fractured and faulted. Burial of the productive sand beneath a great thickness of shale is equivalent to diminution in faulting, since the faults may be sealed or may not persist to great depths. The more numerous and open the faults, the greater the tendency toward an oil pool or to barrenness through more or less complete leakage. The fewer and tighter the faults, the greater the tendency toward a gas pool or to barrenness through lack of migration and accumulation.

With the Lost Soldier and many other oil and gas fields of the Rocky Mountain region as confirmatory evidence, it may be said that, in the post-Paleozoic strata of the region (1) water in an upper sand is not indicative of what is to be expected in lower sands, whether oil, water, or gas; (2) gas, as the predominant product in an upper sand, means that lower sands may be expected to be primarily gas sands; and (3) oil, as the predominant product in an upper sand, indicates that lower sands are likely to be primarily oil sands.(FOOTNOTE 3)