Abstract 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 (i) 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. 1 3 J. S. Irwin, “Faulting in the Rocky Mountain Region,” Bulletin Amer. Assoc. Petrol. Geol, Vol. 10 (1926), pp. 127-28.

Abstract Prediction of hydrocarbons ahead of the drill is our highest goal. Every factor relating to it has been studied extensively, except the role that traps play. I suggest that this role is important, and in some cases pivotal. Traps act as “ valves,” controlling what they retain, leak, and spill. Closure (vertical distance between spillpoint and top of the trap) vs. seal strength (height of the hydrocarbon column the seal can retain before leaking) controls this. Three classes of trap are essential to cover the possibilities. Two key hydrocarbon columns are used as thresholds for the classification: the highest possible gas column and the highest possible total column the seal of the trap will allow. The spillpoint may be above (class 1), below (class 3), or between (class 2) these two thresholds. The three classes distribute hydrocarbons differently. Class 1 traps spill rather than leak gas, finally spilling oil from the trap. Class 3 traps spill neither fluid, but accumulate oil until it balances seal strength; thereafter, gas plus excess oil leaks. Class 2 traps leak gas but spill oil, and have gas/oil contacts suspended in midtrap. The argument that oil has a lower interfacial angle and entry pressure applies only until leakage starts. Then gas’ s higher position, smaller molecules, and lower viscosity make it more mobile. Traps equilibrate based on this principle: full of gas (class 1), partly full of oil (class 3), or filled with a mixed charge (class 2). To the extent that the best provinces are exposed to an excess of both fluids, this principle controls world distribution of oil and gas. More practically in an economy-favoring oil, higher-class traps leak gas, preserving oil in themselves and traps updip. The trap class of a discovery has implications that may modify drilling sequence. During uplift, class 1 traps may be gas-flushed and thus degrade. Class 3 traps should change minimally. Class 2 traps should vent additional, now more buoyant gas, and make more space for oil, improving their economics. A slightly different scenario applies if seal strength decreases during uplift. In the North Sea, Gullfaks, Snorre, and most of the Ekofisk group of chalk anticlines are class 3, Oseberg is class 2, and Troll East, is class 1. Turner Valley in Alberta, which has a two-phase fill, perpetual gas flare, great closure, and is surrounded by gas fields with less closure, is class 2. The Mahakam Delta in Indonesia has seemingly random distribution of oil and gas fields, but on closer inspection, the class 1 gas fields may have the less-damaged seals and the class 3 oil fields the more-damaged seals. Despite historic interpretation as a class 1 trap, the Little Lost Soldier Field in Wyoming appears to be class 3.

Abstract The Lost Soldier district in south-central Wyoming contains 3 oil fields, 4 gas fields, 1 prospective gas field, and 3 anticlines with structural closures that have been proved barren of gas or oil in the formations which produce gas in the gas fields. The gas fields, Wertz, Mahoney, West Ferris, and Middle Ferris, have produced 114 billion cubic feet of gas. In the Wertz gas field only does the gas carry natural-gas gasoline in an appreciable quantity. It is estimated that 2–4 million gallons of natural-gas gasoline have been produced in this field. Commercial amounts of oil are not found in the zones containing gas. The Mahoney gas field has one small oil well in the deeper Tensleep sand. Bunker Hill anticline is the only prospective gas or oil field, and it is being drilled. There are no prospective, untested zones in the gas fields. It is estimated that the proved gas fields of the district still contain 47 billion cubic feet of gas at a reservoir pressure of 200 pounds per square inch and that the recoverable natural gas in the Wertz gas field contains 10 million gallons of natural-gas gasoline.