Two structural provinces exist in Wyoming. These provinces are the overthrust belt of western Wyoming marginal to an earlier trough, and the mountain range and intermontane basin province over the old foreland. The tectonic provinces were outlined by Cambrian time, and reflected in the later depositional history.

The attitude of the major fault planes within the overthrust belt is a major problem. The attitude near the toe of the thrust is known locally but at depth is hypothetical. Attitude of the thrustplanes will affect the amount of lateral displacement of rock masses in relation to their original sites and environments of deposition, and hence their potential as a source of petroleum. The amount and direction of displacement will affect the degree of cementation to be anticipated in reservoir rocks. Continuity of reservoir rocks and their hydrostatic systems have been interrupted by the thrust planes. The extent of available areas for free migration of hydrocarbons is dependent upon structural conditions.

The foreland province of Wyoming covers a large area and contains the majority of the state's oil fields. Localization of the major tectonic features in the province has been controlled by several factors such as: pre-Laramide fracture systems, possibly as old as pre-Cambrian; heterogeneity in the crystalline basement; and Laramide flexures independent of inherited control. Lesser structural features localize as the result of adjustment of rocks to available space in the developing intermontane

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basins. Differentiation between different types of controlling influences may lead to recognition and prediction of a basic and systematic arrangement of structural features. It appears that the controlling factors varied with time.

Faults in the foreland province are of several types. These include primary fractures in the basement complex which control the localization of folds of both major and minor dimensions, faults secondarily related to the growth of folds; faults developed due to local space accommodation; and rather extensive late fractures related to a period of regional tension. Problems arise in attempting to differentiate between systems at any single locality.

The relative age of structural traps in Wyoming influences their effectiveness. Some problems related to time and rate of structural development are: nature and amount of the hydrostatic head in aquifers; time of effective closing of a trap; subsequent tilting of a trap; time of faulting relative to accumulation. Criteria for the age of folds should be investigated.

Basic understanding of the mechanics of deformation will broaden the frontiers in exploration. Interpretation of new data is best made on the basis of an understanding of old data. Critical review of the following problems in structural geology is needed to expand our thinking.

(1) Is there a curve which best fits the majority of cross sections of Wyoming foreland folds? (2) Is parallel folding too often assumed in constructing cross sections? (3) Further study of the minor faults and their patterns in folds, and (4) quantitative study of the kind and amount of fracturing in folds relative to the amount of potential reservoir void space developed.

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