Faulting in the Rocky Mountain Region

Abstract

Reverse faults, generally of the thrust type, are of little or no importance in their direct effect on oil and gas accumulation in the known fields of the Rocky Mountain region, but close association with certain oil occurrences suggests that oil of commercial value may yet be found dependent upon thrust-fault relationships. Structural closures effected primarily by normal faults, similar in principle to those which are productive in Texas, have only recently been given attention. Thus far all tests have been failures, but it is probable that success will yet be attained. Normal faults, which occur characteristically in parallel and radial systems on domes and anticlines in the Cretaceous strata of the Rocky Mountain region, are normal accompaniments of the uplifts. They are confined to the uplifts and formed at the same time as the uplifts. For such features the name epi-anticlinal is suggested. The dual role of faults as avenues of, and barriers to, migration and accumulation of oil and gas is discussed. Unfaulted upfolds are rare and abnormal. Usually apparent absence of faulting is to be ascribed either to insufficient or incompetent observation, to absence of resistant strata in shale outcrops, or to burial beneath surface deposits. Unfaulted structures are to be regarded, generally, with less favor than faulted ones, the chances apparently being somewhat greater that the former will be barren or will carry gas largely to the exclusion of oil. Increase in depth of a given horizon by its tendency to cause tightness of fault planes and fissures tends toward the same effect as lack of faulting.