A History of Changing Concepts in Petroleum Exploration and Development,

Dakota Group, Rocky Mountain Basins

By

Robert J. Weimer

Colorado School of Mines, Golden, CO

The history of petroleum exploration for the Cretaceous Dakota Group follows industrywide patterns of evolution from simple to more complex geological concepts, and application of new technologies. The story of changing concepts for the Dakota Group is representative of those observed for interpreting other rock sequences in the Western Interior Cretaceous basin and the Laramide structural basins. Only the most important topical subjects related to oil and gas fields are discussed and illustrated in this talk.

Search for 4-way closures on anticlinal trends dominated early exploration and resulted in important discoveries on flanks of structural basins. Extensive wrench-faulting, a concept still being developed, is now known to control fault traps, and fracturing, as well as reservoir distribution, and compartmentalization. Recurrent movement on basement faults also influenced sedimentary patterns and intra-formational erosion.

Interpretation of stratigraphic sections originally followed the“layer cake geology” concept with hopes by the explorer that charged reservoir rock would be present within a structural closure. With the introduction of the facies, progradation, onlap, and pinch-out concepts, the search for stratigraphic, or combination traps, became dominant. Basin-centered accumulations were recognized and became economical by use of hydraulic fracing. Depositional models from studies of modern coastal areas improved stratigraphic analyses.

The recognition of basin-wide unconformities, some with preserved paleosols indicating subaerial exposure, had a revolutionary affect in interpretations, and introduced the new concepts of sequence stratigraphy. Erosional relief related to younger incised valley fills changed the traditional methods of correlation.

The total petroleum system concept integrates ideas about source rock, temperature causing generation and over-pressured cells, migration, reservoir, diagenesis, seal, and trap. When the petroleum system is integrated with the available technologies in geophysics, geochemistry, drilling and logging, the explorer and developer are creating new ideas and plays to sustain production in basins long regarded as highly developed.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming