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Recurrent movement on Precambrian-age basement-fault systems has influenced the origin, thickness, and distribution of Phanerozoic strata in the interior of the North American continent. In this structural setting, tectonic and sedimentation patterns are well known for major orogenies of the late Paleozoic, late Mesozoic-early Cenozoic (Laramide orogeny), and post-Oligocene. The influence of basement-fault movement on sedimentation and erosion during anorogenic times is generally not as well known. Subtle structural movement controlled topography in the depositional basins, which in turn influenced the distribution of high energy (reservoir) and low energy (nonreservoir) deposits. In addition, extensional fractures in strata overlying basement faults provided pathways for petroleum migration, either vertically or horizontally. Inversions of structural movement along fault zones are commonly observed, and they sometimes mask the recognition of exact time of sporadic fault movement.
The most common petroleum reservoirs are deposited during rising or high sea level stands and are fluvial-deltaic, estuarine, shoreline, shelf, and lacustrine sandstones, and shallow marine and tidal-dominated carbonates. Depositional topography, which formed within the depositional basin during highstands, controls the distribution of unconformities during lowstands. Associated lowstand reservoirs are either base of slope or shelf sandstones, or secondary porosity in carbonates associated with paleokarst or dolomitization. Porosity in sandstones deposited during highstands may be infilled with diagenetic clays because of subaerial exposure during lowstands. Thus, recognition of the lowstand unconformity is of vital importance in exploration.
All rock systems record sea level changes, but those most important to petroleum exploration in western cratonic basins are in the Ordovician, Devonian, Permian-Pennsylvanian, Jurassic, Cretaceous, and Tertiary. A major problem is to determine whether structural topography, or depositional topography and eustatic changes, exerted the primary control on sedimentation. Petroleum occurrences in well-documented sequences related to sea level changes are discussed for the Rocky Mountain and Mid-Continent regions.
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