Pennsylvanian Delta Patterns and Petroleum Occurrences in Eastern Oklahoma

Abstract

Data from modern deltas have made it possible to interpret the origin of the Pennsylvanian sandstone in northeastern Oklahoma. Objective criteria from cores and outcrops used to define specific areal and vertical depositional patterns include: (1) vertical patterns of sedimentary structures, bedding, and grain size; (2) clay mineralogy and detrital clasts; (3) trace fossils; and (4) detailed analysis of textures. The historical development of a delta provides the insight for interpreting the depositional patterns observed in ancient deltaic sandstone bodies. The processes of progradation and maturation are used to develop a four-dimensional deltaic model. Six subdivisions are distinguishable: (1) lower alluvial plain; (2) upper deltaic plain; (3) lower deltaic plain; (4) subaqueous sand sheet; (5) marginal basin; and (6) marginal depositional plain. The lower alluvial plain is characterized by stream meandering, point bars, and confined channel flow. Stream gradients are reduced in the upper deltaic plain, distributary bifurcation occurs, and marsh, natural levee, and floodplain deposition is predominant. Development of the lower deltaic plain is controlled by crevassing of natural levees, tid s, and floods. The subaqueous sand sheet is characterized by shallow water currents. It is modified by progradation and commonly replaced by deltaic-plain environmental units. The marginal basin and depositional plain are produced by longshore drift and reflect a balance between subsidence, sediment supply, and wave energy. These environmental units may be modified or replaced by other deltaic elements. This model has been applied to the Bluejacket-Bartlesville sandstone of eastern Oklahoma. This stratigraphic interval was selected because of its importance as an oil reservoir, the large amount of available subsurface data, and the simplicity of the stratigraphic and structural framework. Lower and Middle Pennsylvanian sediments were deposited during an overall transgression, but the transgression is marked by extensive regressions. These are widespread sandstone sheets generally underlain and overlain by marine shales or limestones. The sandstones are distributed over thousands of square miles, but locally they are lenticular. Each regression is in response to the outbuilding of sediment under static sea level conditions. The supply of sediment is genetically related to a single river system, and deltaic patterns related to this system can be traced through Morrowan, Atokan, and Desmoinesian strata.