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Abstract
The Upper Pennsylvanian Lansing and Kansas City Groups of much of the western Midcontinent consist of cyclic terrigenous clastic and carbonate strata. Individual cycles less than 30 meters in thickness were deposited in northwestern Kansas as a response to fluctuations of sea level and progradation of sediments. Each cycle in this area is characterized by four basic components: a thin but distinctive basal transgressive unit (deposited as sea level rose), overlain by a marine shale, followed by the regressive carbonate and a regressive shale (regression the result of sea-level drop).
Analysis of a few strategically located full-diameter cores allows lithologic characterization of the cycles and permits correlation of cycles to signatures on geophysical logs. After a stratigraphic framework is developed with cores, interpretation of logs and samples provides details of the depositional environment through the preparation of maps, cross sections, and cross-plots that illustrate spatial variations and trends of facies and thickness. Furthermore, maps are used to display other log-derived reservoir characteristics such as structural configuration, porosity-feet variation (an estimate of the reservoir quality), and apparent water resistivity (a quick-look method of recognizing hydrocarbon saturation).
Although most strata were deposited in a simple layered fashion over the study area, thickness and facies changes are mappable and can be related to porosity development and sometimes petroleum accumulation. Subtle bathymetric relief and variation in bottom slope produce local changes in rock strata that can be resolved using the techniques described in this report. General landward (northward) shallowing and terrigenous clastic influx control facies distribution in the area of study, but on a larger scale.
Patterns exhibited by carbonate depositional facies vary in shape from elongate and trend-forming to irregular and isolated. Furthermore, the size of these facies patterns varies from square miles to tens of square miles. Maps of the thickness of cycle components defined on geophysical logs help to define these facies patterns.
Extensive early freshwater diagenesis results from subaerial exposure that occurred late in a cyclic sequence as a result of the withdrawal of the sea to the south. This has substantially modified porosity and permeability. Porosity and permeability were enhanced by partial dissolution of carbonate skeletal debris and lime mud producing moldic, granular, fractured, and sometimes brecciated carbonate textures. Freshwater diagenesis apparently was prolonged and more intense landward (northward) and locally basinward over and adjacent to areas with bathymetric relief. Secondary porosity development or destruction, critical to reservoir development, is generally erratic although large-scale patterns of diagenesis can be distinguished. Intense diagenesis results in occlusion or disruption of most porosity by shale infiltration as the carbonate disintegrates into rubble. This does not appear to be a significant problem basinward over western Kansas. Whole cores are the primary means of accurately defining this stage of diagenesis.
Significant porosity formation in the carbonates of each cycle is the result of processes of both sedimentation and diagenesis that occurred prior to deposition of the younger overlying cycle. Thus it is important to describe each individual cycle in order to use this subsurface information most effectively in an exploration program.
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Kansas Geological Survey, Cyclic Sedimentation
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Web version Oct. 2004. Original publication date Oct. 1980.
URL=http://www.kgs.ku.edu/Publications/Bulletins/220/02_abs.html