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Conclusions
1. The Missourian Pennsylvanian sediments in the study area represent cyclic deposition on a portion of a broad epeiric shelf.
(a) Fluctuations in sea level over the Kansas shelf and variation in terrigenous clastic influx are proposed as the major processes that produced the Lansing-Kansas City alternating sequence of carbonate and clastic sediments representing marine, shoreface, and continental environments.
(1) Thin but extensive deposits of a transgressive carbonate unit initiate the cycles. The base of this unit closely approximates a time surface. Marine shale accumulation records the period of maximum marine transgression. Regressive deposition in the upper part of a cycle represents, in part, accretion and progradation of facies which are diachronous, building outward (basinward) with time.
During regression, patterns of sediment accumulation are combined with relative sea-level fall, eventually resulting in deposition of shallow-water facies throughout the study area. Facies belts varying from 10 to more than 100 miles wide are established across the shelf by this process. Local bathymetric relief or slope changes cause recognizable variations in facies distribution.
The later regressive phase of the cycle is dominated by clastic accumulation deposited under continental conditions. Subaerial exposure and freshwater diagenesis of the regressive carbonates occurred at this time.
(2) Climate was probably semi-arid or included prolonged dry periods, as inferred from the paleolatitude of the area and the type of age-equivalent deposits in the surrounding western Midcontinent. Regressive shale deposits are also thin and suggest a lack of significant runoff. Paleosoil development is similar to soils developed in the southwestern United States.
2. Early freshwater diagenesis significantly altered the regressive carbonate, such that primary porosity was frequently enhanced and effective secondary porosity was produced.
(a) Porosity trends, controlled by the depositional environment, generally extend beyond the oilfield limits. Untested areas "along trend" are prospective locations, particularly in areas with structural closure on the top of the prospective reservoir.
(b) The extent and form of diagenetic alteration and pore cement distribution are functions of the conditions of diagenesis and the composition of the host carbonate. The accessibility and chemistry of diagenetic waters affecting these sediments are important. Similarly, the relative abundance of unstable carbonate components such as phylloid algae (aragonite) versus more stable (less easily dissolved) crinoid debris (Mg-calcite) can significantly change the end product of diagenesis, i.e., porosity enhancement or porosity occlusion.
(c) Intensity and probably duration of freshwater diagenesis increases northward toward the landward shelf margin. The relative degrees of diagenesis are expressed by formation of subaerial crusts, in situ brecciation, and erosion of the regressive carbonate, and by soil development and oxidation within the regressive shale.
3. Subsurface methods including core and sample description and petrophysical well-log interpretation can be integrated to define and understand prospective hydrocarbon trends. Even though well cores may be few in number, the benefits of geological and engineering understanding of the reservoir and its surrounding rock facies more than offset the infrequency of taking cores.
Occasional cores are justified in a drilling program. A thorough well-site geological description followed by selective laboratory core-analysis measurements proves most useful to the immediate well and to a timely analysis for additional drilling. Likewise a core from an unproductive zone can also be important by providing economically useful information. A water-wet but very porous interval still fulfills one of the reservoir requirements. Knowing why the carbonate is porous or tight is very useful when deciding what the next step will be. Using this "dry-hole" core data may provide insight for map modification and suggest additional drilling locations.
Acknowledgments
W. J. Ebanks, Jr., and John Doveton were instrumental in initiating this project through their encouragement and support. I thank all the individuals and companies who supplied correlation log data along with their support for the project. Particular thanks go to those who donated to the Survey core that was used in this report, namely Cities Service Company, John Farmer, Ted Gore and Joe Rakaskas (Empire Drilling Company), Murfin Drilling Company, and Skelly (Getty Oil Company). Appreciation is particularly extended to W. J. Ebanks, Jr., for his stimulating discussion and suggestions throughout the course of the study and his critical review of the manuscript. Thanks are given to Donna Saile and Teresa Jewell for typing the manuscript and to Beverly Ohle for preparing the illustrations.
Conversion Table
1 mile= 1.609 kilometers
1 foot= 30.480 centimeters= 3.048 meters
1 inch= 2.540 centimeters
1 square mile= 2.590 square kilometers
1 acre= 0.405 hectares= 4047 square meters
1 gallon= 3.785 liters
1 barrel (petroleum) = .1589 cubic meters
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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/220/07_conc.html