CYCLOTHERM CHARACTERIZATION FOR THE EASTERN EDGE OF THE CHEROKEE BASIN

Well log analysis is one of the most useful methods to correlate lithologic units below the surface especially when cyclotherms are interpreted. The purpose of this project is to characterize the cyclicity of sedimentary layers of rock, located in eastern edge of the Cherokee Basin, in the northwest quarter of section 17-36N-33W, Vernon Co., Missouri. Geophysical well logs (Gamma Ray (GR) Photo-electric (PE), Density logs (RHOB)) data and geochemical data (XRD and XRF) were analyzed to characterize Formation facies. Well log and geochemical samples were obtained from eight wells drilled to depths of approximately 350’. All the wells studied lie above the Mississippian Unconformity. Results show that GR values across the wells range from less than 20 API to greater than 300 API, PE values range from 1.5 to 5.5 barns/electron, with median values between 2 and 3 barns/electron suggestive of highly mixed facies. Density values range from 1.5 to 2.8 g/cc suggesting the presence of heavy minerals like dolostone. From facies analysis, fifteen distinct rock units were identified from the well log data. These were composed of thin beds of coal, fine and coarse grain sandstone (based on GR trends), shales, and limestone layers. High GR values greater than 300 API observed, indicate hot shales. High density values of greater than 2.8 g/cc suggest dolomitic type facies. Characterizing each well vertically by lithology shows that most of the well samples have a repeating pattern of thin coal beds interbedded between thick sandstone beds shales and silty limestone layers indicating cyclicity. From the GR log trends there is a serrated pattern through the sandstone unit and a coarsening pattern of sediment towards a thin coal bed, that may indicate that this study area was once located within a transitional continental-marine environment. The succession of thick sands deposits and repeating coal beds and shale layers suggest fluctuating low energy water levels. Geochemical data will be interpreted to characterize different coal beds and lithofacies in terms of their mineral and elemental content.