Kansas Geological Survey, Subsurface Geology 12, p. 55-56
by
John H. Doveton
Kansas Geological Survey
The use of wireline logs in subsurface studies is all too often restricted to the correlation of selected stratigraphic horizons. The value of such correlations to sedimentary modeling lies in their definition of three-dimensional surfaces which express the large-scale geometry of sedimentation units. However, the set of correlation surfaces is purely a topological skeletal framework, as it is based entirely on depths and geographic coordinates. Explicit geological information linked with the magnitudes of the log measurements can be used to infill the body of the framework. The traces of older logs reflect primarily shale content and pore volume and provide crude but effective indicators of gross geological variation. The demands of modern reservoir engineering have stimulated the development of new wireline tools whose measurements are sensitive to mineral and elemental compositions in clastics, carbonates, and shales. These tools are now run commonly and their logs can be transformed to continuous and quantitative profiles of interpreted composition and geochemistry.
The lithodensity-neutron tool combination simultaneously records apparent bulk density, photoelectric absorption, and neutron porosity, together with a conventional gamma-ray measurement. The contrast between the neutron and density curves has been used successfully for a number of years in the distinction of quartz, calcite, and dolomite, as well as the recognition of some evaporite minerals. The more recent determination of photoelectric absorption provides a measure of aggregate atomic number. This is a valuable additional variable in the resolution of complex mineral mixtures. It also appears to be useful in the distinction of clay-mineral facies within shale sequences, although the controlling parameter is probably clay-mineral iron content. The spectral gamma-ray tool partitions the total natural gamma ray flux of the subsurface formation between contributions attributed to isotopes of potassium-40 and the uranium and thorium series. The logs are scaled in units of parts-per-million (uranium and thorium) and percent (potassium). The computation of a thorium-potassium ratio has proved useful in the qualitative distinction between potassium-rich illite/mica/feldspar and potassium-poor kaolinite/smectite/ chlorite. A log of thorium/uranium ratio accentuates zones of relative uranium enrichment or impoverishment. This ratio can often be interpreted in terms of redox potential, either due to depositional processes or diagenetic modification.
Graphic profiles of lithofacies and KUT (potassium-uranium-thorium) geochemical facies were generated by computer processing of lithodensity-neutron and spectral gamma-ray logs from two Kansas subsurface sections. The two logged sections were chosen as case-study demonstrations of the great potential of modern wireline logs to be used in conjunction with sedimentary modeling. The first section is from a hydrologic observation well in central Kansas which was drilled through the Cretaceous into the Permian Cedar Hills Formation (fig. 1). A drastic shift on the thorium potassium ratio log highlights the basal Cretaceous unconformity, caused by the higher feldspar content of the Permian sandstones. Fluctuations of the ratio within the deltaic sandstones and shales of the overlying Dakota Formation appear to be broad reflections of contrasts between illite-rich marine regimes and more kaolinitic freshwater environments. The photoelectric absorption factor is an additional aid in the subdivision of shale facies, probably linked with iron content of different clay minerals. Relatively high uranium zones indicate fixation under reducing conditions which are often marine, as attested by glauconite and other marine indicators seen in the drill-cuttings. Mobilization of uranium under oxidizing conditions in nonmarine and subaerial environments would account for the repetitions of thin uranium-poor zones which characterize much of the Dakota Formation. The repetitive character probably reflects high lateral variability in deltaic facies and interplay between mostly brackish and freshwater regimes. By contrast, the uranium enrichment of the overlying limestones and shales shows a long-term (115-m [380-ft] wavelength) cyclic pattern which is interpreted to be the product of regional transgression/regression alternations on an open-marine shelf.
Figure 1--Gamma-ray, lithology, and thorium-potassium-uranium ratio logs of a Permian to Cretaceous Section in Ellis County, Kansas.
The second case study is taken from a gas well in southwestern Kansas which penetrated the Permian Chase Group. The section consists of an alternating sequence of carbonates and shales. The carbonates are variable, consisting of dolomites with some anhydrite, ranging to limestones, which can have a high chert content. Processing of the lithodensity-neutron log combination generated continuous composition profiles of dolomite, chert, calcite, anhydrite, and shale. The thorium-potassium ratio log indicates that the shales are primarily illitic in character. Core studies classified subdivisions in terms of the depositional environments, ranging from shallow marine to supratidal, and these are ordered in distinctive transgressive/regressive sequences. The thorium-uranium ratio curve shows good concordance with the classification cyclic pattern, oscillating between the extremes of relatively enhanced uranium marine limestones and uranium-poor supratidal shales. However, realistic geochemical interpretations must incorporate considerations of the role of diagenesis, especially linked with dolomitization.
The two case studies provide useful demonstrations of the kind of geological information that is now available from computer processing of modern wireline logs. The data can be incorporated in either forward- or reverse-modeling modes in the simulation and analysis of sedimentary sequences. In addition to their geologic-information content, they have the useful property of being quantitative, and so they can be used easily as input for numerical-modeling programs. In addition, the logs provide lengthy and continuous records of sections of interest. This contrasts with many traditional sources of geological observations which are often short, prosaic, and discontinuous.
Kansas Geological Survey
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Web version May 11, 2010. Original publication date 1989.
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