GEOLOGICAL LOG INTERPRETATION TUTORIAL Text and Figures by Geoff Bohling and John Doveton The following pages will familiarize you with the basics of the geological interpretation of common logs as they appear on hard-copy blue-line logs (or their electronic raster copies). The lithologies considered are those common in sedimentary successions, with particular emphasis on sandstones, limestones, and dolomites, which have potential for oil, gas, or water production. However, other log keys are given of evaporite sequences and coal-bearing clastic successions. The logs used in interpretation are all measurements of nuclear properties, which are sensitive to both fluids and gases within the pore space and the mineral composition of the rock framework. They are the gamma-ray, neutron porosity, bulk density, and photoelectric index logs. These logs are available for thousands of wells across Kansas, either as paper copy, raster or digital logs. Mastery of the principles described in this tutorial and practice with the Oz Machine will give you a start with the skills to "read the rocks" from these wireline logs. The Gamma Ray Log A majority of the elements are found in a variety of isotopic forms. Many of these isotopes are unstable and decay to a more stable form, while emitting radiation of several types. Gamma rays have significantly high penetrations and can be measured by simple devices such as Geiger counters or scintillation detectors on logging tools. Of the many radioactive isotopes which are known, only three types occur in any appreciable abundance in nature: The uranium series The thorium series, and The potassium-40 isotope The measurement scale of the gamma-ray log is in API (American Petroleum Institute) units, accepted as the international reference standard that allows consistent comparisons to be made between a wide variety of gamma-ray counting devices. The API standard is set by the primary calibration test pit at the University of Houston where a radioactive cement calibrator is assigned a value
of 200 API units and conceived originally so that a typical Midcontinent shale would register at about 100 API units. Analyses of the North American Shale Composite (NASC) reference standard reported values of Th 12.3 ppm, U 2.66 ppm., K 3.2%, which converts to an equivalent gamma-ray log reading of 121.7 API units. Although higher than the vague assertion that a typical Midcontinent shale should read about 100 API units, the hypothetical log value of the NASC standard is a good match with the gray shales of the Pennsylvanian succession shown in the figure at right. The black shales, however, are prominent as thin anomalously radioactive zones. Their markedly different character is produced by a high U content that supplements radioactive sources in gray shales of 40 K contained in illite and other K-bearing minerals, and Th contained in monazite in the silt and clay fraction and adsorbed at clay-mineral surfaces. In the majority of stratigraphic and petroleum geological applications, the gamma ray log is used as a "shale log", both to differentiate shales and "clean" formations and to evaluate shale proportions in shaly formations. Typical sandstones, limestones and dolomites have relatively low concentrations of radioactive isotopes as contrasted with shales. Most carbonates show very low levels of radioactivity unless they contain disseminated shale or have been mineralized by uranium-bearing solutions. Simple orthoquartzites show similarly low values, although relatively high readings may be introduced by significant amounts of shale, felspar, mica or heavy minerals such as zircon.
Density and Neutron Log Overlay The gamma-ray log generally allows a basic distinction of shales from non-shales but is not usually diagnostic of the rock type in hydrocarbon reservoir or aquifer formations. Neutron and density logs are used to evaluate porosity in these units but are also affected by the neutron moderating characteristics and densities of the formation minerals. By overlaying the two logs on a common reference scale, a true volumetric porosity can be estimated and the formation lithology interpreted. A scale of equivalent limestone percentage porosity is the most commonly used reference because limestone is intermediate in its neutrondensity properties between sandstone and dolomite. A hypothetical overlay is shown of neutron and density logs for some common reservoir lithologies and a shale in the figure. Shales show a high gamma-ray reading, a high neutron reading, and a moderate density reading. Limestones generally have a low gamma-ray value, and a coincident density and neutron response, because of common calibration to an assumed limestone porosity scale. Dolomites have a low gamma-ray value, a relatively low density porosity (because the grain density of dolomite is higher than calcite) and a relatively high neutron reading (because the neutron moderating character of dolomite is higher than calcite). Sandstones have a low gamma-ray value, a relatively high density porosity (because the grain density of quartz is less than calcite), and a relatively low neutron reading. The true, effective porosity of shale-free zones in the reservoir lithologies is approximately midway between the two extremes of the neutron and density porosities.
The Photoelectric Index The photoelectric index (Pe) is a supplementary measurement by the latest generation of density logging tools, and records the absorption of low-energy gamma rays by the formation in units of barns per electron. The logged value is a direct function of the aggregate atomic number (Z) of the elements in the formation, and so is a sensitive indicator of mineralogy. The values are less sensitive to pore volume changes than either the neutron or density logs, so that the index is an excellent indicator of mineralogy. The common reservoir mineral reference values are : quartz 1.81 ; dolomite 3.14 ; calcite 5.08 barns/electron. The photoelectric index log is commonly scaled on a range between 0 and 10 barns/electron, and a generalized interpretation guide is given in the figure. The variable compositions of clay minerals means that their position on the scale should only be taken as a broad indication. The ordering of clay minerals on the index is almost entirely a function of their likely content of iron. The photoelectric index log is particularly useful when considered in conjunction with the neutron/density porosity overlay as an additional input to resolve mixtures of minerals such as commonly occur in "complex carbonates" as cherty dolomitic limestones or anhydritic dolomites. Successful interpretation requires the disposition of the neutron/density porosity traces to be considered simultaneously with the Pe curve. The photoelectric factor curve should be watched most carefully, because it has a finer vertical resolution (about half a foot) than the neutron/density curves (about 2 feet). As a result, the Pe character can give a better reading on lithology in thin beds, where the averaging effect of adjacent thick beds may smooth the neutron and density responses adversely
Multiple Log Overlay The addition of the photoelectric index curve to the gamma ray and neutrondensity log overlay provides both additional validation of simple lithology picks and the resolution of ambiguities in interpretation of "complex" (multimineral) lithologies. The generalized expectations of log patterns for shales and endmember reservoir lithologies of limestone, dolomite, and sandstone are shown in the figure. In the examination of the neutron-density log overlay, dolomites and siliceous rocks (either sandstones or cherts) can be recognized by the curve separations. However, the close overlay of the two could be caused either by a limestone or a cherty dolomite (or a cherty dolomitic limestone!). The inclusion of the photoelectric index can be used to choose between these alternatives. Similarly, a dolomite reading on the photoelectric index curve could also be caused by a cherty or sandy limestone. The simultaneous consideration of the neutron-density log overlay resolves the more likely of these two interpretations. An example of the log interpretation of simple and "complex" lithologies is described on the next page.
Example from Central Kansas The example section has a variety of common rock type mixtures, which makes it a good demonstration of the interpretive power of the combined photoelectric index and neutron/density overlay. The Cherokee "Burgess Sandstone" is picked out by the density/neutron crossover which is shown to be a silica matrix effect rather than "gas effect" by the Pe value. As with the other log measures, the photoelectric index does not distinguish whether the silica mineral is quartz sand or chert. This additional distinction must be made either from drill-cuttings information or inferred from petrographic experience of correlative units. The carbonates in the underlying Mississippian have zones with almost every mixed lithology drawn from the three endmembers of limestones, dolomites and cherts. Limestones are easily recognized as segments of Pe curve trace which are about 5 barns/electron, and a close match of the neutron and density porosity curves. At higher porosities, limestones will show minor drifts in the Pe value below 5, but these zones could equally represent either dolomitic limestone (recognized by systematically higher neutron than density porosity, but "clean" gamma-ray response), shaly limestones (higher neutron than density porosity, but indications of shale from the gamma-ray logs), or cherty limestones (density porosity higher than neutron porosity). Cherty dolomites are sometimes difficult to discern on the neutron/density overlay alone, and may even look like limestones, because of the conflicting effects of dolomite and quartz. However, cherty dolomites are marked on the photoelectric index by distinctive shifts downward from the dolomite towards the quartz value. As with so many visual processes, the pattern recognition of lithologies from these logs is easier to do than to describe in words! After some limited practice, log sequences can be "read" for rocktype very rapidly, particularly since most zones are simple lithologies
Logging Properties of Evaporites The geological interpretation of log overlays is easily extended to sedimentary lithologies other than the common reservoir lithologies of sandstone, limestone, and dolomite. The common evaporite minerals of gypsum, anhydrite, and halite (listed in the order of their evaporitic appearance in the Usiglio Sequence) have highly distinctive logging properties as shown in the table: Pe neutron porosity density porosity (bulk density) Halite 4.7-3 39 (2.04) Anhydrite 5.1-2 -16 (2.98) Gypsum 4.0 60 21 (2.35) and on the figure. Halite and anhydrite have markedly low and high bulk densities, respectively, while the very high neutron porosity of gypsum is caused by hydrogen in its water of crystallization
Logging Properties of Coal Sequences Clastic successions containing coals were commonly developed in deltaic environments with clastic deposits of shales, siltstones, and sandstones, as well as occasional ironstones (typically siderite). The clay mineralogy of the finergrained rocks is quite variable and can show elevated contents in kaolinite, particularly in paleosols. The logging properties of coals vary according to their rank, but typical figures are: Pe neutron porosity density porosity (bulk density) Anthracite 0.16 38 72 (1.47) Bituminous 0.17 60 86 (1.24) Lignite 0.20 52 89 (1.19)