Total organic-carbon (TOC) content present in potential source rocks significantly affects the response of various well logs. This paper discusses and illustrates well-log anomalies caused by TOC as observed on various wireline measurements, including resistivity (or conductivity), acoustic, nuclear (density and neutron), gamma ray, natural gamma ray spectra, and pulsed neutron [sigma and carbon/oxygenField examples of these well-log responses in open and/or closed wellbores are presented from several countries. Several correlations between TOC and individual and/or combinations of various logging responses are also reviewed.

During the past quarter century, the exploitation of oil and gas reserves, associated with thick sequences of very fine-grained and coarse-grained rocks in Tertiary basins, have become increasingly important for fulfilling the world's energy needs. Many exploration and reservoir development problems have arisen which demand an analytical solution. The scientific problems have arisen which demand an analytical solution. The scientific and technological problems associated with these geologically young basin sediments include the origin, maintenance and distribution of abnormally high pore-fluid pressures, chemical changes induced in the interstitial water by compaction, origin and migration of hydrocarbons, temperature gradients, clay minerals phase changes and subsidence of the surface. Successful drilling to depths greater than 20,000 feet in these sediments and the amounts of the hydrocarbons discovered and produced depend to a great extent on our knowledge of the physical and mechanical properties and deformation characteristics of the sediments and the interrelationships between their various petrophysical and fluid properties. This paper is an historical review of studies dealing with the effects that gravitational compaction of sediments has on hydrocarbon reservoirs and source beds. Specific attention will be given to the generation of abnormal pore-fluid pressures, chemistry of interstitial fluids, compaction models, pressures, chemistry of interstitial fluids, compaction models, compressibility of the reservoir rocks and surface subsidence. Introduction It has been postulated that gravitational compaction of sediments is directly related to the following parameters and can be functionally represented in the following manner: C = (1) Where C is the degree of compaction, is the stress on the sediment system, is the velocity parameter for solids and interstitial fluids in the system, is the density, is the bulk volume of the sediments, is the porosity, is the permeability of the system, is the burial depth, t is the time, T is the geothermal temperature, and c represents the compressibility relationships. Gravitational compaction of sediments under the influence of their own weight has long been a recognized geologic phenomenon. In the seventeenth century Steno attributed variations in the attitude of sedimentary formations to compaction.

Gamma ray spectral logging devices, in addition to total gamma ray counts, record the individual contributions of potassium-40 isotope, uranium series nuclide bismuth-214, and thorium series nuclide thallium-208. Application of these data to identify fractured shale reservoirs and source-rock characteristics of argillaceous sediments is discussed. Introduction Highly radioactive, black, organic-rich, and gaseous shales are encountered in several U.S. geologic provinces. Such organic-rich shales are not only potential source rocks but frequently owe their localized but significant production potential to natural fracture systems in an otherwise impermeable rock. These natural fracture systems normally are concentrated in the interbedded brittle, calcareous, cherty, or silty zones.Conventional logging and interpretive techniques are not adequate to evaluate satisfactorily the complex and frequently fractured shale reservoirs. Novel applications of gamma ray spectral logging data for characterizing these shale formations as to their reservoir properties and source-rock potential (SRP) are discussed here.Calcareous and silty zones, both characterized by low values of potassium and thorium but excessively high values of uranium, are located easily with natural gamma ray spectral information obtained from highly sensitive scintillation spectrometer logging tools. These interpretive concepts already have assisted in many successful gas- and oilwell completion and recompletion attempts in the more permeable and/or fractured intervals of such shale formations.Such logging information also allows a continuous monitoring of the SRP of shales in open and cased boreholes. Hence, both vertical and lateral SRP variations can be studied using appropriate mapping techniques. Gamma ray spectral data also assist in detailed stratigraphic correlations, because in addition to total gamma ray counts, individual gamma rays emitted by potassium-40 (K(40)), the uranium series nuclide bismuth-214 (Bi(214)), and the thorium series nuclide thallium-208 (TI(208)) are measured.K(40) emits gamma rays at 1.46 MeV, Bi(214) emanates gamma rays at 1.764 MeV, and TI(208) emanates gamma rays at 2.614 MeV. These nuclides are of particular interest to the oil industry because all are found, in various amounts, in subsurface formations as constituents of potential reservoir rocks. Based on an extensive literature search and on recent field observations, a data compilation has been published to document potassium, uranium, and thorium distributions in various rock types.This discussion focuses on the use of gamma ray spectral logging to interpret the reservoir pore structure present in shales. JPT P. 2053＾

Log-derived spectral gamma ray data have been correlated to core-derived Qv data (i.e., cation exchange capacity per total pore volume) in a Texas coast Tertiary sand, east Texas Jurassic sands, and an Alaskan elastic formation. Such correlations can be used to provide a continuous in-situ water saturation estimate in shaly sands based on the Waxman-Smits equation. Introduction Exceptionally few hydrocarbon-bearing clastic reservoir rocks are essentially free of clay minerals. The significant effect of the latter on important reservoir properties such as porosity, water saturation, and permeability and on most geophysical well log responses is well-established.In clastic reservoirs, various types of clay minerals may occur in dispersed, laminated, or structural form. The types of clay distribution, each with a differing effect on effective porosity, can be inferred from crossplots of well logs, visual study of cores, or detailed SEM investigations. The latter distinguish dispersed-clay occurrences such as discrete particles (patchy kaolinite), pore lining, and pore bridging (illite, chlorite, and smectite), each of which have a pronounced but different effect on reservoir permeability.Clay minerals may be characterized in several ways. Table 1 lists composition, density, hydrogen index, cation exchange capacity C ec, and distribution of potassium, thorium, and uranium based on spectral gamma ray information for some of the more common clay minerals.Furthermore, numerous log-derived clay content (shaliness) indicators are reviewed and discussed (Table 2). These techniques basically assume identical properties for clay present in clastic reservoir rocks and adjacent shales. However, this assumption is often unrealistic.With the advent of the Waxman-Smits model to calculate reliable water saturation in shaly sands, emphasis has been focused on log-derived evaluation of C ec per total pore volume, Qv. The novel application of spectral gamma ray data will be discussed in detail. Water Saturation Calculation Models and Associated Parameters Archie, in his classic empirical equation, relates formation conductivity Ct, formation-water conductivity Cw, and the formation resistivity factor F (a function of porosity phi and cementation exponent m) to the formation-water saturation Sw. Ct =Sw CwF(-1), where F= phi (-m). Archie's equation satisfactorily applies to clean sands. The presence of clay minerals, however, has a detrimental effect on Sw calculations. Since such Sw results are often too pessimistic, several clay/quartz distribution models and Sw calculation concepts have been proposed. JPT P. 1641＾

Reliable evaluation of hydrocarbon resources in shaly clastic reservoir rocks is an important, although difficult, task. This paper briefly reviews the wide variety of interpretive models that has evolved. The discussion focuses on digital shaly-sand evaluation techniques based on the Waxman-Smits model to provide information on total and effective reservoir porosity and fluid distribution, silt content, volume, type (smectite, illite, and chlorite/kaolinite), and distribution modes (dispersed, laminated, and structural) 'of the clay minerals present in subsurface formations. Case studies from different geologic environments present field experiences in clastic reservoir rocks that exhibit a wide range of porosity and permeability and various amounts, types, and distribution modes of clay minerals. A log-derived formation damage index is also discussed. Finally, emphasis is placed on improved reservoir evaluation of thinly laminated shale/sand intervals through the integration and enhancement of resistivity data, short-spaced dielectric measurements, and/or analytical data-blocking routines.S w telates to total interconnected pore space.Journal of Petroleum Technology, February 1987 R ellR t from 0.5 to 1.0. ReI approaches R t.