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Summary. 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/oxygen (C/O) ratio]. Field 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.

Introduction

Several geophysical well logs can be used to evaluate the organic-carbon content of subsurface strata, such as shale formations, and to provide an indicator for the source rock potential. In the earliest stages of oil and gas exploration, one must be certain that sufficient source rock is present for the generation of hydrocarbons. Adsorption of uranium (U) from U-bearing formation waters is controlled by pH; Eh; temperature; activities of U+6, U+4, and other cations and anions; and the type(s) of adsorbers present. Uranium adsorption by organic matter depends largely on its type, amount, and degree of maturity; and pH/Eh conditions. Uranium is easily adsorbed in the presence of carbonaceous material present in layers or in a dispersed form, particularly in a reducing environment. Such processes involve the reduction of hexavale uranium (U+6) from solution to U +4 adsorbers based-on their high ion-exchange capacity and formation of uranium/organic colloids and/or compounds. Organic compounds present in subsurface formations, therefore, play a significant role in the accumulation and concentration of uranium. Although marine black shales, which occur worldwide, usually exhibit U concentrations in the range of 15 to 60 ppm, some of these shales contain enough uranium to be considered as future uranium resources. For example, the massive black shale deposit at Ranstad, located in Southern Sweden, has an average U308 content of 300 ppm. This translates into about 300,000 tons (272 x 10(3) Mg] of uranium, which according to Jankovic represents about 15% of the world's uranium reserves. Generally, dark, organic-rich shale intervals exhibit increased natural radioactivity compared with lighter-colored shales that contain less organic matter. Both laboratory studies and natural gammaray spectral logging in open and/or cased wellbores indicate this increase in radioactivity to be primarily a result of increased U concentrations. In addition to suitable temperature and time conditions, the type and quantity of organic material and associated depositional environment will affect both volume and properties of the hydrocarbons generated. Wide variations exist in both organic matter and hydrocarbon contents of source rocks, as compiled by Hunt from many investigations. According to Jones, a majority of the world's major oil accumulations originated in source rocks with a TOC content in excess of 2.5 wt%-i.e., the Kimmeridgian shale in the northern North Sea; the upper Miocene Monterey formation of California; the Cretaceous black shale overlying the major unconformity of the North Slope (Prudhoe Bay) in Alaska; the Bakken and Woodford shales of the Paleozoic in the U.S. midcontinent region; the Silurian shales of Algeria; the Duvenay shale of the Devonian in Alberta, Canada; the Cretaceous La Luna formation in Venezuela; and the source rocks of the Middle East and Siberia, USSR. In contrast, in several Tertiary delta systems-e.g., the Mississippi River of the U.S. gulf coast and Niger River of Africa-the source rocks underlying potential reservoirs exhibit TOC content ranging from 0.3 to 0.5 wt%. The minimum concentration of organic-carbon content required for a source rock to become a source of commercial hydrocarbon accumulations has frequently been stated as being 0.5 wt%. A prerequisite for the generation and accumulation of commercial hydrocarbon reserves is the optimum interaction of numerous parameters, including source rocks, reservoir rocks, migration, seals (caprocks), timing, and sequence of geologic events.

Geophysical Well-Log Response to TOC Content

Natural Gamma Ray Spectral Logs. Since the late 1930's, natural gamma ray logging has been an integral part of formation evaluation, when it was recognized that natural gamma ray intensities vary with lithology. Today, natural gamma ray spectrum 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. Natural gamma ray logging devices, commercially available to the industry, are scintillation spectrometers that detect and measure the natural gamma rays. Gamma ray count rates from multiple energy windows are used to determine concentrations of K, U, and Th. The measured concentrations are corrected for the effects of spectral interference and Compton scattering. Various data-filtering techniques are also used. Furthermore, natural gamma ray logging devices have numerous applications in open and/or cased wellbores and can be run individually or in combination with neutron, compensated-neutron, or density/compensated-neutron/dual-induction logging devices. Highly radioactive, black, organic-rich, and gaseous shales are encountered all over the world. Such organic-rich shales are potential source rocks and frequently owe their localized but significant hydrocarbon-production potential to natural fracture systems in an otherwise impermeable rock. These natural fracture systems are normally concentrated in interbedded, brittle, calcareous, cherry, or silty zones. Calcareous and silty zones, both characterized by log values of potassium and thorium but excessively high values of uranium, are easily located with natural gamma ray logs. These interpretive concepts have already assisted in many successful oil and gas well completion and/or recompletion attempts in the more permeable and/or fractured intervals of such shale formations. Field experiences include investigations in the Eagle Ford shale of the Cretaceous carbonate trend of south Texas; the Cretaceous Niobrara and Pierre shales of Colorado; the Woodford shale of the Lower Mississippian and Upper Devonian Ages of Oklahoma and west Texas; the Devonian shales of the Appalachian basin; the Miocene Monterey shale of California: the Ordovician black shales of Quebec, Canada; and the Upper Jurassic Bazhenov shale in the mid-Ob/Irtysh region of western Siberia, USSR.

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