Gamma Ray Spectral Evaluation Techniques Identify Fractured Shale Reservoirs and Source-Rock Characteristics

Abstract

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＾

Comparative Study for the Interpretation of Mineral Concentrations, Total Porosity, and TOC in Hydrocarbon-Bearing Shale from Conventional Well Logs

“…To obtain mineral concentrations, most studies rely on multi-mineral solvers included in commercial petrophysical software. Analysis of natural gamma-ray and spectral gamma-ray log responses in organic-rich shale has been published extensively in the open technical literature (Fertl, 1979;Fertl and Rieke, 1980;Schmoker, 1981;Fertl and Chilingar, 1988;Lüning and Kolonic, 2003). Natural gamma ray logs respond primarily to clay, which commonly exhibits high potassium (K) and Thorium (Th), and to TOC, which very often has very high uranium (U) concentration.…”

“…However, the relationship between TOC and gamma-ray reading is often nonlinear and other sources of radioactivity may affect gamma-ray logs, thereby making it more difficult to accurately estimate TOC. The invention of the spectral gamma-ray tool triggered further studies to better understand the relationship between the uranium log and TOC (Fertl and Rieke, 1980). Nevertheless, the relationship between uranium log and TOC is also nonlinear and not universal.…”

Total Organic Carbon Content Determined From Well Logs

“…These natural fracture systems are normally concentrated in interbedded, brittle, calcareous, cherty, or silty zones. 4 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.…”

Dual-shale-content method for total organic carbon content evaluation from wireline logs in organic shale

“…Gamma-ray spectral logs were first introduced to identify organic-rich rocks and their relationships with total gamma ray, uranium, and thorium-potassium ratio were analyzed by Fertl and Rieke (1980) [5] and Fertl and Chillngar (1988) [6]. Then the density log was used to estimate TOC content widely since Schmoker and Hester (1983) [7] proposed the technology.…”