GAMMA RAY LOGGING OF CHALK

Gamma ray logging of chalk

Gamma ray logging of chalk

Introduction

The gamma ray wire-line log has for more than 30 years been a standard tool in well logging. The tool measures the total natural radioactivity of the formation intersected by the well. The gamma radiation is recorded within a certain energy interval which is typically from 0.3 to 3 MeV. The natural radioactivity from potassium, uranium and thorium bearing minerals varies in accordance with the mineral assemblage in the sedimentary "rock type. The gamma log is therefore used as a primary "lithological log", and quantitatively as an indicator of the shale content. Interpretation of the total gamma-ray log {GR log) possesses problems when several radioactive minerals are present in a rock. If the radioactivity of sandstone is due to mica and K-feldspar content rather than clay content, the shale volume may be erroneously calculated and the shale effect on the log suite misinterpreted.

The gamma-ray spectral log (NGS log) which has be increasingly used since 1975 makes up for some of the difficulties as it enables the interpreter to distinguish between log responses from U, Th and K bearing minerals (Suau & Spurlin 1982, Marett et al. 1976). The logging industry has presented a number of cross plots, eg. the photoelectric effect Pe from the Litho-Density log (LDT) versus the Th/K ratio, which provide a direct identification of the predominant radioactive minerals. The NGS log is comparatively expensive and its use is often restricted to reservoir intervals.

During 1979-1985 five deep exploration wells were drilled in Denmark with the purpose of storage of natural gas and exploitation of-low temperature geothermal energy. None of these were logged with the natural gamma-ray spectral tool. The wells intersected Mesozoic stratigraphic intervals of considerable thicknesses in the Danish sedimentary sub basin and in the North German Basin. Lithologically the wells encompass a very large range of sedimentary rocks with respect to both composition and depositional environment.

The purpose of the present radiometric study was to present the typical variations of naturally occurring radioactive minerals within the Nesozoic and Cainozoic sequences in the Danish Subbasin and the North German Basin. The radiometric profiles are constructed on the basis of gamma-ray analyses of bulk cutting samples. They are intended to serve as preliminary geochemical reference profiles for major lithostratigraphical units. They may improve the lithological, stratigraphical and geochemical interpretation of all intervals which have not been or will not be logged by gamma-ray spectrometry.

Downhole in-situ analysis is influenced by attenuation of the gamma rays in the drilling fluids and by radioactive constituents in the mud. These constituents may be partly mud chemicals and partly solid particles from the penetrated formations. Similarly the bulk cutting samples are to a varying degree contaminated by mud chemicals and by "fall down" from the shallower formations in the hole. These effects have been studied and quantified in a separate laboratory experiment which is summarized in Chapter 3. A total of 1329 samples have been analysed from the wells ...