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Abstract

AAPG Bulletin, V. 88, No. 9 (September 2004), P. 1239-1254.

Copyright copy2004. The American Association of Petroleum Geologists. All rights reserved.

Distinguishing faults from flooding surfaces on spectral gamma-ray logs

A. Ruffell,1 J. M. McKinley,2 R. Evans3

1School of Geography, Queen's University, Belfast, Northern Ireland; [email protected]
2School of Geography, Queen's University, Belfast, Northern Ireland; [email protected]
3Department of Petroleum Engineering at Curtin University, Curtin, Perth, Western Australia; [email protected]

AUTHORS

Alastair Ruffell has a Ph.D. (geology) from the University of Birmingham (United Kingdom). Prior to this, he was a geophysicist in Rio Tinto Zinc and Elf Aquitaine. An Imperial College (London) postdoctoral research led to a teaching position at Queen's University in Belfast. He is currently working on various aspects of clay mineralogy, including quantitative x-ray diffraction, uses of clay mineralogy in paleoclimate studies, the geophysics of clays (gamma-ray and ground-penetrating radar responses), and forensic geoscience.

Jennifer McKinley has a Ph.D. (geology) from Queen's University (Belfast) on the petrography and geochemistry of reservoir variation. She is currently working on a postdoctoral research on spatial modeling of permeability in visibly homogenous sandstones. Research interests include use of geostatistics in the spatial analysis of mineral cements and fluid-flow properties of rocks.

Richard Evans is a research fellow in reservoir characterization at Curtin University of Technology, where he also lectures on formation evaluation and 3-D reservoir modeling. He was awarded a Ph.D. from the Queen's University of Belfast (1998) for research on Carboniferous outcrop analogs in the north of Ireland region. This was followed by two Shell/Exxon-sponsored postdoctoral projects at Imperial College of London and Cardiff University on the seismic architecture, evolution, and faulting of the Tay Fan, United Kingdom, in the North Sea. Before joining Curtin in early 2003, he worked as a consultant geoscientist for Reservoir Management Limited in Aberdeen, working on a variety of industry projects in the North Sea and west of Shetland basins. He is interested in the multidisciplinary characterization of reservoir and aquifer rocks from pore throat to basin scale, to investigate the controls on reservoir architecture, porosity, permeability, and fluid flow for static and dynamic modeling. He is currently working closely with Commonwealth Scientific and Industrial Research Organization and Woodside Energy Limited on several joint research projects.

ACKNOWLEDGMENTS

We thank Lindsay Proudfoot, Brian Whalley, and Luis Pomar for their help at Estellences in Majorca. We thank Evgenij Baraboshkin and Andre Guzikov for organizing the Ukraine trip. We also thank Richard Worden, Chris Cornelius, and John Meneely for help with fieldwork at Mullaghmore and laboratory analyses. The Neoproterozoic study was carried out by Matt Hadley and instigated by Graham Leslie and Bernard Anderson, the latter provided other fault details, for which he is thanked. Many thanks to Pedro Rios and Dale Leckie for providing information on the Guando field. This paper was improved by the helpful reviews of Laura Foulk, Roger Slatt, and Laird Thompson.

ABSTRACT

Elevated gamma-ray emission from discrete beds in sedimentary deposits may conventionally be interpreted as representing flooding surfaces or transgressive beds. Potassium (K), uranium (U), or (less commonly) thorium (Th) concentrations in outcrop or borehole successions cause such elevated gamma-ray emission and may be linked to the presence of specific mineral hosts. Furthermore, specific Th/K and Th/U ratios occur at correlated stratigraphic surfaces and form part of a pattern that reflects a sequence-stratigraphic hierarchy. Spectral gamma-ray logs from uncored boreholes or weathered cliffsides and acquired without full petrographic descriptions can intersect low-angle faults such as thrusts. Our study demonstrates that bedding-parallel faults can be mistaken for flooding surfaces. We document the spectral gamma-ray response through a range of visually obvious and cryptic faults that may serve as proxy examples for other areas. Finally, we derive a preliminary generic model for the origin of spectral gamma-ray variations in faulted sandstones, limestones, and metamorphic successions. This shows why fluid-rock interactions along bedding-parallel zones of deformation generate elevated K and U and depressed Th/K and Th/U. Our observations may aid subsurface studies of the complex stratigraphy below thrusts.

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