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New insights from jurassic shale characterization: Strengthen subsurface data with outcrop analogues

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Author: Zijp, M. · Veen, J. ten · Ventra, D. · Verreussel, R. · Laerhoven, L. van · Boxem, T.
Type:article
Date:2014
Publisher: Society of Petroleum Engineers
Source:European Unconventional Resources Conference and Exhibition 2014: Unlocking European Potential, 25 February 2014 through 27 February 2014, Vienna, 1, 472-479
series:
Society of Petroleum Engineers
Identifier: 516499
ISBN: 9781632660817
Keywords: Geosciences · Fracture · Fracture mechanics · Gamma rays · Geochemistry · Geomechanics · Kaolinite · Lithology · Minerals · Pyrites · Quartz · Resource valuation · Shale gas · Thorium · Uranium · Fracture analysis · Fracture behavior · Geochemical data · Geomechanical properties · Posidonia shales · Spectral gamma rays · Strong correlation · Uranium contents · Shale · Geological Survey Netherlands · Energy / Geological Survey Netherlands · Earth / Environmental · PG - Petroleum Geosciences · ELSS - Earth, Life and Social Sciences

Abstract

This study shows the first results of an ongoing project on the characterization of variations in lithological- and geomechanical properties of targetted gas shale formations. The aim of this study is to get an increased insight into both the fraccability and productivity of a shale formations. Ultimately, this can lead to better localization of shale gas "sweet spots" and prediction of mechanical reservoir behaviour. Since subsurface data is scarce and often widely spaced to achieve this, we did an outcrop study along the Yorkshire (UK) coast on the time and depositional analogue of the Jurassic Posidonia Shale Formation, the Jet Rock Member of the Whitby Mudstone Formation. High resolution (10 cm) sedimentological, geochemical and geobiological sampling, natural fracture analysis and Spectral Gamma Ray measurements are performed. The mineralogy of the Jet Rock Member is based on geochemical data and thin section point counting and shows more or less equal amounts in Quartz, Kaolinite and Illite with only Pyrite and Calcite changing substantially. Within high TOC layers the percentage of Pyrite increases at the cost of Kaolinite. Fracture analysis of the Jet Rock Member shows a change of fracture density and length on a meter scale and is confined to the bedding. Comparison of the fracture density and mineralogical ratios shows a strong correlation suggesting that identification of the mineralogical signature can be of importance when targeting zones with preferential geomechanical behaviour. Geochemical test show that of the eight meters of black shale studied only one zone of half a meter has a measured TOC value of 15-20%. All other measurements on the other 7.5 meters of the member show an average organic content of 3-5%. Spectral Gamma Ray (SGR) analysis of the Whitby outcrops shows gradual changes in potassium, thorium and uranium content, while the variation in thorium content is most significant. A direct link between SGR and fracture density is not visible although it is clear that the altering SGR signature is result of the changing minerology and thus possibly linked to the fracture behavior. When comparing the Whitby outcrop results with the SGR results of the Posidonia Formation in the Dutch well Moerkapelle-14 it stands out that the Dutch subsurface values are in general lower than the English outcrop ones. Both thorium and uranium are, for almost the entire formation, lower than the English time-equivalent. Shell