Multidisciplinary Approach for Detailed Characterization of Shale Gas Reservoirs: A Netherlands Showcase
Authors: Schavemaker Yvonne Anja , Zijp Mart , ter Heege Jan , Nelskamp Susanne , Ten Veen Johan 
Published: June 2014

Abstract
The success of shale gas exploration and production in the United States has triggered other countries around the world to look into possibilities of producing gas from different shales. As it turns out, one of the main difficulties when looking for shale gas is obtaining an in depth understanding of the shale formations and interpret where the sweet spots for gas production will be. This is mainly caused by the large differences in properties of shale formations.
To improve our understanding of the vertical and lateral property changes that determine high and low productivity zones in shales, TNO has proposed a workflow including well log interpretation, mud log interpretation, depth modeling, organic geochemistry, basin modeling, palynology, electron microscope analyses, mineral typing and porosity identification and characterization. This method serves as a solid basis for sweet spot determination and proper resource estimates.
The proposed method has been applied to the two main possible shale gas reservoirs in the Netherlands; the Lower Jurassic Posidonia Shale Formation (PSF) and the Carboniferous (Namurian) Epen Formation. The latter is of particular interest as it contains the highly organic basal Geverik Member (GM). For the two formations detailed and accurate depth maps have been produced.
Research on the PSF shows that a number of areas are expected to have reached gas maturity, together around a fifth of the total area of the formation. Besides shale gas there appears to be potential for shale oil. High productivity zones within these shales are expected in the deepest part of formation and orientated along the distribution axis within the West Netherlands Basin. In the multidisciplinary workflow a 3D faciesmodel of the formation is created using palynology and organofacies to detect prospective layers within the formation. This faciesmodel is then used in combination with log-interpretaion, seismic interpretation and sedimentological interpretation to detect most prospective zones and areas. FIB-SEM microscopy shows that coccoliths present in the formation can have large favourable effects on the porosity and permeability providing for a micro-permeability pathway for oil and gas when interconnected.
When the proposed workflow is applied to the Geverik Member, a correlation between silica content and Total Organic Carbon (TOC) is found, i.e. log-derived TOC is in general close to 10% when the dominant mineral type is silica. Geomechanical analysis of rock properties shows that the lower zone of the Geverik Member is the favourable layer to target, i.e. the layer is dominantly build up of quartz and silica suggesting preferential conditions for hydraulic stimulation..

Introduction
Exploration and production of shale gas has proven to be a game changer in North America. In reaction to the success and corresponding drop in gas price, European countries are investigating if there is shale gas potential and if shale gas can be extracted in economically. In the Netherlands two formations are expected to be the most prospective. These are the Posidonia Shale Formation (PSF) of Lower Jurassic age and the Lower Geverik Member of the Epen Formation of Carboniferous age (Bouw 2012, van Bergen et. al. 2013).