Shale gas formation research: from well logs to outcrop - and back again

Mart Zijp, Johan ten Veen, Roel Verreussel, Jan ter Heege, Dario Ventra and John Martin show the added value of outcrop analogue studies to understand and characterize the heterogeneity of gas shale formations. Intense exploration activities over the past 75 years have resulted in an impressive amount of publically available subsurface data for the Netherlands. About 100 exploration wells penetrate the Early Jurassic Posidonia Shale Formation (PSF) in the Netherlands, which is considered the most important oil source rock in the Netherlands. More recently, this approximately 30m-thick black shale has attracted attention as a possible target for shale gas. Since outcrops and tangible subsurface data are virtually absent, Dutch geoscientists face the reality that shale-gas research typically takes place on a work station. For instance, petrophysical analysis shows that the PSF stands out from the surrounding shales by conspicuous patterns on wire-line logs: high gamma ray, high sonic, low density and high resistivity. Higher order trends in the well logs can be traced across large distances and are thought to represent a remarkable variation in lithology that is important for the characterization of potential shale gas targets. Ideally this reservoir characterization should be carried out at different scales: from an analysis of fault networks and responses to local stress regimes at reservoir scale, to analyses of hydro-mechanical properties and fracture networks at bed scale, finishing at a characterization of compositional and sedimentological heterogeneity at the scale of laminae. Thus, the common practice of lithology deduction via petrophysical methods is incomplete and requires ground truthing, which can be best provided by outcrop studies.