Chapter 1: Shale Gas Evaluation of the Early Jurassic Posidonia Shale Formation and the Carboniferous Epen Formation in the Netherlands

Abstract Since the discovery of the Groningen gas field, the Netherlands has been a large producer and consumer of natural gas. Current forecasts show that production from conventional on- and offshore fields will decline noticeably in the next decades. The Netherlands has the ambition to sustain its prominent role in the northwestern European gas market and will have to be able to meet the future domestic demand. Import of natural gas, either through liquid natural gas import (North Africa, Middle East) or from the East (Nordstream), are therefore evaluated and planned. Following the developments in the United States, the question has arisen if there is shale gas potential in the Netherlands that could add to the domestic gas production. A first evaluation in 2009 confirmed this potential, although the uncertainties are huge. Others have, however, challenged this positive view on the potential resource of the Netherlands. The follow-up work that is presented provides more detailed information based on extensive data evaluations and interpretations of potential shale gas targets in the Netherlands. The Netherlands has a long and intense exploration history that led to a very high data coverage that is largely in the public domain. A first assessment of possible shale gas reservoirs in the Netherlands was made using this unique data set. The main target formations for shale gas are the Lower Jurassic Posidonia Shale Formation and the Carboniferous (Namurian or Serpukhovian to Lower Bashkirian) Epen Formation, especially its basal part with high organic content. The Posidonia Shale Formation is known to be present in the onshore West Netherlands Basin from many well penetrations and its distinct seismic character. Gas logs indicate the presence of gas. Fault-bounded tectonic blocks were identified on three-dimensional seismics with relatively undisturbed deposits. Calculations of gas initially in place were performed for these individual blocks, based on total organic carbon (TOC) and porosity. TOC values were calculated from logs and cross-checked on actual measurements. The deposit is probably brittle (and therefore susceptible for fracturing) throughout most of the area as indicated by log-derived Young’s Moduli. The evaluation showed favorable conditions for shale gas prospects, that is, TOC content of about 6%, porosity of 5-9.5%, and an average thickness of 30 m (98.42 ft). The evaluation of three example fault-bounded blocks indicated total gas volumes of 0.26-0.46 billion cubic meters per square kilometer (STP; 23-42 billion cubic feet of gas per square mile), which merits further investigation into the viability of this gas play. The evaluation of the Carboniferous Epen Formation is more complicated because it is generally deeper. The Geverik Member (~50-70 m (164.04-229.65 ft) thick) is the lowest unit of the formation and is considered a main target for shale gas exploration because of its high organic content (TOC ~7%, Type II). Recent mapping and newly released wells indicate that instead of a uniform basin, the deposition of the formation was controlled by the existing paleogeography that was probably formed by the presence of carbonate platforms. Exploration challenges of the Geverik Member include the scarce data due to limited well penetrations, its present-day depth, and its high maturity (>3.3 % Ro) for most of the Netherlands, as indicated by modeling and measurements. In conclusion, the presented study provides background information on the geological setting for potential shale gas developments in the Netherlands. The need for such information has become very relevant over the past two years because as of 2010 a total of four exploration licenses have been granted to different companies. This challenges policymakers to take a position on shale gas development in the Netherlands, where public concern has also risen about the environmental impact and safety of shale gas operations.