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European Gas Shales in Time and Space - New Insights from GASH

Brian Horsfield¹, Andy Aplin², Bernd Krooss³, Sylvain Bernard4, Klaus Bauer¹, Oliver Ritter¹, Georg Dresen¹, Ralf Littke³, Rolando di Primio¹, Volker Lüders¹, William Sassi5, Hans Doornenbal6, Niels Schovsbo7, and Reinhard Sachsenhofer8
¹GFZ German Research Centre for Geosciences, Potsdam, Germany
²Newcastle University, Newcastle, UK
³RWTH Aachen, Aachen, Germany
4MNHN, CNRS, Paris Cedex 5, France
5IFPen, Rueil Malmaison, France
6TNO, Utrecht, The Netherlands
7GEUS, Copenhagen, Denmark
8Leoben University, Leoben, Austria

GASH is the first major research initiative in Europe that is focussed on shale gas; it is broad ranging in scientific scope and unites leading European research groups and geological surveys with industry. It comprises two main elements: the compiling of a European Black Shale Database (EBSD) and conducting research on the interplaying factors governing shale gas formation and occurrence. The research projects are focussed on the two basic geological variables establishing viability, namely gas in place (GIP) and the delivery of gas to the wellbore. The Cambrian Alum Shale from Sweden and Denmark, the Lower Jurassic Posidonia Shale from Central Germany, and Carboniferous black shales from the UK in the west via the Netherlands to Germany in the east are the natural laboratories for the research programme. While producing shale gas is very much a reservoir scale exercise, the key to sweet spot recognition begins at the basin scale. Deposition, diagenesis and maturation together determine the amount of gas source material in the shale, the extent of gas generation (from numerous organic precursors), the amount of gas retained or lost, as well as the fabric and strength of the rock. All these vary for better or for worse in the geological depressions and structures that underlie Europe.

In this presentation we focus mainly on the Posidonia Shale of Germany. We have developed a fully integrated 3D high resolution numerical petroleum systems model (over 20 Mio. grid cells, PetroMod v.2011.1 software suite) covering an area of 200x150 km incorporating the Lower Saxony Basin and parts of the Pompeckj Block and Münsterland Basin. Using a compilation of erosion maps calibrated by means of a large amount of vitrinite reflectance and downhole temperature data a differentiated image of the maturity distribution of the source rock members could be achieved.

The kinetics of primary and secondary gas generation was used to predict yields. Mass balance modelling reveals expulsion efficiency is very high. An additional late gas charge (Ro > 2%) was shown to significantly contribute to total gas. The phase behaviour of cumulative and instantaneous fluids was shown to play an overriding control of GOR in the late oil window (Ro = 0.88 – 1.45%) in response to regional inversion.

Porosities decrease through the oil window as a result of compaction and filling of pores with bitumen. In the gas window, Posidonia porosities increased to 9-12%, ascribed to cracking of bitumen and linked generation of pores. Electrical resistivity initially increases, and then decreases to the lowest values measured. The development of pores due to OM degradation and gas generation is reflected in the existence of a strong positive correlation between the TOC and total porosity. There is no similar correlation at lower maturity. In the immature Posidonia, 96% of porosity was found to be present in the mineral matrix with only 4% related to the OM particles. On the other hand, for the shale at Ro = 1.45%, 75% of visible porosity is associated with organic matter. Advances were made in the investigation of single- and two-phase (water/gas) fluid transport properties of low-permeable shales. Concerning sorption, the effects of moisture content, temperature and clay mineral composition on sorption capacity were studied systematically, and the exercise revealed how sensitive the data are to storage conditions and preparation methods. The relationship between organic matter structure and adsorptive properties has been revealed using thermal analysis. Links between these physical attributes and chemical behaviour (richness, gas-oil ratio, kinetic parameters) have been revealed for the first time.

The Posidonia shale shows strength anisotropy, i.e. the strength normal to bedding planes is up to 200% higher compared to the strength parallel to bedding. Posidonia shale with high clay content and high porosity shows a transition from brittle to ductile behavior already at small confining pressures (<35 MPa), whereas low porosity shale shows brittle behavior over the entire pressure range. Mechanical data and microstructure observations reveal that closure of pores and compaction are important for deformation. Geomechanical parameters were correlated with the mineral content, porosity (θ), density (ρ), and ultrasonic p-wave velocity (vp)..

Fluid inclusions in fracture-fill mineralization hosted by the Posidonia Shale have been studied for δ¹³C isotopic compositions of methane and, when present, CO2 using a new developed on-line method for simultaneous measurements for stable isotope ratios of N2, CH4, and CO2 of fluid inclusions. The data obtained were used to estimate the maturity of source rocks at the time of CH4 generation using calibration curves of δ¹³C(CH4) of natural gases in relation to maturity. The data suggest significant gas generation within the oil window and formation of horizontal fracture-fill mineralization due to overpressure.

We have documented geochemical and mineralogical heterogeneities down to the nanometer scale within the investigated samples as a function of their level of thermal maturity. In particular, authigenic albite crystals containing nanometric halite inclusions have been documented within the investigated mature and overmature samples. The presence of such tracers of palaeobrine and carbonate interactions supports a maturation scenario for the Lower Toarcian Posidonia Shale intimately related to ascending brine fluids rather than a maturation scenario solely resulting from high heat flows.

AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012