BIB-SEM pore characterization of mature and post mature Posidonia Shale samples from the Hils area, Germany

Abstract Organic-matter porosity is an important component in mature organic-rich shales providing storage capacity for liquids and gases. This study presents 2D microstructural characterization of porosity and in particular organic-matter porosity of two organic-rich Posidonia Shale samples with different maturity using Broad Ion Beam milling and Scanning Electron Microscopy (BIB-SEM). The organic-matter porosity is investigated in a mature sample and a postmature sample. Results show that both samples show a similar trend in pore size distributions, though the pores in the oil-mature sample are less frequent, resulting in a lower visible porosity of 0.82% compared to 2.47% in the postmature sample. This difference is interpreted to be a combination of enhanced cementation and compaction in the mature sample and to secondary organic-matter porosity generation in the gas window for the postmature sample. The organic-matter pores as found in the postmature samples are mostly absent in the mature sample. The latter shows only crack-type porosity at the organic-matter–mineral interface which is interpreted to be related to either shrinkage because of devolatilization or hydraulic fracturing because of hydrocarbon generation. The transition from the crack-type porosity along organic-matter into intra-organic-matter pores is related to thermal maturation. The total BIB-SEM visible porosity is compared with bulk Mercury Intrusion Porosimetry (MIP) porosity to assess the pore connectivity and evaluated using Wood's Metal Injection (WMI). Pore orientations suggest, and WMI demonstrates, that the preferred transport pathways are sub-parallel to bedding. Comparison between MIP porosity and BIB-SEM inferred porosity indicate that most of the pores are connected with pore throats below 10 nm. Overall, crack-porosity is minor, although it may be important on a larger scale as suggested by previous studies.

[1]  E. W. Washburn.  Note on a Method of Determining the Distribution of Pore Sizes in a Porous Material. , 1921, Proceedings of the National Academy of Sciences of the United States of America.

[2]  A. Schimmelmann,et al.  Porosity of Devonian and Mississippian New Albany Shale across a maturation gradient: Insights from organic petrology, gas adsorption, and mercury intrusion , 2013 .

[3]  D. Welte,et al.  Organic matter maturation under the influence of a deep intrusive heat source: A natural experiment for quantitation of hydrocarbon generation and expulsion from a petroleum source rock (Toarcian shale, northern Germany) , 1988 .

[4]  R. Loucks,et al.  Scanning-Electron-Microscope Petrographic Evidence for Distinguishing Organic-Matter Pores Associated with Depositional Organic Matter versus Migrated Organic Matter in Mudrock , 2014 .

[5]  R. Loucks,et al.  Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale , 2009 .

[6]  János Urai,et al.  Multi-scale characterization of porosity in Boom Clay (HADES-level, Mol, Belgium) using a combination of X-ray μ-CT, 2D BIB-SEM and FIB-SEM tomography , 2015 .

[7]  János Urai,et al.  Nanoscale imaging of pore‐scale fluid‐fluid‐solid contacts in sandstone , 2015 .

[8]  X. Sillen,et al.  Nanometer-scale pore fluid distribution and drying damage in preserved clay cores from Belgian clay formations inferred by BIB-cryo-SEM , 2014 .

[9]  János Urai,et al.  Morphology of the pore space in claystones - evidence from BIB/FIB ion beam sectioning and cryo-SEM observations , 2009 .

[10]  Paul C. Hackley,et al.  The nature of porosity in organic-rich mudstones of the Upper Jurassic Kimmeridge Clay Formation, North Sea, offshore United Kingdom , 2012 .

[11]  U. Mann.  Veränderung von Mineralmatrix und Porosität eines Erdölmuttergesteins durch einen Intrusivkörper (Lias epsilon 2–3: Hilsmulde, NW-Deutschland) , 1987 .

[12]  R. Loucks,et al.  Comment on “Formation of nanoporous pyrobitumen residues during maturation of the Barnett Shale (Fort Worth Basin)” by Bernard et al. (2012) , 2014 .

[13]  C. Ostertag-Henning,et al.  Shale oil potential and thermal maturity of the Lower Toarcian Posidonia Shale in NW Europe , 2015 .

[14]  R. Littke,et al.  Hydrocarbon-bearing fluid inclusions in calcite-filled horizontal fractures from mature Posidonia Shale (Hils Syncline, NW Germany) , 1995 .

[15]  János Urai,et al.  BIB-SEM characterization of pore space morphology and distribution in postmature to overmature samples from the Haynesville and Bossier Shales , 2015 .

[16]  Thompson,et al.  Fractal sandstone pores: Implications for conductivity and pore formation. , 1985, Physical review letters.

[17]  János Urai,et al.  BIB-SEM study of the pore space morphology in early mature Posidonia Shale from the Hils area, Germany , 2012 .

[18]  R. Littke,et al.  Geochemical and petrographic characterization of Campanian–Lower Maastrichtian calcareous petroleum source rocks of Hasbayya, South Lebanon , 2015 .

[19]  L. Vernik.  Hydrocarbon‐generation‐induced microcracking of source rocks , 1994 .

[20]  D. Jarvie,et al.  Unconventional shale-gas systems: The Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment , 2007 .

[21]  R. R. Berg,et al.  Primary Migration by Oil-Generation Microfracturing in Low-Permeability Source Rocks: Application to the Austin Chalk, Texas , 1999 .

[22]  J. Urai,et al.  Pore morphology and distribution in the Shaly facies of Opalinus Clay (Mont Terri, Switzerland): Insights from representative 2D BIB–SEM investigations on mm to nm scale , 2013 .

[23]  C. Ostertag-Henning,et al.  Mineralogical changes in organic-rich Posidonia Shale during natural and experimental maturation , 2016 .

[24]  Milind Deo,et al.  Characterization of oil shale pore structure before and after pyrolysis by using X-ray micro CT , 2013 .

[25]  Paul Meakin,et al.  A 4D Synchrotron X-Ray-Tomography Study of the Formation of Hydrocarbon- Migration Pathways in Heated Organic-Rich Shale , 2013 .

[26]  B. Schröppel.  Multi-scale analysis of porosity in diagenetically altered reservoir sandstone from the Permian Rotliegend (Germany) , 2016 .

[27]  J. Espitalie,et al.  Méthode rapide de caractérisation des roches mètres, de leur potentiel pétrolier et de leur degré d'évolution , 1977 .

[28]  H. Wenk,et al.  Mineral Preferred Orientation and Microstructure in the Posidonia Shale in Relation to Different Degrees of Thermal Maturity , 2012, Clays and Clay Minerals.

[29]  C. Peach,et al.  Microstructural characteristics of the Whitby Mudstone Formation (UK) , 2016 .

[30]  R. Littke,et al.  Microscopic and sedimentologic evidence for the generation and migration of hydrocarbons in Toarcian source rocks of different maturities , 1988 .

[31]  K. Thomas,et al.  High-pressure methane adsorption and characterization of pores in Posidonia shales and isolated kerogens. , 2014 .

[32]  R. Littke,et al.  Application of BIB–SEM technology to characterize macropore morphology in coal , 2013 .

[33]  Velimir Radmilovic,et al.  Focused ion beam assisted three-dimensional rock imaging at submicron scale , 2003 .

[34]  M. Curtis,et al.  Relationship Between Organic Shale Microstructure and Hydrocarbon Generation , 2013 .

[35]  M. Curtis,et al.  Development of organic porosity in the Woodford Shale with increasing thermal maturity , 2012 .

[36]  J. Urai,et al.  A comparative study of representative 2D microstructures in Shaly and Sandy facies of Opalinus Clay (Mont Terri, Switzerland) inferred form BIB-SEM and MIP methods , 2014 .

[37]  Benjamin Bruns,et al.  Petroleum system evolution in the inverted Lower Saxony Basin, northwest Germany: a 3D basin modeling study , 2013 .

[38]  G. Cody,et al.  Minimizing damage during FIB sample preparation of soft materials , 2012 .

[39]  Y. Gensterblum,et al.  Experimental study of fluid transport processes in the matrix system of the European organic-rich shales: II. Posidonia Shale (Lower Toarcian, northern Germany) , 2014 .

[40]  R. Littke,et al.  Reflectance of dispersed vitrinite in Palaeozoic rocks with and without cleavage: Implications for burial and thermal history modeling in the Devonian of Rursee area, northern Rhenish Massif, Germany , 2012 .

[41]  Bernhard M. Krooss,et al.  Geological controls on the methane storage capacity in organic-rich shales , 2014 .

[42]  M. D. Rudnicki,et al.  Organic matter–hosted pore system, Marcellus Formation (Devonian), Pennsylvania , 2013 .

[43]  B. Horsfield,et al.  Geochemical evolution of organic-rich shales with increasing maturity: A STXM and TEM study of the Posidonia Shale (Lower Toarcian, northern Germany) , 2012 .

[44]  R. Littke,et al.  High thermal maturity in the Lower Saxony Basin: intrusion or deep burial? , 1999 .

[45]  Anders Malthe-Sørenssen,et al.  4D imaging of fracturing in organic-rich shales during heating , 2011, 1101.2295.

[46]  Stephen C. Ruppel,et al.  Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores , 2012 .

[47]  J. Urai,et al.  The connectivity of pore space in mudstones: insights from high-pressure Wood's metal injection, BIB-SEM imaging, and mercury intrusion porosimetry , 2015 .

[48]  S. Chipera,et al.  Improving Our Understanding of Porosity in Source Rock Reservoirs through Advanced Imaging Techniques , 2013 .