Sedimentological and Petrophysical Characterisation of the Lochaline Sandstone, Upper Cretaceous, Nw Scotland

SEDIMENTOLOGICAL AND PETROPHYSICAL CHARACTERISATION OF THE LOCHALINE SANDSTONE, UPPER CRETACEOUS, NW SCOTLAND - DETERMINATION OF MICROSTRUCTURAL PARAMETERS IN A PURE QUARTZ SANDSTONE - By Gregor Bächle Diploma Thesis 1998 Department of Geology University of Tübingen (Laboratory work carried out at the University of Aberdeen) 2 Abstract Two major periods of marine transgression are recorded in NW-Scotland. The Lochaline sandstone represents a deposition during a minor transgressive phase of Cenomanian age which is interrupting the first trangressive phase. The Lochaline sandstone crops out along a small north-south striking band ranging from Morvern in the north to Mull in the south. Sedimentological analysis of five lateral discontinuous outcrops at Beinn na h-Uamha in Morvern (previously unpublished) yield to a subdivision of the sandstone into four distinctive units. A near-shoreline depositional environment was concluded from the analysed sedimentological facies, the rare ichnological findings and the mineralogy. The range of recorded sedimentary facies may provide useful analogue information for extrapolation into the subsurface, in particular with regard to the hydrocarbon potential of offshore Cretaceous sections. The pore structure of 22 samples of the Lochaline Sandstone was investigated, employing thin section analysis combined with SEM-image analysis, mercury intrusion, helium porosimetry and complex electrical conductivity measurements, from cores with porosity ranging continuously from 4 - 21%. This large variation was achieved without any change in composition: The Lochaline Sandstone is composed of homogeneous grain size (~240 µm). The low porosity samples were taken from cemented horizons within the formation. Examining thin sections and outcrops revealed that the cemented horizons are caused either by pre-Tertiary surface or subsurface (groundwater) silification, they are genetically not related to the overlying Tertiary basalts. The apparently complete “pore fraction distribution” relevant for transport was determined using a combination of data from SEM-image analysis and mercury intrusion which enables to measure pores from 0.01 micrometers up to several millimetres in diameter. Transport properties were studied measuring Klinkenberg corrected gas- and brine permeabilities at 400 psi confining pressure and electrical conductivities at 1kHz. The results showed an excellent correlation between permeability and measured porosity for all samples. There existed two regimes with distinctly different poroperm relationships: for low helium porosities (? = 4% - 12%), permeability (in mD) was 7*10-4?5.59, while for high helium porosities (? = 12% - 21%), permeability (in mD) was given by 12.78?1.66. The correlation was also excellent between electrical conductivity and porosity. Normalised conductivity (1 / FF) changed with porosity reduction following a single power law (1 / FF = ?-a) with an exponent of a = 1.88. The clear correlation between hydraulic and electrical properties with porosity was related to constant grain size, while the two regimes for the hydraulic conductivity emphasised the importance of the microstructure for this process. “Effective” pore structure parameters like the pore radius related to the capillary threshold or to the entry pressure were derived and used to predict permeabilities and to verify models in permeability prediction in a pure sandstone. Models dealing with characteristic pore length (Pittman, 1992, Katz and Thompson, 1987) underestimate permeability, while the permeability in low permeable samples (K < 10 mD) is overestimated. Predicted permeabilities from models incorporating the grain size (Revil et al., 1997; Van Baaren, 1979; Berg 1970) correlated very well with measured permeability in the low permeable samples, but overestimated permeability as well. Different techniques in porosity determination were examined, too. The core plug porosity was measured most accurately by helium porosimetry as proved by the grain density, which was closest to the density of quartz. Fluid imbition methods (calliper, Archimedes) slightly underestimated the helium porosity. Porosities measured by mercury injection showed a weak correlation with the core helium porosity. As well, the two-dimensional porosities derived from SEM image analysis and light microscopy point counting were less reliable and tend to underestimate, respectively overestimate in the latter case, the helium porosity. From the petrophysical point of view, it is established that the Lochaline sandstone has excellent reservoir characteristics. Acknowledgement 3 I would like to express my thanks to Dr. Paul Glover for offering me this interesting project and for his unreserved attitude confronted with ideas. I was also very pleased that he had always the time to give me advice and to discuss things. Thanks also to the help of the technical staff, to Douglas - the fastest man in preparation of thin sections, to John Still for his introduction in the use of the SEM and his help in the interpretation of the data. In particular I would like to thank Colin Taylor for his continuous advice and help during the mercury injection measurements and Renate for her help and hints concerning the record of logs in the field and for her introduction in Corel applications. I am deeply indebted to Ursula for her lasting nature in rebuilding my staying power and her time- and nerve-consuming corrections of my English writing. Also Urs, Jochen and Gernot are thanked for the weekly dietern... My parents are thanked for their financial help throughout my years of studying Geology, in particular abroad, without which this thesis could not have been realised. Thanks also to Student Awards Agency for Scotland for their prompt payment of my course fees. The German Office of Financial Support of Studies Abroad (Bafoeg) has tremendous confirmed their right to exist - now it is two years after my application and I am still waiting for their final decision over my legitimate claim of financial support. Tilcon Ltd. and their employees at the sand mine are thanked for their co-operation concerning the probe permeameter measurements. Uli Aspiron is thanked for his help in decompressing the data files. Finally, I would like also to thank Prof. Tom Aigner for his time to supervise the project in Tübingen. Declaration I am assure that I made this thesis on my own and without any unauthorised support. 4 Nomenclature ? Calliper porosity Hg Mercury porosity ?Cal Point counting porosity ?He, ? Helium porosity ?PC ? SEM porosity ?Arch Archimedes porosity SEM λ Wavelength θ Contact angle τ Electrical tortuosity σ Surface tension D Fractal dimension σw Electrolyte conductivity d Dominant grain diameter σR Rock conductivity FF Formation Factor Q Flow rate r Pore capillary radius v Viscosity G Pore geometrical factor Fb Buoyant force p Pressure δ Density of fluid α Phase angle ω Angular frequency q Cole-Cole exponent σs Surface conductivity APore Area of pore space Adetritalgrain Area of detrital grain Acement Area of cement Aintergranular Intergranular area Pc Mercury / air capillary pressure Pd Mercury / air extrapolated displacement pressure SHg, SRV Specific surface (surface area per rock volume) SPV Specific surface (surface area per pore volume) SGV Specific surface (surface area per grain volume) Sel Specific surface electrical weighted SFrac Fractional mercury saturation SPerc Percentage of mercury saturation [%] ρ'' Imaginary part of the complex resistivity ρ' Real part of the complex resistivity σ'' Imaginary part of the complex conductivity σ' Real part of the complex conductivity κ'' Imaginary part of the complex relative dielectric permittivity κ' Real part of the complex relative dielectric permittivity -12 κ0 Permittivity in vacuum = 8.85419*10 KKl Gas permeability, Klinkenberg corrected K Gas permeability, Klinkenberg corrected KApex Permeability predicted by empirical equation after Pittman KKC Permeability predicted by Kozeny-Carman model KB Brine permeability KP Permeability measured by probe permeameter RApex Characteristic pore radius calculated after Pittman RThres Characteristic pore radius calculated after Katz and Thompson R Hydraulic radius (equivalent to Rthres) RAv Average pore diameter of the pore size distribution derived from mercury injection RSEM75 Pore radius at the third quartile (75%) of the SEM pore size distribution curve R35 Pore radius at 35% mercury saturation after Winland RSEM50 Mean SEM pore radius l1,Hg lower length (radius) limit of the fractal regime calculated from mercury injection l2, Hg upper length (radius) limit of the fractal regime calculated from mercury injection l1, SEM lower length (radius) limit of the fractal regime calculated from image analyses l2, SEM upper length (radius) limit of the fractal regime calculated from image analyses dHg Average grain diameter derived from mercury injection ((SPerc)Pc/Pc)max Maximal ratio of percentage mercury saturation divided by capillary pressure 5 Contents I. INTRODUCTION................................................................................................. 8 1. 1 OBJECTIVE.......................................................................................................... 8 1. 2 EXPOSURE AND STRATIGRAPHY......................................................................... 9 1. 2 REVIEW: SEDIMENTOLOGY OF THE LOCHALINE SANDSTONE.......................... 11 1. 2. 1 Mineralogy and Grain Size.................................................................................

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