Unconventional resources and play analysis
Shale section in southern Sweden.
Selection criteria for biogenic and thermogenic shale plays. Only maximum thermal maturity and maximum depth cut-offs are applied since both biogenic gas and thermogenic oil and gas plays may be relevant.
Photo by A.T. Nielsen.
Unconventional resources are hosted in fine-grained reservoirs that are enriched in organic carbon. The play can be either thermogenic, where the shale has been deeply buried and thus matured to oil and gas level, or biogenic, where the shale has been degraded by microbes whereby gas was generated. A systematic description of the shale layers is a pre-requisite for a uniform and scientific-based assessment of the shale resource systems. Below is provided screening criteria for evaluating any shale as a first important step. The selection criteria in the table cover both thermogenic and biogenic shale gas plays.
For thermogenic plays, the selection criteria are chosen to ensure that the right TOC type is present (marine types I-II) in significant volumes (>2% and 20 m thick), that the unit has a significant thickness (>20 m), that the thermal maturity is high enough to ensure generation of HC (at least oil maturity), and that the present-day reservoir has maintained its integrity (present-day depth at least 1.5 km and low to medium structural complexity). Present-day depth larger than 7 km is used to exclude shales that are not within reach of the well bore.
For biogenic plays, the criteria are similar to those used for thermogenic plays with respect to TOC and thickness cut-offs. The preferential maturity range for this play is immature to wet-gas maturation since the biogenic gas generation depends on depth, biodegradable kerogen and/or bitumen within the shale. Loss of reservoir integrity is typical a requirement for biogenic gas production. This is typically formed in structural complex settings, such as in glacier-induced fractured shales, where microbial activity occurs due to addition of freshwater into the shale system. Accordingl,y basins with biogenic gas shale plays will be rather shallow and hence screened by applying a depth cut-off of 1 km as maximum depth. The structural complexity is typically medium to high for this play type.
- Characterisation of shales and play definition
- Estimation of unconventional resource base and identification of main risk components
- Sweet-spot analysis
- Organisation of field trips to Lower Palaeozoic shale outcrops in southern Sweden (Scania) and on the Danish island of Bornholm
Ongoing projects with GEUS contribution:
Completed projects with GEUS contribution:
- EUOGA project: European Unconventional Oil and Gas Assessment
- M4ShaleGas project: Measuring, Monitoring, Mitigating, Managing the environmental impact of shale gas; funded by the European Union’s Horizon 2020 Research and Innovation Programme
- GASH project: the first European interdisciplinary shale gas research initiative
- Gautier, D.L., Schovsbo, N.H., Nielsen, A.T., 2014. Resource potential of the Alum Shale in Denmark. Unconventional Resources Technology Conference (URTeC). DOI 10.15530/urtec-2014-1931754.
- Krüger, M., van Berk, W., Arning, E.T., Jiménez, N., Schovsbo, N.H., Straaten, N., Schulz, H.-M., 2014. The biogenic methane potential of European gas shale analogues: Results from incubation experiments and thermodynamic modelling. International Journal of Coal Geology 136, 59–74.
- Schovsbo, N.H., Anthonsen, K.A., Pedersen, C.B., Tougaard, L., 2016. Overview of relevant shale layer characteristics. Report for DG JRC in the Context of Contract JRC/PTT/2015/F.3/0027/NC.
- Schovsbo, N.H., Nielsen, A.T., Gautier, D.L., 2014. The Lower Palaeozoic shale gas play in Denmark. Geological Survey of Denmark and Greenland Bulletin 31, 19–22.
- Schovsbo, N.H., Nielsen, A.T., Klitten, K., Mathiesen, A., Rasmussen, P., 2011. Shale gas investigations in Denmark: Lower Palaeozoic shales on Bornholm. Geological Survey of Denmark and Greenland Bulletin 23, 9–12.
- Schulz, H.-M., Biermann, S., van Berk, W., Krüger, M., Straaten, N., Bechtel, A., Wirth, R., Lüders, V., Schovsbo, N.H., Crabtree, S., 2015. From shale oil to biogenic shale gas: retracing organic-inorganic interactions in the Alum Shale (Furongian-Lower Ordovician) in southern Sweden. AAPG Bulletin 99, 927–956.
Rock typing in shales and shaly reservoirs by multivariate data analysis
The HH-XRF apparatus in function.
Photo by N.H. Schovsbo.
Shale intervals are naturally heterogenetic so it requires careful analysis of the rock sequence to select the right interval for optimal well bore placement and stimulation. For such purposes, GEUS provides principle component analysis (PCA) rock typing defined from properties such as XRF-XRD and/or wireline-log responses. The PCA is a multivariate data analytical approach. The rock types are not distinguished by their sedimentary facies (i.e. process dependent) but are rather defined by a series of functional properties. A PCA model transforms a matrix of measured values into sets of projection sub-spaces delineated by principal components (each a linear combination of all P variables), which display variance-maximised interrelationships between samples and variables, respectively. The PCA-defined rock types help to:
- Reduce the lithological heterogeneity of the reservoir-section classes
- Provide the highest possible rock-type resolution in prospective intervals
- Reduce the rock-type resolution in non-prospective intervals
- Geochemical characterisation of relevant sections by high-resolution scanning of cores, cuttings and outcrop samples using handheld XRF apparatus
- Rock typing by means of principal component analysis (PCA) on geochemical and/or wireline-log data such as flex log or lithoscanner log
- Calculation and prediction of log responses including prediction of dynamic rock modulus
- Esbensen, K., Schovsbo, N.H., Kristiansen, L., 2015. Down-hole permeability prediction – a chemometric wire-line log feasibility study from a North Sea chalk well. Geological Survey of Denmark and Greenland Bulletin 33, 13–16.
- Schovsbo, N.H., Esbensen, K.H., Nielsen, A.T., Derbez, E., Gaucher, E.C., Poirier-Coutansais, X., Riou, A., Tallone, P., Milton-Taylor, D., 2015. Rock types in the Scandinavian Alum Shale resource play: Definitions and predictions. 77th EAGE Conference & Exhibition 2015, IFEMA Madrid, Spain 1–4 June 2015. Extended abstract Th N101 13, 1–5.
Processes at the oil/water contact and water type characteristic in reservoirs
General description of the topic
Microbial mediated carbonate cementation is a result of degradation of organic carbon and the subsequent production of bicarbonate. The process provides an additional source of carbonate cement that may form localised around sites of high microbial activity such as the oil/water contact in a hydrocarbon reservoir or even at the gas/water contact. The source of microbial mediated cement has a characteristic isotope signature with respect to δ13C, which makes it possible to identify this source of cement. In combination with δ13C, the oxygen isotope signature (δ18O) of the carbonate cement may provide a relative timing of the cementation process with respect to the thermal history (reservoir temperature) or hydrodynamic events. The presence of a cemented oil/water and/or gas/water contact may have huge impact on the reservoir performance since the cementation may create baffling zones and may lead to compartmentalisation of the reservoir. Therefore, knowledge of the microbial processes that are likely to have occurred in a reservoir is of importance when production is planned and when history matching is made in a reservoir simulation.
GEUS has since 2009 worked on characterising the microbial mediated carbonate cement in sandstone reservoirs in the North Sea and along the Norwegian continental shelf. The principal senior staff involved from GEUS’ side has been Niels Schovsbo that holds a PhD degree in geochemistry and have more than 10 years of experience in using geochemistry in reservoir characterisation. Relevant competences comprise:
- Regional understanding of microbial isotope signature in the Danish part of the North Sea
- Best practice of sampling of isotopically heterogenic carbonates
- Isotope database of microbial end-member characteristics from the reservoirs in the Danish North Sea
- Mu, N., Schulz, H.-M. Fu, Y., Schovsbo, N.H., Wirth, R., Rhede, D., van Berk, W., 2015. Berthierine formation in reservoir rocks from the Siri oilfield (Danish North Sea) as result of fluid-rock interactions: Part I. Characterization. Marine and Petroleum Geology 65, 302–316.
- Schovsbo, N.H., Holmslykke, H.D., Kjøller, C., Hedegaard, K., Kristensen, L., Thomsen, E., Esbensen, K.H., 2016. Types of formation water and produced water in Danish oil- and gasfields: implications for enhanced oil recovery by injection of ‘smart’ water. Geological Survey of Denmark and Greenland Bulletin 35, 43–46
- Svendsen, J., Poulsen, M.L., Schovsbo, N., Friis, H., Lindgreen, H., 2009. Enhanced reservoir performance by microbial carbonate cementation – results and application from a producing oil field, Danish North Sea. IAS 2009 meeting.
- WiPS (external collaborator)
Niels H. Schovsbo
Phone: +45 9133 3759