P2-2-21 Inorganic elemental geochemistry a conventional tool for an unconventional play: an elemental approach to the Bowland Shale Formation David Riley, Tim Pearce, John Martin Chemostrat Ltd, Welshpool, UK Introduction The Namurian Bowland Shale Formation has been identified as the premier contender for development as a UK unconventional play (Andrews, 2013). Inorganic geochemical data has been proven to be fundamental to the completion and understanding of these plays, particular within the developed fields of North America (Hildred et al. 2011; Ratcliffe et al. 2012; Sano et al. 2013). However, to date there has been no similar developments within the UK and mainland Europe; while the use of elemental stratigraphy, chemostratigraphy, is commonly used for offshore conventional plays very little attention has been payed to the wealth of information to be obtained from elemental data. In addition to correlation techniques elemental data may also be used to predict mineralogy, a fundamental requirement for well completion, and if a predicted mineralogy log exists for the study well then a relative brittleness may also be calculated. Predicted mineralogy provides a fast and cost effective alternative to XRD. Elemental data is also advantageous for identifying zones enriched with biogenic silica, which while invisible to conventional X-ray diffraction (XRD) is measured in the samples total SiO 2 , and can greatly affect the reservoir quality. The techniques was best illustrated by Ratcliffe et al. (2012b) on the Woodford Formation, which characterised by high proportions of biogenic silica. Elemental data has also been demonstrated to reflect organic matter preservation, and conditions of palaeoredox. It has long been known that under reducing conditions, typically promoted by the decay of organic matter, elements such as uranium, molybdenum, nickel and vanadium will become enriched (Tribovillard, 2006), a function of a change in their oxidation state and solubility. However, the use of these redox sensitive elements can be used to highlight potential zones of organic matter preservation, and high grade samples for additional total organic carbon (TOC) analysis. The initial phases of this study focused on the Gainsborough Trough, England. This has now been increased to cover the majority of the East Midlands, with excursions into the West Midlands, and type sections that occur within the majority of the key basins. Figure 1 illustrates the thirty four study wells analysed, with over two thousand three hundred samples taken for geochemical analysis, over two thousand of those samples were analysed for whole rock geochemistry via inductively coupled plasma optical emission (ICP-OES) and mass spectrometry (MS), which provides data on 50 elements. However, the majority of these wells are legacy wells, and where sample volumes were depleted preventing destructive analysis, data were acquired via non- destructively using handheld X-ray fluorescence (HHXRF), which provides enough key elements to for fill the objectives of the study. Over three hundred samples were analysed by HHXRF providing data on the samples while preserving the sample at the UK national geological repository.
Transcript
P2-2-21
Inorganic elemental geochemistry a conventional tool for an unconventional play: an elemental approach to the Bowland Shale Formation
David Riley, Tim Pearce, John Martin
Chemostrat Ltd, Welshpool, UK
Introduction
The Namurian Bowland Shale Formation has been identified as the premier contender for development as a UK unconventional play (Andrews, 2013). Inorganic geochemical data has been proven to be fundamental to the completion and understanding of these plays, particular within the developed fields of North America (Hildred et al. 2011; Ratcliffe et al. 2012; Sano et al. 2013). However, to date there has been no similar developments within the UK and mainland Europe; while the use of elemental stratigraphy, chemostratigraphy, is commonly used for offshore conventional plays very little attention has been payed to the wealth of information to be obtained from elemental data.
In addition to correlation techniques elemental data may also be used to predict mineralogy, a fundamental requirement for well completion, and if a predicted mineralogy log exists for the study well then a relative brittleness may also be calculated. Predicted mineralogy provides a fast and cost effective alternative to XRD. Elemental data is also advantageous for identifying zones enriched with biogenic silica, which while invisible to conventional X-ray diffraction (XRD) is measured in the samples total SiO2, and can greatly affect the reservoir quality. The techniques was best illustrated by Ratcliffe et al. (2012b) on the Woodford Formation, which characterised by high proportions of biogenic silica.
Elemental data has also been demonstrated to reflect organic matter preservation, and conditions of palaeoredox. It has long been known that under reducing conditions, typically promoted by the decay of organic matter, elements such as uranium, molybdenum, nickel and vanadium will become enriched (Tribovillard, 2006), a function of a change in their oxidation state and solubility. However, the use of these redox sensitive elements can be used to highlight potential zones of organic matter preservation, and high grade samples for additional total organic carbon (TOC) analysis.
The initial phases of this study focused on the Gainsborough Trough, England. This has now been increased to cover the majority of the East Midlands, with excursions into the West Midlands, and type sections that occur within the majority of the key basins. Figure 1 illustrates the thirty four study wells analysed, with over two thousand three hundred samples taken for geochemical analysis, over two thousand of those samples were analysed for whole rock geochemistry via inductively coupled plasma optical emission (ICP-OES) and mass spectrometry (MS), which provides data on 50 elements. However, the majority of these wells are legacy wells, and where sample volumes were depleted preventing destructive analysis, data were acquired via non-destructively using handheld X-ray fluorescence (HHXRF), which provides enough key elements to for fill the objectives of the study. Over three hundred samples were analysed by HHXRF providing data on the samples while preserving the sample at the UK national geological repository.
Figure 1. Study wells with Carboniferous basins and platforms (after Andrews, 2013)
The Scaftworth-B2, Gainsborough Trough, is presented as the type well for the study, with over one hundred and fifty samples analysed by ICP-OES and ICP-MS. To provide a mineralogical control eighteen samples are analysed by XRD, thirty two samples are analyse by TOC. Furthermore, thirty seven samples are analysed by δ
13Corg stable isotopes. The upwell changes in
the elemental geochemistry are used to subdivide the study interval into eight chemostratigraphic sequences, of which four are subdivided further into thirteen chemostratigraphic packages (Figure 2), the chemostratigraphic zonation is derived on perceived changes in heavy mineral assemblages, clay mineral and organic matter preservation. Predominant geochemical features highlight specific chemostratigraphic boundaries, for example chemostratigraphic sequence CC1 corresponds to the Dinantian limestones, the top boundary is characterised by a sharp decrease in carbonate content, corresponding to a lithological change (Figure 2). While the Bowland Shale, equivalent to chemostratigraphic sequence CC2, is characterised by higher redox and organic matter preservation (Figure 2), the top of this sequence is recognised by the sharp decrease within these geochemical markers. While changes within heavy mineral related elements, and clay mineralogy characterised the overlying succession, which corresponds to deposits of the Millstone Grit Group.
Figure 1. Chemostratigraphic sequence zonation for the type well
Elemental data is the reflection of the mineralogy present within the sample, therefore it is possible to model the mineralogy from the elemental data, and there are numerous studies on this (e.g. Rosen et al. 2004). The calculated mineralogy is defined using the XRD data as a guide, which provides a type of calibration to the process. Normative mineralogy logs provide an easier means to interpret E-log responses, and relate the changes to changes in mineralogy. As the physical properties of the rock are a function of the mineralogy, if the mineralogy has been calculated then a relative indication of brittleness can be calculated. The brittleness index is based on a modification of the formula proposed by Jarvie et al, (2007), based on previous experience on the North American unconventional plays.
Work previously carried out on the North American Woodford Formation (Ratcliffe et al. 2012b) demonstrates the importance of biogenic silica, which can greatly impact reservoir quality. Useful because due to its amorphous nature, biogenic silica is not detected using traditional XRD analysis. Elemental data provides a means of high grading sections for SEM or petrographic analysis. Ratcliffe et al. (2012b) demonstrate the modelling of biogenic silica with the empirical relationship between elemental Si and Zr which highlights excess silica, thought to be of a biogenic origin rather than a terrestrial one.
Figure 3 demonstrates the typical log generated, the Chemical derived mineralogical column is normalised mineralogy, demonstrating the distribution of quartz, clay minerals and carbonate minerals. From the calculated mineral proportions the relative brittleness of the sample is calculated (Figure 3) the samples calculated to be more ductile are green, while those that are brittle appear red. Elemental data provides the quartz modelling, which shows an upward decreasing trend in the biogenic silica, and increases in the terrestrial quartz; the changes in modelled here can be attributed to the advancement of the Namurian delta systems which lead to the deposition of the rocks of the Millstone Grit Group.
Figure 2. Modelled mineralogy, brittleness and quartz proportions
Redox sensitive elements can be used to look at the palaeoredox conditions of the environment of deposition, which is important within unconventional plays. High organic matter preservation is typically associated with environments where the bottom waters were anoxic. Tribovillard et al. (2012) demonstrated how open marine setting become enriched in concentrations of uranium and molybdenum during times of anoxia. In addition, Tribovillard et al. (2012) demonstrated how the degree of restriction of a sedimentary basin can be identified using TOC and molybdenum values. Using this method the Bowland Shale is shown to be deposited within a weakly restricted basin, equivalent to the modern day analogue of the Saanich Inlet. Furthermore, uranium and molybdenum may be calculated as environmental enrichment factors (Tribovillard et al, 2006) and presented on the plot proposed by Algeo and Tribovillard (2009) illustrates that the samples from the Bowland Shale plot within the anoxic field, on the unrestricted trend (Figure 4).
A key benefit of a large, stable and repeatable standardised data set, is the ability to compare different occurrences of the Bowland Shale. Figure 5 demonstrates the completed dataset for the Bowland Shale study, with individual basins highlighted. The aforementioned plot demonstrates that the shale encounter within the Widmerpool Trough contains higher proportions of clay minerals, compared to the Lancaster Basin, which appears coarser with higher proportions of sand. While the Edale and Cheshire Basins contain greater proportions of carbonate, suggesting the proximity of a carbonate system not seen elsewhere. In addition, the Lancaster Basin may also contain a greater proportion of biogenic silica in comparison to the other sedimentary basins, which plot on a more terrestrial trend. This is essential in identifying different facies which appear to be complex and variable. Small and subtle changes are reported. These changes can have an impact is the characteristic of the reservoir and affect the hydrocarbon production significantly. The rapid change in nature of these facies could have an adverse effect on the extrapolation model employed.
Figure 4. Geochemical comparison of different occurrences of the Bowland Shale
Conclusion
This study demonstrates the value of inorganic elemental data as a primary geological data resource. Firstly, elemental data has been used to establish a robust chemostratigraphic correlation for the Bowland Shale. Secondly, elemental data have also been used to produce accurate mineral logs for all samples from the succession providing an accurate, fast and cost effective alternative to XRD. Thirdly, new high resolution mineral logs can be used to calculate a high resolution brittleness index and to provide synthetic gamma and sonic logs for shale successions. Finally, geochemical data model anoxia trends and TOC for sweet spot identification. In addition, is critical to stress that elemental data is quantitative and Chemostrat’s robust data QC protocols monitor sample and data quality to ensure that subtle variations in shale composition can mapped on a field and regional scale in the Bowland
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