OR/15/013 Modelled surfaces/volumes

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Woods, M A, Newell, A J, Haslam, R, Farrant, A R and Smith, H. 2015. A physical property model of the Chalk of southern England. British Geological Survey Internal Report, OR/15/013.

Although well picks have been made for all Chalk lithostratigraphical boundaries and many other marker beds, for reasons of simplicity and time availability, horizon modelling has been limited to four lithostratigraphical boundaries (Table 1). A priority for future work is to generate model surfaces for all the units listed below, as well as for key marker-beds.

Table 1 Formation tops which have been modelled (highlighted in green) and their correspondence to standard BGS lexicon descriptions and codes.

The Base Chalk and Top Chalk unconformities encapsulate the Chalk Group, while the Top Zig Zag and Top Lewes Nodular Chalk formations provide internal control on the alignment of grid cells within the main body of the Chalk. Note that the Top Chalk Unconformity is a complex polygenetic horizon that includes, (1) subaerially exposed Chalk eroded to all stratigraphic levels, (2) Chalk that has been eroded to all stratigraphic levels and covered by Quaternary deposits (e.g. head and alluvium), and (3) Chalk that has been eroded to relatively shallow stratigraphic levels and covered by in situ Palaeogene deposits. Additional horizons can be incorporated at a later stage if they are deemed necessary to add further control to the alignment of grid cells or the proportion of lithofacies within modelled regions. The modelled horizons were selected largely because of, (1) their distribution across the full thickness of the Chalk, (2) each has a relatively high number of borehole picks and (3) they enclose regions within the Chalk of broadly similar lithology (Figure 2). Thus the Base Chalk to Top Zig Zag Chalk interval contains many marls and marl-rich chalks, the Top Zig Zag to Top Lewes Nodular Chalk interval contains common nodular chalks, and the Top Lewes to Top Chalk is dominated by soft marl-free chalks. These lithological regions provide broad constraints on the proportion of lithofacies at different stratigraphical levels within the model during the interpolation process.

Figure 2 The Sandhills 2 well on the Isle of Wight, southern England, as an example of the how the four modelled horizons (Table 1) subdivide the Chalk into three broad lithological regions.

Surfaces have been modelled using SKUA-GOCAD 2013.2 software, which following petroleum-industry conventions, models the top of formational units. Although this departs from some BGS practice which models the bases of geological units, there is no practical impact on model outputs. The rationale for the SKUA approach is that it is the tops of geological formations that are modified by processes before later geological units are deposited.

In addition to the above formational surfaces, a large number of intra-formational marker-beds have been coded in boreholes and outcrop successions. A list of these is provided at Appendix 1. These are not currently modelled, but the intention is that these surfaces will be capable of being generated in future model versions. These marker-beds include flints, marl seams and hardgrounds. In many, although not all cases, they form isochronous surfaces that divide up formations, allowing detailed analysis of thickness and facies variation within parts of formations, and in the future, perhaps also including stratigraphical analysis of fracture data when this becomes available. Planned future incorporation of outcrop scan data (i.e. chalk cliff sections) will use the marker-beds as high resolution tie points to facilitate linking with modelled formational surfaces. One of the major aims of this work has been to not only model formation boundaries but to model the internal distribution of facies within the Chalk Group. At present we use a simple sevenfold classification of Chalk facies, with the aim of creating a uniform and consistent subdivision across the UK Chalk. The facies scheme must reflect the limited data that are often available from geophysical logs and simple borehole descriptions. The main facies types recognised for modelling are:

  • Chalk
  • Marl (stylolitic marl)
  • Marly Chalk
  • Hardground
  • Hard Chalk (Nodular Chalk)
  • Faulted/Fractured/Channelised Interval
  • Sandy/Silty Chalk

Terms in parenthesis indicate additional terms recognised in facies coding of outcrop sections that have been combined with more generic terms for the purpose of facies modelling (see 4 below). The facies term 'Chalk' is a default used when there is no data to indicate the presence of other facies types.