OR/17/037 Method

Hannis, S, and Gent, C. 2017. Petrophysical interpretation of selected wells near Liverpool for the UK Geoenergy Observatories project. British Geological Survey Internal Report, OR/17/037.

Data for the wells was sourced and prepared for interpretation (Data types and sources) and then interpreted in specific software (Interactive Petrophysics (IPTM, Version 4.2.2015.61, LR-Senergy software, used under academic licence). This describes the general method. The process required for individual each well is recorded in the relevant appendix, as they may have incorporated fewer, or additional steps, from those listed here, depending on data availability, and any anomalies discovered and dealt with during the data checks.

Data types and sources

A number of data types and sources are required for, or contribute to, the petrophysical interpretation of each well. Specific data available by well are reported in the appendix for each well. The main data types and sources used for the ESIOS project are listed here, tabulated in Table 2 and more details including compressed log plots are shown in the relevant appendix for each well (Appendix 1 - Kemira 1 (SJ47NE/101)) for Kemira 1 (SJ47NE/101) and Appendix 2 - Ince Marshes 1 (SJ47NE/100) for Ince Marshes 1 (SJ47NE/100)).

• Digital geophysical log curve data, mainly in LAS format (or sometimes LIS or DLIS) were extracted from the BGS storage holdings. Data origin includes company data via the DECC data storage agreement and also legacy BGS data e.g. curves machine-digitised from company field prints.
• Associated well data from scanned company reports available from the DECC data store, mainly in *.PDF or *.Gif format:

- Composite logs. Used to check well location, depths, curves scales, spliced intervals etc

- End of well reports. Used to cross check well location, logged intervals etc

- Tabulated core sample analysis (available as excel sheets or digitised for this project from report PDFs). Includes laboratory measured values such as Total Organic Carbon (TOC) content, mineralogical or elemental concentrations (from X-ray Ray Diffraction (XRD) and X-ray Fluorescence (XRF) analysis respectively) as well as porosity and permeability measurements. Used to cross check or, in some cases, calibrate log curve responses, depending on sample density and analysis results available.

- Borehole orientation data (available as excel sheets or digitised for this project from report PDFs). These are usually tables recording borehole depth, borehole inclination and azimuth. Used to be able to accurately position subsurface features such as the location of formation tops intersecting the well bore.

• Stratigraphy, i.e. formation tops (known as ‘well tops’) from various sources (Appendix 3 - Stratigraphic interpretations):

- Interpreted by BGS geologists for this (and previous) projects based on correlation with multiple wells in the region and in some cases in combination with examination of borehole rock core sections. 2 iterations available, one by N. Smith, one by C Waters (Table 12). These were compared with:

- The company stratigraphy, as recorded on the composite log, or in the end of well report (Table 13).

- The digital seismic interpretation, i.e. picked formation tops (identified using synthetic seismic sections using stratigraphy from surrounding wells (in PetrelTM, e.g. by J Williams, J White, D Evans).

• Cored intervals based on BGS digital core-holdings database query. This was used to indicate core locations on log plots to help to distinguish intervals where data was derived from core, or from, for example, side wall cores or cuttings, where this information was not readily available in the material in b).

Table 2    Data available for each well. More details are provided in the appendix for each well (1 & 2 respectively)

Well	General indication of amount of data available	Origin of digital curves used in the interpretation	Company composite log?	Stratigraphy
Kemira 1 (SJ47NE/101)	Full log suite only available over part of the main hole. Partial data for the remainder. No core or supplementary analysis.	Company (via BGS storage of DECC data).	Composite log scan, basic end of well report, field prints of most (not all) services. No borehole orientation data.	3 interpretations available (Appendix 3 - Stratigraphic interpretations): BGS (Table 12) 1 Company (Table 13) Edited BGS (N Smith version) used to output results (Table 14).
Ince Marshes 1 (SJ47NE/100)	Full log suite over main hole section including ancillary services and core sample analysis.	Legacy BGS, digitised from company field prints.	Composite log scan, end of well report and core sample reports. Multiple other reports, including TVD survey.

Data preparation

Steps to import and prepare the data prior to interpretation (Petrophysical interpretation of lithology and porosity ) are described:

Data load, depth and quality checks

1. Geophysical log curve data was loaded (to match the seismic petrel project). These were loaded and displayed in metres, but depth in feet was also calculated and displayed to help compare depths on the company logs.
2. The loaded curve types and intervals were checked against the company data (Data types and sources) to ensure that the expected curves were present. Curve response values at selected depths were then cross checked in more detail to ensure that data from each run was ‘on depth’, i.e. that the data had loaded and displayed at the correct value and depth.
3. Loaded LAS headers were compared to company data (Data types and sources) to ensure that the well location data matched and that elevations of the ground and the depth or height of the log starting position (usually ‘kelly bushing’ or ‘drill floor’) also matched.
4. Curves were examined more generally that their responses were within the normal ‘expected range’ and any possible log quality issues were noted. Any small data gaps were filled (to allow software calculation of Net to Gross and curve averages, Petrophysical interpretation of lithology and porosity).
5. Company data was checked for mud type (water or oil-based) and other parameters. If no opposing information was provided, it was assumed that suitable environmental corrections had already been applied to logs. Table 3 and Table 9 include some quality control comments and assumptions for individual wells.
6. Geophysical log curve data was loaded (to match the seismic petrel project). These were loaded and displayed in metres, but depth in feet was also calculated and displayed to help compare depths on the company logs.
7. Stratigraphic interpretations were examined and discussed to select the most appropriate to output Petrophysical summaries for (Appendix 3 - Stratigraphic interpretations). N Smith’s interpretations were used and slightly modified to match the intervals with petrophysical results (Table 14).

Table 3    Data load, depth and quality checks for each well.
More details are provided in the appendix for each well (1 & 2 respectively)

Well	Curve type and intervals loaded?	Curve values and depths OK?	Location/elevation data match?	Curve response ‘normal’?	Parameter availability?	Core data?
Kemira 1 (SJ47NE/101)	Run 2 Sonic inserted from more recently machine-digitised data. DRHO nota vailable digitally for runs 2 & 3. No resistivity curves available over runs 1 & 3	Yes. Given the machine-digitised nature of the data in this well is perhaps less precise than the original	Some minor discrepancies	Broadly yes (see parameter availability column)	Reasonable. Neutron recorded in sandstone units not limestone	None acquired
Ince Marshes 1 (SJ47NE/100)	All main curves available in standard and high resolution modes. Also data from FMI, DSI and ECS services	Yes	Some minor discrepancies	Yes	Good	Available as excel sheets. Reformatted (and depths converted to m) for IP load

Core sample data notes

The usual procedure for matching core and logs on a field-scale would be to first depth shift the core to the logs and then if necessary correct the core measurements for downhole in-situ conditions (particularly applies to porosity and permeability measurements, to allow for the different fluid phases and different confining pressures, for example, to understand the degree of overburden stress correction to apply). Core data could then be used to aid the interpretation as per Data types and sources.

However, for the wells examined here with core data available, (Ince Marshes 1, (SJ47NE/100)), details about core treatment, depth shifts to apply and the measurement method(s) were not generally captured. Therefore, within this report scope, the ‘usual’ steps to correct the core data described above are not fully implemented.

Other points of note for log-core matching include:

• Sample scale — the vertical resolution of geophysical logs are much larger than the few centimetres-across core samples retrieved. Thus in very heterogeneous formations, average log response over an interval may be very different to the ‘point’ data measurements on core;
• Core treatment history: once the cores are removed from their downhole environment, depending on their treatment, fluids and other core features (e.g. clay structures etc) may not always be usefully preserved. This is because of technical difficulties in preserving or simulating the down hole temperatures and pressures in a core. Preparation of the core prior to analysis may also include cleaning and drying processes, which can further alter the measured parameters from downhole conditions (this can apply particularly to porosity and permeability measurements).
• Core collection method: Sidewall core samples (wells 112/15-1 and 110/09a-2) of sandstones are often more affected by damage and drilling mud contamination than full cores (because of their smaller size relative to conventional core, which may also be affected by drilling and mud invasion damage around the outsides).

Table 4    Core and analysis available for Ince Marshes 1

Well	Core types	Analysis and number of measurement depths	Comments
Ince Marshes 1 (SJ47NE/100)	~200 m of conventionally drilled core. ~98 sidewall cores recovered.	47 XRD 8 XRF 82 TOC	Analysis appears to be on a combination of sidewall cores and rock cuttings. Depth shifting for sidewall cores should not be necessary (shot on wireline). Sample density and lack of logs over the lower section means the cuttings derived samples cannot be depth checked (or depth shifted if necessary).

Temperature gradient derivation

The software requires a temperature profile down the well for the processing (to allow recalculation of water and mud filtrate resistivities at downhole temperatures). Temperature data for each well and for the region was examined to determine a suitable temperature gradient to apply. Data sources include the ‘Maximum Recorded Temperature’ from the wireline logging tools, coupled with ‘time since circulation’. This can be used to calculate the downhole temperature undisturbed by drilling (e.g. using methods described in ZetaWare, 2016). In some cases downhole fluid production temperatures may also be available. Busby et al. (2011) examined temperatures in the top 1000 m of the UK based on data from a variety of sources and produced contour maps for the results. Results from the closest contours at each depth was included as was the UK-wide average temperature gradient (Appendix 4 - Temperature gradient). This was 2.8°C per kilometre and as this provided a broad fit to the few data points available for the wells examined, it was used in this interpretation.

Petrophysical interpretation of lithology and porosity

Curves describing reservoir parameters were interpreted using deterministic petrophysics workflows. The curves were used in combination to identify appropriate reservoir cut offs for the calculation of Net to Gross and average porosity values for the main formations (Calculation of thicknesses, average values and ranges) and for the TOC interpretation (Total Organic Carbon (TOC) calculation results).

Volume of clay curve (V_CL)

A Volume of Clay (V_CL) curve was interpreted for each well. This gives a continuous, geophysical log-derived volume of clay for the intervals investigated. Input curves were the Gamma Ray (GR) and a combination of the Neutron, Density and Sonic curves where available and of suitable quality. These curves were used to select end points representing 0% clay and 100% clay for zones of the log, subdivided based on changing log character and curve responses with depth, to create a (V_CL) log scaled from 0 (0% clay, i.e. 100% clean reservoir) to 1 (100% clay). This ‘quick-look’, interpretation of clay volume is based on curve responses only for Kemira 1, but for Ince Marshes 1, additional curves were available from the elemental capture spectroscopy (ECS) log and also sample analysis data for particular depths. This was used to help guide parameter selection. More details are described in the appendix for each well.

Coal identification curve (V_COAL)

A coal identification curve (V_COAL) was interpreted for each well, where ‘coal indicated’ = 1, and ‘no coal indicated’ = 0. This gives an indication of whether coal is thought to be present at each depth, based on the log response, and certain cut off values. This is because coal intervals could otherwise by mistakenly identified as part of the ‘net’ reservoir intervals by the other cut-off criteria. See Gross and net thicknesses. The cut off values were selected based on a combination of the curve responses (based on knowledge of expected responses in coal and other minerals) and where the composite log lithology track indicated coal to be present. Thus slightly different cut offs were used in each well (see Table 9, Summary of parameters and quality of the output interpreted curves). The additional ECS data for the Ince Marshes 1 well was used to cross-verify identification parameters. Well-specific details are described in the appendix for each well.

Porosity curves

Porosity curves were interpreted for each well. Input curves included the (V_CL) curves (Calculation of thicknesses, average values and ranges), Neutron, Density and Sonic curves. (Resistivity and Photoelectric Factor curves were used as visual aids to interpretation where required and data appeared to be reading within expected ranges). Areas of poor log quality were identified using primarily the Density Correction and Calliper curves.

Effective Porosity (PHIE) and Total Porosity (PHIT) curves were computed using the Neutron–Density method*. Where Density or Neutron data was unavailable, or its quality was poor, porosity was calculated using the sonic curve. These computations take into account tool measurements and interpretations of clay, mud filtrate and rock matrix properties.

* Using IP variable matrix density logic. IP solves the tool response equations for PHIE (corrected for wet clay volume). PHIT is then back-calculated by adding back in the clay bound water. Intervals that required sonic porosity calculations utilized the Wyllie equation.

Well-specific details are described in the appendix for each well.

Other curves: lithology curves and permeability indicators

Where suitable curves exist (i.e. dependent on data availability and pre-calculation of some curves) it may be possible to derive likely lithology from the curve responses. For example, the ‘multi- mineral lithology’ interpretation workflow in IP requires curves for the photo electric effect (Pef), and invaded zone resistivity (Rxo) curves. This interpretation was therefore only implemented for the Ince Marshes 1 well and the results were compared to ECS processed lithology output curves (see Appendix 2 - Ince Marshes 1 (SJ47NE/100)). However, the ‘simple’ mixed lithology calculations were also used to be able to cross compare results with the Kemira 1 well, which did not have the required curves available for the ‘multi-min’ interpretation. The mixed mineral plot is derived from the V_CL, V_COAL, V_SALT and PHIE curves already described, and a Vsilt curve, a silt index, created by the software to indicate silt content (i.e. it is not an accurate, calculated volume. It is purely for display in the plot, to indicate that the rock can be thought of as containing clean sand of a certain porosity, non-porous silt and clay and it is not used in the interpretation methodology).

Permeability can be inferred by the relative responses of particular curves, for example, by observations in the Spontaneous Potential (SP) curve deflections (not available for either well) or by separation in resistivity curves which have different depths of investigation away from the borehole (see Summary of parameters and quality of the output interpreted curves) for discussion on this). If hydrocarbon exists in the well and a water zone beneath it is also present, then residual water saturation can be interpreted from log responses and permeability calculated using various empirical relationships e.g. Timur Coates equations etc. This method was also not applicable to the wells in this study.

Petrophysical interpretation of Total Organic Carbon content (TOC)

The Total Organic Carbon TOC was calculated using the Passey-method inbuilt into the IP TOC calculator for the shale intervals of Ince Marshes 1, in a similar methodology to those used by Gent et al. 2014. In the Passey method, scaled sonic and deep resistivity curves were made to overlay giving a vertically continuous wt % TOC curve (Passey et al. 1990). Ince Marshes was split into two maturity zones to represent the increasing maturity with depth based on the vitrinite reflectance profile (Harriman, 2011). The bulk density and neutron porosity curve overlay plots were used to verify those of the sonic.

Kemira 1 was not chosen for TOC calculations as the required resistivity curve only covers a 640 m interval over typically reservoir and barren units. Further to this there is a lack of geochemistry data with which to calibrate the TOC calculation.

The objective of this study was to produce TOC curves Ince Marshes 1 accompanied by statistical TOC outputs for the well. To be able to calculate the TOC, level of maturity (LOM) values had to be established. In addition, clay volume curves (VCl) with a suitable cut-off value were required to be able to distinguish potential shale source rocks from clean reservoir rock. Coal identifiers and TOC curve cut off values were also applied to the final calculations

Volume of Clay (VCl): As discussed in Volume of clay curve (V_CL). The VCl curve was used as a discriminator in subsequent calculations, to remove intervals with less than 50% clay (i.e. those considered unlikely to be a source rock). Coal Discriminator: The Passey method is accurate for calculating TOC in shale intervals but not in coals; if coals are not removed they give inaccurate spikes on the calculated TOC curve. The coal signal has to be removed using discriminators discussed in Coal identification curve (V_COAL). The final results presented do not incorporate coals and account only for shale intervals.

Level of Maturity (LOM): A key parameter in the Passey equation for calculating TOC is the level of maturity (LOM). This can be calculated from Ro values, measured on core and cuttings samples (Hood et al. 1975). The Ro values were taken from released geochemical reports (Harriman, 2011).

Assumptions and limitations

The following assumptions and limitations should be considered when analysing the results and graphical TOC log plots:

• The Level of Maturity parameter required for the Passey method TOC calculation is well defined from the Ro values in the geochemical report (Harriman, 2011; Appendix 5 - Technical information for BGS internal use). It is assumed that these values are correct, no sensitivity analysis has been run on these values in this study.
• The Passey method also requires the selection of a ‘lean shale’ point where a shale is assumed to have no organic carbon. No sensitivity on this parameter has been done for this study, so this should be taken into consideration when examining the absolute TOC values reported here.
• A volume of clay (VCl) cut-off of 0.5 has been arbitrarily applied to remove intervals with a low clay content.
• The vertical resolution of the calculated TOC is limited by the resolution of logging tools. This means that, for example, sharply varying TOC values across thinly interbedded shales, coals and sands intervals may not be distinguishable and is likely to be presented as a smoother ‘average’ TOC curve response. By contrast, each TOC measurement from cores or cuttings samples represent a single point in the succession. In addition it was not always possible to precisely depth shift the core depths to log depths. Therefore there may be some small depth differences between core TOC measurement points and the calculated TOC curve. The sample-derived TOC measurements are assumed to be correct, but these in themselves may have their own limitations, which are not discussed here.
• Petrophysical log analysis has been used as a screening tool to highlight potential TOC rich source rock intervals (shales), over larger depth ranges than is available for core/cuttings sample data. Given time constraints, data availability and the variable nature of the Carboniferous sedimentation, kerogen types have not been taken into consideration. To further the work presented in this report, investigation of the kerogen type in conjunction with the calculated TOC will give a more complete understanding of the hydrocarbon sources
• Units shallower than the Westphalian A (<700 m TVD) have not been assessed for TOC wt% as they are immature for hydrocarbon production and also have no measured TOC values for calibration.