OR/17/037 Results

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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.

Results are based on the method broadly described in the Method section, using Interactive Petrophysics (IPTM, Version 4.2.2015.61) LR-Senergy software, used under academic licence. Output log plots are shown by well in the relevant appendices to this report. Output digital *.LAS files, excel tables of input data and 1:200 log plots are provided separately. All outputs should take into account the data quality comments provided in Table 9. This can give an indication of the confidence in output curve results.

Interpreted curves[edit]

Digital output curves were interpreted using the method described in the Method section. These were:

  • Volume of Clay curve (VCL);
  • Coal Identification curve (VCOAL);
  • Evaporite Identification curve (VSALT);
  • Effective Porosity curve (PHIE);
  • Total Porosity curve (PHIT);
  • Total Organic Carbon content curve (TOC).

Plots of data for each well are available as a ‘quick-look’ output in (Figure 2) and Figure 9 (in Appendix 2 - Ince Marshes 1 (SJ47NE/100). Table 5 lists the curves included in the output *.LAS files (Appendix 5, Section 5.1).

Table 5    Curves included in output *.LAS files.

Calculation of thicknesses, average values and ranges[edit]

Gross and net are in metres, measured depth. This means that they represent measured depth thicknesses along the borehole, which is not necessarily the true stratigraphic, or true vertical thickness. Net to gross and porosities are provided as fractions.

Gross and net thicknesses[edit]

The total thickness of the interval of interest along the borehole is the ‘Gross’ provided here.

The Net interval is the sum of the thicknesses of those parts of the reservoir that meet a set of cut- off criteria (applied to one or more curves). These parameters (the cut off criteria that define the Net) will, at the field scale, be based on operator preferences or field observations of reservoir productivity that may be refined through time. However, for this ‘quick-look’, generic cut-offs have been applied to give a broad indication of the Net where:

  • Clay volume is less than 50% (i.e. where VCL <0.5);
  • Porosity is more than 5% (i.e. where PHIE > 0.05);
  • No coal or salt intervals are identified (i.e. where VCOAL = 0, or VSALT = 0).

Net to gross[edit]

Net to Gross (NTG) in this report gives an indication of the amount of reservoir (Net) within an interval of interest (Gross). It is expressed as a fraction from 0 to 1, where a NTG of 0 means that no reservoir has been interpreted within the interval and a NTG of 1 means that all of the rock within the interval has been interpreted to be composed of 100% reservoir. The NTG equation is shown below.

Net to Gross (NTG) = Total thickness of reservoir (net)


Total thickness of interval (gross)


NTG values were calculated for each stratigraphic unit in each well (and by stratigraphic unit (for all wells) and by well (for all stratigraphic units)).

Average porosity and range[edit]

Average porosities and ranges were calculated for each stratigraphic unit in each well. These are based on arithmetic average calculations and curve statistics of the interpreted effective porosity (PHIE) curve (Section 2.3.3 over the intervals defined as net reservoir (Net: see NTG, Section 3.2.1)).

Summary of reservoir petrophysical results[edit]

Summary results (based on interpreted curves, Interpreted curves) are given for the whole well by individual formation in each well according to the stratigraphy in Table 14.

Table 6    Results of petrophysical calculations listed by formation for each well
(see Calculation of thicknesses, average values and ranges ) for an explanation of the column headings).
For each column, the best and worst values are highlighted on the colour spectrum from dark green to dark red respectively
Well Unit name Unit code Top Base Gross Net NTG Av Phi
Kemira 1 (SJ47NE/101) Bromsgrove Sandstone Formation Bromsgrove 39 281 242 241 0.99 0.22
Kemira 1 (SJ47NE/101) Sherwood Sandstone Formation Sherwood 281 523 242 242 1.00 0.25
Kemira 1 (SJ47NE/101) Kidderminster Formation TSZ_or_KDM 523 618 95 95 1.00 0.19
Kemira 1 (SJ47NE/101) Manchester Marls or Bold Formation MM_Bold 618 738 120 120 1.00 0.18
Kemira 1 (SJ47NE/101) Collyhurst Sandstone Formation CS 738 1042 304 304 1.00 0.21
Kemira 1 (SJ47NE/101) Halesowen Formation Halesowen 1042 1221 179 73 0.41 0.15
Kemira 1 (SJ47NE/101) Westphalian C West_C 1221 1248 27 6 0.24 0.23
Kemira 1 (SJ47NE/101) Westphalian B West_B 1248 1401 153 58 0.38 0.14
Kemira 1 (SJ47NE/101) Westphalian A West_A 1401 1433 32 2 0.08 0.10
Ince Marshes 1 (SJ47NE/100) Westphalian C West_C 368 548 180 38 0.21 0.10
Ince Marshes 1 (SJ47NE/100) Westphalian B West_B 548 700 152 35 0.23 0.10
Ince Marshes 1 (SJ47NE/100) Westphalian A West_A 700 945 245 21 0.09 0.13
Ince Marshes 1 (SJ47NE/100) Millstone Grit Group MG 945 1452 507 110 0.22 0.08
Figure 2    Kemira 1 compressed well plot to show stratigraphic options (far left tracks) input data availability (middle tracks) and output petrophysical results (2 right hand tracks).
Figure 3    Ince Marshes 1 compressed well plot to show stratigraphic options (far left tracks) input data availability (middle tracks) and output petrophysical results (2 right hand tracks).

Total Organic Carbon (TOC) calculation results[edit]

The results for Ince Marshes are presented graphically in Figure 5 and have been tabulated for no minimum shale thickness (Table 7) and with a 2 m minimum shale thickness (Table 8). Graphical presentation includes logs plots and histograms of TOC calculated for each formation. When assessing the absolute values and quality of the results reported here, the assumptions and limitations outlined in section Assumptions and limitations should be taken into consideration. In the presented TOC plots reservoir intervals and coals have been removed from the calculated TOC curve leaving only the shale intervals.

Based on the available data, TOC has been calculated for the Westphalian A and Millstone Grit intervals. The shallower formations have no measured TOC values to which the calculated TOC curve can be calibrated, combined with the low maturity (<0.5% Ro) any predicted TOC values could be drastically over predicted.

The siltstone and sandstone dominated succession of the Millstone Grit has caused the VCl discriminator to highlight the formation as mainly ‘clean’ rather than ‘shaley’. This is especially evident in the shale thickness to gross formation thickness ratio change of 0.41 to 0.28 with a 2 m minimum thickness applied.

Table 7    Shale thickness and TOC rich shale thickness to Gross Formation thickness
summary for Ince Marshes 1. (No minimum shale thickness)
Formation G, Gross Formation Thickness (m) S, Shale Thickness (m) S/G Avg TOC of S thickness (calculated wt %) T, TOC rich (>1.5% TOC) Thickness (m) T/G Avg TOC of T thickness (calculated wt %)
Westphalian A 233.4 161.1 0.70 1.05 34.2 0.15 3.4
Millstone Grit 486.4 199.5 0.41 2.09 118.8 0.24 2.9
All Formations 719.9 360.6 0.50 1.54 153.0 0.21 2.9
Table 8     Shale thickness and TOC rich shale thickness to Gross Formation thickness
summary for Ince Marshes 1. (2 m minimum shale thickness)
Formation G, Gross Formation Thickness (m) S, Shale Thickness (m) S/G Avg TOC of S thickness (calculated wt %) T, TOC rich (>1.5% TOC) Thickness (m) T/G Avg TOC of T thickness (calculated wt %)
Westphalian A 233.4 125.2 0.54 1.18 18.6 0.08 4.1
Millstone Grit 486.4 134.3 0.28 2.18 52.3 0.11 3.1
All Formations 719.9 259.5 0.36 1.51 70.9 0.10 3.1

The Westphalian A in Ince Marshes 1 is 233 m thick consisting of 161.1 m of shale. Of this thickness of shale 34 m of it has a TOC >1.5 wt% at an average of 3.38 wt% TOC. If a minimum thickness of 2 m of shale is included in the calculation the total TOC rich shale thickness is reduced to 18.6 m. Subsequently reducing the overall formation thickness to TOC rich shale thickness, fraction from 0.15 to 0.08.

Intervals within the Westphalian A at 870–876 m, 897–901 m and possibly 790–795 m show typical curve responses for a mature source interval containing hydrocarbons (Passey et al. 1990[1] schematic, Appendix 5 - Technical information for BGS internal use, Conclusions), these reach TOC values up to 9.2 wt% (Figure 4).

Figure 4    Mature TOC rich shale intervals at 875 m and 900 m in the Westphalian A. The black line in the track on the right is the calculated TOC with a maximum value of 9.2 wt%. Grey shading indicates intervals with TOC >1.5 wt%.

The Millstone Grit has been split into two for the calculation based on the increasing maturity outlined in Harriman, 2011[2] Appendix 5 - Technical information for BGS internal use (Ancillary information available for BGS reference only). As a complete unit the Millstone Grit in Ince Marshes 1 is 486.4 m thick consisting of 199.5 m of shale. Of this thickness of shale, 118.8 m of it has a TOC >1.5 wt% at an average of 2.9 wt% TOC. If a minimum thickness of 2 m of shale is included in the calculation the total TOC rich shale thickness is reduced to 52.3 m. The resultant change in the T/G (TOC rich shale thickness (T) to gross formation thickness (G) fraction) is a reduction from 0.24 to 0.11.

The measured sidewall core and cuttings TOC values in this well extend beneath the base of the geophysical well logs to 1575 m (MD). There are 44 measured values averaging 2.89 wt% TOC with a maximum of 6.93 wt% TOC.

Figure 5    Ince Marshes 1 section of well plot to show the calculated TOC curve (pink) and TOC rich (>1.5 wt%) intervals (grey shading in track 8). Measured TOC values are represented by black dots. Interpretation guide diagram available in Passey et al. 1990[1] and Appendix 5 - Technical information for BGS internal use (Ancillary information available for BGS reference only).

Summary of parameters and quality of the output interpreted curves[edit]

Table 9 documents key log interpretation and quality notes regarding each well. Individual data quality checks by well are expanded on in the relevant appendix, as referred to in the table below. Parameter sets for (VCOAL) are also included. Parameters for VCL and porosity modules for each well are available separately as *.set files (Appendix 5 - Technical information for BGS internal use (Ancillary information available)).

Table 9    Summary of VCOAL parameters and interpretation comments

Well

Coal ID parameters

General interpretation/data quality comments
Density Neutron Sonic Comments refer to the ‘fixed logs’, i.e, once the data load & preparation processes have prepared the best possible starting dataset. It was assumed that appropriate borehole corrections had already been applied to all curves, (except where otherwise mentioned) Density correction curve (DRHO or HDRA) in tolerance was assumed to be -0.1 to 0.1.
Calliper logs (HCAL or CALI etc) were compared to bit size to identify washouts or zones of potential poor pad-tool contact.
All curves were compared to their expected responses and to the company composite pdf logs where available.
IP defaults:

1.8      0.5      120

Kemira 1 (SJ47NE/101) 2.1 0.45 75 Outputs are lower confidence for this well than for Ince Marshes 1, resulting from initial input data quality and lack of additional data to cross-validate output curves during the interpretation process. There is low confidence in absolute neutron porosity (NPOR) values, relating to its transformation to limestone units (see Appendix 1, data load notes). This affects both VCL and PHI outputs. The density correction curve (DRHO) was not available digitally (although shown in field prints), so it was not possible to auto-replace porosities derived using the poor quality density data, with sonic derived porosities, which in any case did not match with the neutron-density derived porosities particularly well over the parts of the log (see Appendix 1 - Kemira 1 (SJ47NE/101), PHIE interpretation notes). Calliper data shows that the hole was rugose and washed out in places, but callipers are rarely open to their maximum extent (i.e. pad tools are mainly not ‘floating’ and assumed to be in contact with borehole wall, suggesting that density-neutron data should be otherwise reasonable). No other information is available to cross check results apart from the lithology plot of the company composite log. Selection of VCOAL parameters was guided by the coal bed list in the end of well report (it is not known by what method these were identified). The cut-off parameters selected were able to detect 4 of the 15 recorded coal beds (see Appendix 1 - Kemira 1 (SJ47NE/101), PHIE interp notes). High porosity spikes occur either side of identified coal beds, because porosity is nulled over the identified coal, but the coal content of adjacent coal-rich sediments (below the VCOAL cut-off criteria parameters) is not able to be accounted for in the software.
Ince Marshes 1 (SJ47NE/100) 1.8 0.5 70 Data quality for this well appears to be not too bad from CALI and HDRA evidence. A few washouts, hole rugosity (CALI spikes) and areas where HDRA is out of tolerance, in some cases correspond to coals (e.g. around 620 m), but not always. The calliper doesn’t appear to be open to its maximum extent anywhere, i.e. pad tools were not ‘floating’ suggesting that density-neutron data should be otherwise reasonable. The hole is ovalised in the top section above 434 m to casing shoe (378 m) based on the FMI dual callipers, C1, C2) but the data generally appears to be OK. Ancillary data from the Elemental Spectroscopy Log (ECS) provided a processed output for a suite of minerals. In addition, XRD, XRF analysis was available from samples at some of the sidewall core depths, or from rock cuttings Both these datasets were used to cross check input and interpreted output curves and in some cases used to guide parameter selection. VCOAL created porosity spikes where porosity is nulled over the identified coal bed. (see Appendix 2 - Ince Marshes 1 (SJ47NE/100), Interpreted output data quality check notes and PHIE interpretation notes).

References[edit]

  1. 1.0 1.1 PASSEY, Q R, CREANEY, S, KULLA, J B, MORETTI, F J, and STROUD, J D. 1990, A Practical Model for Organic Richness from Porosity and Resistivity Logs. AAPG Bulletin. 74. 1777–1794
  2. HARRIMAN, G. 2011. Ince Marshes-1 VR report Modified.pdf. A GHGeochem Ltd report for iGas.