OR/17/037 Conclusions
| 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. |
Two onshore wells near Liverpool UK were petrophysically evaluated to contribute to the geological characterisation of a Cheshire site for the UK Geoenergy Observatories project. The two wells are Kemira 1 (SJ47NE/101) and Ince Marshes 1 (SJ47NE/100).
Different types, quality and amounts of data were available for the two wells. Far less data is available for the evaluation of Kemira 1. For both wells, various limitations and assumptions are taken into account during the interpretation (particularly for the TOC calculations) and these should be taken into account when examining the petrophysical results.
The Kemira 1 well was logged from the Triassic Bromsgrove Sandstone Formation down to the Carboniferous Westphalian A unit, the base of which is not penetrated (~1400 m logged between 32–1433 m). Data available comprises parts of a standard log suite, machine-digitised from the legacy log field prints
The Ince Marshes 1 well was logged over the Carboniferous interval comprising the Westphalian C-A and the Millstone Grit Group, the base of which may be drilled, but was unable to be logged due to hole difficulties (~1084 m logged between 368–1452 m). In addition to standard log suites, data from more advanced logging tools such as imaging, dipole sonic, and elemental spectroscopy tools were also recorded. Sidewall cores were collected and analysed using various techniques to determine mineral, elemental and total organic carbon content.
Outputs for this evaluation include interpretations of clay volume and porosity. These interpreted curves were used to examine the proportions of reservoir rock and shale for each formation in each well and their respective properties. Net reservoir intervals were defined by those intervals where the clay volume was less than 50%, the porosity was more than 5% and no coal intervals were present.
Permian and Triassic age formations were only present in Kemira 1. These all have high reservoir net to gross (NTG) values of 0.99 or 1 (i.e. 100% net reservoir). Their average porosities range from 18–25% and the Sherwood Sandstone Formation shows the highest average porosity at 25%. The older, Carboniferous formations have much lower NTG values, all containing less than 50% reservoir rocks (NTG ranging from 0.08–0.41). Their porosities are also lower, ranging from 8–15%, apart from the Westphalian C unit in the Kemira 1 well, which are anomalously high (23%) resulting from the presence of coal intervals and porosity artefacts adjacent to them (a software/parameter selection limitation).
Total organic carbon (TOC) was calculated for the rocks beneath the Westphalian B unit in Ince Marshes 1. Shales with TOC values calculated as greater than 1.5 wt% were considered ‘TOC- rich’. The ratio of these to the total formation thicknesses are generally low: 0.08–0.15 for the Westphalian A and 0.11–0.24 for the Millstone Grit. The lower end of the range represents the ratio when a minimum shale thickness cut offs of 2 m is considered. The TOC rich shale in the Westphalian A has an average of 3.38 wt% TOC. However, individual intervals within the unit show typical curve responses for a mature source interval containing hydrocarbons and reach TOC values up to 9.18 wt%. The TOC rich shales in the Millstone Grit have an average of 2.9 wt% TOC. The measured sidewall core and cuttings TOC values extend beneath the base of the geophysical well logs to 1575 m. There are 44 measured values averaging 2.89 wt% TOC with a maximum of 6.93 wt% TOC.