Gamma Ray response from Jurassic shale offshore Norway. Can the Th/K ratio indicate Provenance, or do we need mineral analysis?
by Alexey Deryabin, NPD
was presented on Wednesday the 7 th of June, 2017.
A total of 386 wells from the Norwegian shelf have been analysed. 88 from the southern North Sea, 220 from the northern North Sea and 78 wells from Haltenbanken.
In the Jurassic section the volume of clay (Vcl) calculation was done using the Neutron-Density cross plot. For consistency, the same parameters for the 100% clay point was used for all wells. The Vcl calculated from the Neu-Den log was used to calibrate the gamma ray (GR) response for clean sand and 100% clay for each well. The selected GR clean and GR 100% clay was then used higher up the well in the cretaceous and the tertiary. In most cases that produced too much sand and silt. The cut-off values were adjusted to honour the lithological description coming from either the composite log or the mudlog.
In all the wells the Vcl clean sand GR response is between 10 and 25 api. In the North Sea Vcl 100% clay varies from about 100 to 150 api. But in the Haltenbanken, it is much higher. Above 200 api. This suggest that the clay fraction in the Haltenbanken shales are much more radioactive than in the North Sea. Hence, the mineral composition could be fundamentally different.
The first attempt to solve the issue was to identify wells with Spectro GR log to study the thorium-potassium (Th-K) cross-plot. Surprisingly, the K-logs was not logged with consistent units. Although all headings suggest %, quite a few was in fraction v/v.
Having converted all to the same unit, it became apparent that there was no systematic difference in the Th-K cross-plot between Haltenbanken and the North Sea. The resolution is simply not good enough.
It was therefore decided to pick core samples for mineral analysis.
Cost of running mineral analysis is considerable higher than managing to solve the issue with already run logs. It was therefore decided to test the problem by taking only three samples for mineral analysis. NPD assisted in selecting three wells with chronostratigraphically correlatable cores available. 3/7-8S, 25/5-7 and 6506/9-3 had all cores from the Callovian shale about 162 my ago. The three sample were analysed by RockType in Oxford using QemScan. A SEM microscope with a unique software where more than 37 different minerals are identified and its fraction measured.
This analysis show that the Haltenbanken sample have considerable more Muscovite than in the samples from the North Sea. Muscovite has also a very high K concentration and is therefore more radioactive than most other clay mineral. Although this analysis is based on only three samples, the wireline analysis is based on 386 different wells, it does suggest that Haltenbanken Jurassic shales have considerable more Muscovite that the Jurassic shales in the North Sea.
Dr. Carl F. Gyllenhammar, CV
Dr. Carl started his academic career-studying math, physics and chemistry at University of Grenoble, France (1975-78). After three years in France, he returned to Norway and continued studying geology at University in Oslo. In 1984 he became an oil rig well-site geologist. During his time in Conoco (1985) he systematically evaluated the drilling problems associated with drilling through thick clay-stone (Shale) sections before reaching the oil and gas reservoirs. He has received MSc degree in Applied Geophysics, with the title Seismic stratigraphy of the Essouira basin, offshore Morocco and PhD with the title; “A critical review of currently available pore pressure methods and their input parameters. – Glaciations and compaction of the North Sea sediments
Carl joined BP research department in Sunbury and worked with compaction and pore pressure developments in claystone. The last 10 years he has explored for oil and gas in the North Sea (UK, Norway and Denmark), onshore Europe, Morocco, Tunisia, Azerbaijan and Kazakhstan. He is currently focusing on the bridge between geology and geophysics; Rock Physics.