Combining petrophysical and seismic data to assess source
rocks: A case study from the Upper Jurassic of the North Sea
Balazs Badics, Sean Mackie (DEA Norge AS) & Anthony Avu (E.ON E&P UK)
78th EAGE Conference & Exhibition 2016
Vienna, Austria, 30 May – 2 June 2016
78th EAGE Conference & Exhibition 2016
Vienna, Austria, 30 May – 2 June 2016
Introduction
The main proven source rocks of the northern North Sea, the upper Jurassic Draupne and Heather
Formations of the Viking Group (Barnard and Cooper, 1981; Thomas et al., 1985, Kubala et al., 2003)
show large vertical and lateral variations in TOC, kerogen type, and thickness, making the accurate
calculation of a generated petroleum volume challenging. By integrating the available high-resolution
seismic datasets with petrophysical evaluations and organic geochemical data, a better quantitative
assessment of the organic richness can be made and the generated petroleum volume can be calculated
more precisely.
In this paper, we present a workflow for the organic geochemical, petrophysical, and seismic
characterization of the Draupne and Heather Formation source rocks in the Viking Graben in the
Norwegian North Sea (Figure 1). Following standard petrographic and organic geochemical analysis,
a detailed petrophysical analysis was carried out then a well-to-seismic calibration. This workflow
provides the calibration necessary to map the regional distribution of the source rocks on full-stack
seismic data. Finally, post-stack seismic attributes and multi-attribute combination and visualization
techniques were used to map variations in organic-matter content and to highlight fault zones.
Database and Methods
Rock-Eval pyrolysis as well as GC and GC-MS data were available from 17 wells. The data consisted
of measured S1, S2, S3, Tmax, and Total Organic Carbon (TOC) (wt. %), as well as the calculated
parameters such as Hydrogen Index (HI), Oxygen Index (OI), and Production Index (PI). The
petrophysical evaluation utilized 21 wells with standard log suites with shear velocity logs from wells
25/5-7 and 25/6-4. After carrying out the standard petrophysical analysis, we determined the lithology
of the complete Draupne and Heather succession, as well as the Vshale and AI curves. The logs
enabled the calculation of TOC contents using the Passey et al. (1990) method.
The seismic analysis workflow represented a four-step process aimed at highlighting properties in the
seismic data that pointed to the TOC content distribution. It started with data conditioning to improve
the quality of the seismic, then well-log calibration and coloured inversion to highlight TOC
distribution and finally multi-attribute combination and visualization to show the distribution of faults
and organic matter.
Our database was a cropped cube of the PGS 3D Mega-Survey covering an area of 2348 km2 (917
mi2), having a stacking bin size of 25 m × 25 m. The seismic image quality was fair to good quality
above the Base Cretaceous Unconformity (BCU), but below the BCU it was poorer with difficulty
imaging the reflectors in great detail. The data were passed through structurally oriented post-stack
seismic data conditioning to reduce noise, survey edge artefacts, and make it suitable for attribute
extraction.
Seismic interpretation of the study area was performed by creating accurate well ties, then detailed
mapping of the Top Draupne (BCU), Base Draupne (Top Heather), and Base Heather horizons.
Well 25/5-2, which had a good-quality TOC log calculated by the Passey et al. (1990) method, was
selected for further calibration to ascertain the suitability for inversion studies. Løseth et al. (2011)
showed that if seismic data can be inverted to AI data, it is possible to transform the AI values in the
source rock formation to TOC percent values. We cross-plotted the VP∕VS ratio against impedance for
the Draupne to Heather interval of wells 25/5-2, 2-14, 2-7, 3-1, 5-7, 6-1, and 6-2. The cross-plot
coloured with gamma-ray (GR) revealed higher values at the lowest impedance areas. When well
25/5-2 cross-plot was coloured with TOC, a good correlation of high TOC with low impedance was
achieved.
78th EAGE Conference & Exhibition 2016
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Coloured inversion was then performed on the seismic to produce a relative AI cube. Due to eroded
sections of the Draupne in some areas, an exact correlation at the Top Draupne level with observed
TOC logs and petrophysical maps was not achieved. However, map extractions of the Top Draupne
shifted down to 10–30 m below the Top Draupne and in the Middle Draupne level revealed lowimpedance values similar in pattern to the high-TOC areas mapped in the wells.
Figure 1 (a) Location of the study area in Europe. (b) Study area shown as a red rectangle and the
main faults and structural elements of the Viking Graben, North Sea. Structural elements are from
Norwegian Petroleum Directorate (NPD) Factmaps. Main Norwegian fields and discoveries,
Norwegian Blocks, and the UK-Norway medium line (red dashed line) are also shown.
Results
The trough reflection at Top Draupne dims with offset and is classified as an amplitude variation with
offset class 4, which is one of the classic characteristics for top source rock reflections (Løseth et al.,
2011). The Top Draupne reflector has a very high negative amplitude and continuous reflection in the
study area. The base Draupne horizon was particularly difficult to map on the western, deeper part of
the study area, and the thickness uncertainty is therefore largest here. Based on the detailed
biostratigraphy of the wells, and the seismic interpretation of the Top Draupne, Base Draupne, and
Base Heather, it was possible to map in detail the thickness variations of these intervals (Figure 2).
The thickness of the Draupne Formation in the study area decreases from 123 m (400 ft) (in well
25/5-2) closer to the Viking Graben axis to approximately 30 m (100 ft) on the flanks of the Heimdal
Terrace, and then it further decreases to 20–25 m (65–80 ft) or fewer over the Utsira high (in well
25/6-1 and 25/6-2) (Figure 2). Based on the very clear seismic response of the Top Draupne
reflection, and the interpretation of the Base Draupne horizon, the Draupne can be up to 150–400 m
(500–1300 ft) thick in undrilled deep grabens, for example, in the area between the 25/2-5 and 25/2-4
wells. The middle Volgian to Ryazanian upper Draupne is missing due to erosion in wells 25/4-1,
25/5-3, 25/8-5, 25/8-7, which is similar to the results from Justwan et al. (2005). In the other wells,
the upper Draupne is present, usually as a uniformly 10- to 20-m-thick condensed section. The
Oxfordian-early Volgian lower Draupne is thicker in the more basinal wells (25/5-4 and 25/4-6), and
based on the seismic interpretation, the lower Draupne unit represents the thicker Draupne sections in
the undrilled grabens.
The lower Draupne unit is missing in the 25/6-1, 25/8-7, 25/9-1 wells, whereas the upper Draupne
unit is present. Close to the crest of the Utsira high, in 25/5-3 well, the whole Draupne section has
been eroded. The Heather Formation is slightly thicker (Figure 2), typically between 8 and 160 m
78th EAGE Conference & Exhibition 2016
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(25–525 ft) in the wells. It is present in the whole study area, in all wells, except well 25/4-1 on the
Heimdal horst, where it was most likely never deposited. It is up to 500 m (1600 ft) thick in undrilled
Figure 2 Results: (a) Draupne thickness map (m) and (b) Heather thickness map (m). Wells with
Jurassic penetrations are shown. (C) Draupne TOC map (wt.%) and (d) Heather TOC map (wt.%).
The red rectangle shows the area of study. The red rectangle shows the study area.
78th EAGE Conference & Exhibition 2016
Vienna, Austria, 30 May – 2 June 2016
deep parts of the grabens in the north-western part of the area. The TOC maps shown in Figure 2 were
created by first averaging the measured TOC per well per formation. Following this, the TOC maps
were calculated by finite-element interpolation between the wells, which also used the attribute maps
using the TOC-AI relationships (Figure 2).
Conclusions
3D seismic data could also be applied to understand the thickness and richness variations of source
rocks better and ultimately calculate expelled HC volumes with higher accuracy. We have shown a
workflow for the organic geochemical, petrophysical, and seismic characterization of the source rocks
using organic geochemical data, well logs, and full-stack seismic data sets. Detailed petrophysical and
organic geochemical calibration was used to map the regional thickness variations. The data
conditioning, inversion, structural and fault imaging, as well as the multi-attribute combination
techniques used were efficient in highlighting potential TOC distribution, main fault zones, and their
geologic interaction.
Acknowledgements
The authors especially would like to thank their employer, DEA Exploration and Production Norway,
for the permission to publish the results of this study. We would like to thank our colleagues: A.
Cormier for her help with ArcGIS, A. Grindhaug, for some of the figures; J. Herredsvela, for his input
on seismic data analysis; T. Marcus Bauer for his petrophysical work; and R. Davies and R. Hiney for
their positive feedback. We also thank ffA, especially O. Lee, for supplying the software used in this
study, as well as PGS for the PGS Mega-Survey data set, which was the initial 3D seismic data set
used for our investigations.
References
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78th EAGE Conference & Exhibition 2016
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