1. Subsurface biodegradation of crude oil in a fractured basement reservoir, Shropshire, UK
UNIVERSITY
OF ABERDEEN
John Parnell, Mas’ud Baba, Stephen Bowden & David Muirhead
School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK
RESULTS
DISCUSSION
GEOLOGICAL SETTING
SYNOPSIS
The region of study is in Shropshire, part of the ‘Welsh Borderland’. The main components of the hydrocarbon migration story in the Region are as follows:
A block of pre-Carboniferous rocks (Longmyndian and Uriconian metasediments and metavolcanic rocks, and Cambrian to Devonian sediment).
A Carboniferous coal-bearing succession, and as a thin veneer on the pre-Carboniferous block.
A Triassic basin to the north (Cheshire Basin).
From these studies organic geochemistry was found to discriminate between modern and ancient biodegradation, petrography confirmed the presence of microbial activity in the subsurface, and thermal history (fluid inclusion analysis) constrained the level of heating as well as confirmed the migration of bitumen in the geological past. Below is a summary of our findings.
Biodegradation:
All samples except Robins Tump yield 25-norhopanes.
Quartz-Barite samples have relatively low norhopanes reflecting longer history in Subsurface.
Mineralized fractured samples are most degraded.
Recent seepage samples are least degraded.
Evidence for ancient bitumen migration:
Inclusion of quartz in bitumen.
Cross cutting relationship between episodes of bitumen bearing vein.
Evidence of forceful migration between bitumen-supported breccia.
Subsequent faulting of slickensided bitumen.
Evidence of microbial habitat in basement reservoirs:
Framboidal pyrite at Row Brook consequence of microbial sulphate reduction.
Spatial relationship between bitumen and pyrite suggests a possible consequence of microbial sulphate reduction.
Biomarker data in this study offers evidence for basement hosted microbial activity.
Organic Geochemistry (Biomarkers)
This poster describes the use of petrography, organic geochemistry and fluid inclusion techniques on bituminous rocks in fractured basement to investigate;
the potential of basement fractures as a habitat suitable for harboring microbial activities (biomarker analysis).
the timing and extent of biodegradation of bitumen in the subsurface (microscopic analysis).
the interaction between bitumen and mineral precipitation and formation.
the level of heating experienced by rocks in the subsurface during migration (fluid inclusion analysis).
Biomarker ratios for norhopanes/hopanes and steranes/diasteranes were measured to discriminate near surface and sub-surface biodegradation respectively.
Recent seepages are least
biodegraded.
More degraded samples are solid bitumen.
Most degraded samples are mineralised samples from quarries.
Fig. 3. Cross-plot of sterane and hopane maturity parameters. Recent seepages show lower norhopane/hopane ratios than mineralized sample.
Evidence of conversion of hopanes to norhopanes (Fig 4).
This clearly shows microbial involvement during alteration.
Most samples show norhopane formation.
Only Robins Tump sample showed no norhopane formation.
Fig. 1. Outline geological map of region of Shropshire including sample sites. BH, Bayston Hill; D, Downton; E, Emstrey; HC, Hanwood Colliery; HH, Haughmond Hill; NH, Nills Hill, PH, Pitchford Hall, RB, Row Brook; RT, Robin’s Tump; S, Snailbeach; TT, Tar Tunnel; W, Wrentnall.
INTRODUCTION
Fractured basement reservoirs have only recently garnered attention because of their potential to contain heavy oil e.g. West of Shetland, North Africa, Atlantic margin etc.
The buried basement blocks (Fig 1) have been topographic highs when exposed to the surface before burial.
Multiple episodes of uplift provide multiple opportunities for fluid ingress.
Uplift creates a geometry that focusses fluid flow (Fig 1) including hydrocarbons and mineralising fluids towards the basement.
A significant number of near surface petroleum systems can be found in the United Kingdom, which permits investigations into different aspects of petroleum formation and occurrence in the field.
We hypothesize that: subsurface fractured basement reservoirs are a favourable habitat for microbial activity.
Characteristics that would enhance this possibility are as follows:
the basement is typically uplifted to a depth (<2 km) at which microbes can thrive.
the basement is typically fractured, engendering higher permeability than in matrix porosity.
oil-bearing basement provides a food source for microbes.
a basement high is typically a focus for structural reactivation, and accompanying fluid movement and replenishment of nutrients.
Fig 4: Ion chromatograms for m/z 191 showing hopane and norhopane identification.
CONCLUSIONS
Scanning electron microscopy SEM (Petrography)
Backscatter images from SEM studies were used to show evidence of some bitumen migration in the past, as well as reconstruction of a paragenetic sequence established from cross-cutting relationships between bitumen and minerals (Fig 5 and 7).
The observations from the fractured basement in Shropshire add to a growing body of data showing the importance of crystalline basement as a habitat for microbial life, today and in the geological record.
Main points to take away:
Fractured basement reservoir favorable for subsurface habitat.
Cooler and topographically higher basement reservoir more likely support microbial activity.
Elevated temperatures imply thermophiles dominated microbial activity in the subsurface.
Oil reservoirs offer food source for microbial activity in subsurface.
Sulphur in palaeo-reservoirs may be evidence of microbial activity.
This work observers the first instance of both recent and palaeo-degradation in a fractured reservoir.
Fig 5: Scanning electron photomicrographs of bitumen in Longmyndian metasediments. (a) Authigenic quartz (Q) in bitumen (B); (b) barite (Ba) and calcite (Ca) crystals in bitumen (B); (c) chlorite (CL), calcite (Ca), quartz (Q) and barite (Ba) in bitumen (B); (d) pyrite crystals (bright) in bitumen (dark) and mixed potassium feldspar and albite (F); (e) brookite (B) and potassium feldspar (K) in bitumen (dark); (f ) intermixed feldspars (F) and bitumen (B). (a) Bayston Hill; (b–f ) Haughmond Hill.
Paragenetic sequence
Fig. 6. Bitumen occurrence in the field
Fig. 2. Schematic section through potential fractured basement reservoir.
METHODOLOGY
Thermal history (fluid inclusion analysis)
Thermal history was constrained by maturity of organic matter and fluid inclusion data from minerals (Fig 8). .
REFERENCES
CONTACT
Parnell, J., Robinson, N. & Brassell, S Discrimination of bitumen sources in Precambrian and Lower Palaeozoic rocks, southern U.K., by gas chromatography–mass spectrometry. Chemical Geology, 90, 1–14.
Peters, K.E. & Moldowan, J.M The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, NJ.
Parnell, J., Bellis, D., Feldmann, J. & Bata, T Selenium and tellurium in palaeo-oil reservoirs. Journal of Geochemical Exploration, 148, 169–173.
Petford, N. & McCaffrey, K. (eds) Hydrocarbons in Crystalline Rocks. Geological Society, London, Special Publications, 214, /GSL.SP
Wilhelms, A., Larter, S.R., Head, I., Farrimond, P., di - Primio, R. & Zwach, C Biodegradation of oil in uplifted basins prevented by deep-burial sterilization. Nature, 411, 1034–1037.
Trice, R Basement exploration, West of Shetlands: progress in opening a new play on the UKCS. In: Cannon, S.J.C. & Ellis, D. (eds) Hydrocarbon Exploration to Exploitation West of Shetlands. Geological Society, London,
Special Publications, 397, 81–105,
Mas’ud Umar Baba
Department of Geology, University of Aberdeen.
Phone:
Fig. 8. Photomicrograph of two-phase fluid inclusions in calcite, Bayston Hill. Inclusions fluoresce bluish white under UV light.
Fig. 9. Homogenization temperatures for calcite-hosted fluid inclusions, Bayston Hill..
Maturation level for coals confirm carboniferous had entered oil window.
Mineral vein fluid inclusion temperature ranges from C (Fig 9).
Fluid inclusion data were available in vein minerals calcite, quartz and barite cutting the pre-Carboniferous block.
Both oil and aqueous primary fluid inclusions occur in each mineral.
Fig. 7. Paragenesis of bitumen-bearing fractures, based on petrographic study of samples from Bayston Hill and Haughmond Hill quarries.
Pre fracturing phase (rock became tightly cemented and epidotised).
Fracturing phase dominated by quartz-feldspar-barite-bitumen.
Fracturing phase dominated by calcite-chlorite-bitumen
Post-mineralisation fracture phase in which viscous bitumen still seeps today.