New and Revised Vitrinite Reflectance Data
for the Bowser and Sustut Basins, British Columbia
GSC Open File 4669
C.A. Evenchick1, L.D. Stasiuk2,
K.G. Osadetz2, and F. Ferri3
1Geological Survey of Canada
Natural Resources Canada
101 - 605 Robson Street
Vancouver, BC V6B 5J3
2Geological Survey of Canada
Natural Resources Canada
3303 – 33 Street NW
Calgary, AB T2L 2A7
3Ministry of Energy and Mines
Resource Development and
Geoscience Branch
Government of British
Columbia
6th Fl., 1810 Blanshard
Street
PO Box 9323 Stn Prov Govt
Victoria, BC V8W 9N3
Abstract
This
CD contains over 500 analyses of thermal maturity in the form of vitrinite
reflectance and vitrinite reflectance calculated from bitumen, for large parts of the Bowser
and Sustut basins (north-central British Columbia).
Strata are Middle
Jurassic to Late Cretaceous age. The
data are expressed in HTML, Excel, and comma delimited formats. The eastern
limit of data coverage is 127°W, the western limit is 130°W, the northern limit
is 58°N, and the southern limit is 54°S.
Introduction
This report presents new and revised
vitrinite reflectance and vitrinite reflectance calculated from bitumen as an
interim data release in a project to evaluate the level of thermal maturity in
the Bowser and Sustut basins (see also Evenchick et al., 2002). The 286
new analyses are from samples from the Ritchie wells, parts of the northern
two-thirds of the basins with sparse data coverage, a vertical profile in one
region previously recognized as low maturity, and the southern third of the
basin. These are the first data
reported for the southern basin. The
226 revised analyses are from samples previously reported (Evenchick et al.,
2002). Interpretation and integration
with previously reported data will be provided in future reports.
Since the rate of reaction and per cent
reflectance in oil (%Ro) of huminite and vitrinite (hereafter vitrinite is
used) increases with increasing temperature, these data provide an indirect
geo-thermometer for evaluating thermal conditions in a sedimentary basin.
Multiple reaction first order Arrenhius-based kinetics control the reaction
rate in vitrinites, and thus the approximate exponential increase in huminite
reflectance with increasing temperature. Because of this relationship vitrinite
reflectance has been widely used to calibrate thermal history and
time-temperature models for stratigraphic successions within sedimentary
basins, particularly for evaluating the stage of thermal maturation with respect
to oil and gas generation and preservation (Fig. 1). Reflectance studies of
vitrinite have been less commonly used to evaluate the thermal history of high
temperature plutonic and volcanic events in terms of absolute temperature,
although many have reported on the very rapid increase in vitrinite reflectance
within country rock shale beds and coal seams with increasing proximity to
country rock-intrusion contact.
Several
thermal maturity based studies have been published on the correlation between
solid bitumen reflectance, vitrinite reflectance, and Rock Eval Tmax values
(e.g. Riediger, 1993; Landis and Castano, 1995; Potter,
1999) in petroleum-enriched sedimentary basins. These correlations have
been made using either the reflectance of indigenous (Potter, 1999), migra-
(Jacob, 1989) or primary (this study) bitumen. Optical indices of indigenous or
primary solid bitumens provide reliable data for evaluating thermal maturity
when translated in vitrinite reflectance equivalent values.
Hydrocarbon
fluid inclusions (including crude oils) are present in many of the samples
examined from the Bowser and Sustut basins. The majority of data related to
these inclusions will be forthcoming in the next phase of the project. Here we
present some findings from samples from the Ritchie wells drilled in the
central Bowser Basin. Fluorescence properties (e.g. color, emission maxima) of
hydrocarbon fluid inclusions and crude oils are a signature of the gross
chemical composition (i.e. aromatic and saturate hydrocarbons; NSO-bearing
asphaltenes and resins) as well as the density (°API)
of the included oil and thus are useful for assessing oil quality/thermal
maturity, and for exploratory purposes for discerning petroleum migration
pathways (see Stasiuk and Snowdon, 1997). In general, the lightest and most
thermally mature oils have shortest wavelength ‘blue region’ fluorescence, and
the heaviest, least mature oils have longer wavelength ‘red region’
fluorescence. As a result of these relationships, fluorescence studies of oil
inclusions and crude oils can provide valuable information regarding regional
distribution of oil types, quality, and potentially their migration within a
petroleum system.
Methods
Reflectance
Whole
rock outcrop samples were prepared for organic petrographic analysis by
mounting in epoxy, then grinding with carborundum and diamond grit and
polishing on cloth and silk using an alumina-water slurry. Samples were
analyzed using both incident white light and fluorescent light microscopy. Per
cent reflectance in oil (%Ro) analyses were performed using Zeiss and Leitz MPM
II reflected light microscopes equipped with both white and ultra-violet light
sources. Glass (1.025, 1.82 %Ro) and diamond (5.26 %Ro) standards were used to
calibrate the photometer system. Thermal maturation of the sedimentary rocks
and the contained organic matter has been assessed using per cent random
reflectance in oil (%RoR) of wood-derived vitrinite (Organic Type 2,
Table 1) and of primary solid bitumen and primary pyrobitumen (Organic Type 4,
Table 1) macerals. Vitrinite reflectance equivalent (Organic Type 3, Table 1)
was calculated from primary bitumen and pyrobitumen (Organic Type 4) using the
relationship of Jacob, 1989, where %RoV = 0.618 x %Robitumen + 0.40 values.
Evenchick et al. (2002) previously expressed the thermal maturity of
surface samples from Bowser and Sustut basins using primarily %Romax of
vitrinite and calculated %Romax (from bitumen), of which some data was
converted from %RoR measurements. Here, however, vitrinite %Ro and
vitrinite calculated %Ro are expressed as %RoR since this is the
conventional standard for expressing thermal maturity in petroleum exploration
and related burial-thermal history studies.
Two
other types of bitumen and pyrobitumen (Organic Type 60 and 21) were also
measured, which are interpreted as representing solid carbon residues derived
from migrated petroleum and thus are not indicative of thermal maturity. At
locations where there is a pronounced difference between vitrinite or calculated
vitrinite and the reflectance or thermal maturity of the migrated bitumens,
this likely indicates migration of petroleum after maximum burial of the host
strata, during an uplift phase.
Table 1. Key for Organic Types |
|
CODE |
ORGANIC
TYPE |
02 |
VITRINITE |
03 |
VITRINITE
EQUIVALENT (04) |
04 |
PRIMARY
SOLID BITUMEN - PYROBITUMEN |
15 |
HYDROCARBON
FLUID INCLUSION A |
16 |
HYDROCARBON
FLUID INCLUSION B |
17 |
HYDROCARBON
FLUID INCLUSION C |
21 |
PYROBITUMEN(PB)_ISOTROPIC |
60 |
MIGRABITUMEN
ISOTROPIC |
Fluorescence
Visible light region fluorescence of
hydrocarbon fluid inclusions was conducted with a Zeiss Axioplan II microscope
using water immersion objectives (total magnifications up to X2500) and
ultra-violet, blue and green light excitation. Fluorescence microspectrometry
(between 400 and 700 nm) was conducted with a Zeiss UMSP incident light
microscope (03 photomultiplier, a continuous filter monochromator b with 1/2 band width of 14 nm at 540 nm) and
Zeiss UMP pc-base controller system. An epiplan-neofluor water immersion 40x objective
(total magnification x640), HBO 100 ultraviolet source, and an ultraviolet G
365 nm excitation filter (395 nm beam splitter; 420 nm barrier filter) were
used during analysis. Background fluorescence was subtracted and the spectra
were normalized to the wavelength of maximum energy emission. Photochemical alteration of the
“well-sealed” hcfi did not occur during spectral scanning. Two spectral
parameters have been used to characterize the fluorescence properties: (i)
Lambda max (Lmax = wavelength of maximum emission intensity in nm) and;
(ii) R/G quotient (Q = Intensity650nm/Intensity500nm).
Acknowledgements
This project was
funded by the Natural Resources Canada, Earth Science Sector, Geological Survey
of Canada, Northern Resource Development Program. Maria Tomica of Ramtrec
Research Projects and Dr. Judith Potter of JP Petrographics, Calgary, assisted
in petrographic analyses. Kim Dunn and Ryan Diamond of GSC Calgary are thanked
for providing excellent technical assistance.
References
Evenchick, C.A., Hayes, M.C., Buddell, K.A., and Osadetz, K.G.
2002 Vitrinite and bitumen reflectance data and preliminary organic maturity model for the northern two thirds of the Bowser and Sustut basins, north-central British Columbia. Geological Survey of Canada Open File 4343.
Potter, J.
1999: A genetic approach to the characterization of microbitumens; implications for thermal maturity determinations. Sixteenth Annual Meeting of the Society for Organic Petrology, Abstracts and Program 16, p. 11-13.
Jacob, H.
1989: Classification, structure, genesis and practical importance of natural solid bitumen (“migrabitumen”). International Journal Coal Geology, v. 11, p. 65-69.
Landis, C.R. and Castano, J.R.
1995: Maturation and bulk chemical properties of a suite of solid hydrocarbons. Organic Geochemistry, v. 22, p. 137-150.
Riediger, C. L.
1993: Solid bitumen reflectance and Rock-Eval Tmax as maturation indices: an example from the “Nordegg Member”, western Canada sedimentary basin. International Journal Coal Geology, v. 22, p. 295-315.
Stasiuk, L.D. and Snowdon, L.R.
1997: Fluorescence microscopy of synthetic and natural hydrocarbon fluid inclusions: crude oil chemistry, density and application to petroleum migration. Applied Organic Geochemistry, v.12, p. 229-241.