Petroleum Biomarkers Indicative of Source Rock Organic Matter
Input and Depositional Conditions
Biomarkers are a group of compounds, primarily hydrocarbons,
found in oils, rock extracts, Recent sediment extracts, and soil
extracts. What distinguishes biomarkers from other compounds in oil
is that biomarkers can reasonably be called "molecular fossils".
Biomarkers are structurally similar to, and are diagenetic
alteration products of, specific natural products (compounds
produced by living organisms). Typically, biomarkers retain all or
most of the original carbon skeleton of the original natural
product, and this structural similarity is what leads to the term
"molecular fossils".
Biomarkers have a variety of applications in petroleum
exploration. For example:
- When samples of oil and candidate source rocks are available,
biomarkers can be used to make oil-source rock correlations,
or
- When samples of candidate source rocks are NOT available, the
biomarker distribution in an oil can be used to infer
characteristics of the source rock that generated the oil WITHOUT
examining the source rock itself. Specifically, biomarkers in an
oil can reveal (1) the relative amount of oil-prone vs. gas-prone
organic matter in the source kerogen, (2) the age of the source
rock, (3) the environment of deposition as marine, lacustrine,
fluvio-deltaic or hypersaline, (4) the lithology of the source rock
(carbonate vs. shale), and (5) the thermal maturity of the source
rock during generation (e.g., Peters and Moldowan, 1993). Such data
may be key inputs to effective basin
modeling of a prospect or block.
Petroleum Biomarkers Indicative of Source Rock Organic Matter
Input and Depositional Conditions (Table 1)
Below are a few examples of oil biomarker parameters that
provide information about the depositional environment of the
source rock and the origin of the organic matter in the source
rock.
| Source Information |
Biomarker Parameter |
Comments |
| Marine Source Rock |
24-n-propylcholestanes |
Ubiquitous in oils derived from
marine source rocks. (Moldowan et al., 1990) |
| C42-46 Cyclopentylalkanes with odd/even
carbon preference |
(Carlson et al. 1993; Hsieh and Philp, 2001) |
| Lacustrine Source Rock |
Botryococcane |
Presence = lacustrine source. Absence
= meaningless. (e.g., Moldowan et al., 1980, Metzger and Laegeau
1999) |
| b-Carotane |
Presence = lacustrine source. Absence = meaningless. (Hall and
Douglas, 1983; Jiang and Fowler, 1986) |
| Sterane/Hopanes |
Low in oils derived from lacustrine source
rocks. (Moldowan et al., 1985) |
| C26/C25 tricyclic terpanes |
> 1 in many lacustrine-shale-sourced oils. (Zumberge,
1987) |
| Tetracyclic Polyprenoids |
High in oils from lacustrine sources. (Holba
et al., 2000) |
| C42-46 Cyclopentylalkanes with even/odd
carbon preference or with no preference |
(Carlson et al. 1993; Hsieh and Philp, 2001) |
| Higher plant input to Source
Rock |
Oleananes, Lupanes, Taraxeranes |
Biomarkers indicating flowering plant
input to source. (e.g., Ekweozor and Udo, 1988) |
| Bicadinanes |
Derived from Dipterocarpaceae tree resins. (Cox et al.,
1986) |
| Retene, Cadalene |
Biomarkers indicating conifer input to
source. (Noble et al., 1985) |
| Tetracyclic diterpanes |
Biomarkers indicating conifer input to source. (Noble et al.,
1985) |
| C29 steranes |
High relative to total
C27-C29 steranes. (Huang and Meinschein,
1979; Moldowan et al., 1985) |
| Coal Source Rock |
Pristane/phytane |
Very high in coal-sourced oils; e.g., > 3.0
(Hughes et al., 1995) |
| C31 homohopanes |
High relative to total
C31-C35 in some coal-sourced oils |
| Hypersaline Depositional
Environment |
Gammacerane |
High relative to C31 hopanes in oils
derived from sources deposited under hypersaline depositional
conditions. High values indicate stratified water column during
source deposition. (Sinninghe Damste et al., 1995) |
| Pristane/phytane |
Very low values (e.g., < 0.5) in oils
derived from source rocks deposited under hypersaline conditions
(due to contribution of phytane from halophilic bacteria). (ten
Haven et al., 1987; 1988) |
| Anoxic Depositional Environment of
Source Rock |
C35 homohopanes |
High relative to total hopanes in oils derived
from source rocks deposited under anoxic conditions (Peters and
Moldowan, 1991). Abundance of C35 homohopanes in oils
(Relative to C31-C34 homohopanes) is
correlated with source rock Hydrogen Index (Dahl et al.,
1994). |
| Pristane/phytane |
1.0 can indicate anoxic conditions, but the
ratio is affected by many other factors. |
| Isorenieratane & related compounds (2,3,6 and 2,3,4 -
Trimethylaryl isoprenoids), Chlorobacteria |
Presence in oil indicates anoxic photic zone during source rock
deposition, since these compounds are biomarkers for green sulfur
bacteria. (Summons and Powell, 1987; Grice et al., 1998; Koopmans
et al., 1996) |
| V/(V+Ni) Porphyrins |
High = reducing conditions. (Lewan,
1984) |
| 28,30-bisnorhopane |
High in certain reducing environments. (Schoell et al., 1992;
Moldowan et al., 1984) |
| Carbonate Source Rock |
30-norhopanes |
High in carbonate-sourced oils; e.g.,
C29/C30 hopanes ~ 1 (Fan Pu et al., 1987; ten
Haven et al., 1988; Subroto et al., 1991) |
| Diasteranes/steranes |
Low in carbonate-sourced oils. (Rubinstein et al., 1975;
Hughes, 1984) |
| Dibenzothiophene/phenanthrene |
> 1.0 in oils derived from high-sulfur
carbonates. (Hughes et al., 1995) |
| 2a-methylhopanes |
High in carbonate derived oils (Summons et al., 1999) |
| Age of Source Rock Deposition |
Oleanane |
Present in oils derived from Late
Cretaceous or younger sources (Moldowan et al., 1994) |
| (24-norcholestanes)/(26-norcholestanes) |
High in many Tertiary sources. Low values are not
age-diagnostic. (Holba et al., 1998A; 1998B) |
| Dinosteranes, triaromatic dinosteroids |
Absence always means Pre-Mesozoic, while
presence USUALLY means Mesozoic or younger. (Moldowan et al.,
1996) |
| C29 Monoaromatic Steroids |
High in oils derived from sources older than 350 mybp.
(Moldowan et al., 1985) |
| C11-C19 Paraffins |
Odd-carbon-number predominance in oil from
many Ordovician sources. (Douglas et al., 1991; Fowler, 1992) |
|
(24-isopropylcholestanes)/(24-n-propylcholestanes) |
High in oils from pre-Ordovician sources. (McCaffrey et al.,
1994B) |
To characterize charge risk, these biomarker parameters can be
used in a variety of innovative ways. For example, specific
biomarker parameters can be calibrated against specific kerogen
quality parameters in a given basin. Then, the biomarker ratios are
measured in an oil sample from the basin, and the values are
projected onto calibration curves to quantitatively predict
characteristics of the source rock. This approach, pioneered by the
founders of OilTracers, allows explorationists to assess whether an
oil was generated primarily from an oil-prone or gas-prone organic
facies (Dahl et al., 1994; McCaffrey et al., 1994). The information
gained from oil biomarkers (source type, age, maturity, kerogen
quality) when integrated into a basin
model has substantial economic impact because it
provides early estimates of oil quantity and GOR for exploration
targets in the area of interest.
Using Biomarkers in Oil to Assess Source Thermal Maturity
The relative abundances of certain biomarkers in petroleum
change as a function of source rock maturity. As a result, a
variety of biomarker parameters have been identified that are very
useful for characterizing the source rock maturity simply from
analysis of the migrated oil (e.g., Peters and Moldowan, 1993).
Biomarker maturity parameters (e.g., parameters such as those in
Table 2) make use of several processes that occur during source
rock maturation:
- Cracking--large molecules break into smaller molecules
- Isomerization--changes in the 3-dimentional arrangements of
atoms in molecules.
- Aromatization--formation of aromatic rings (loss of hydrogen
from naphthenes)
Petroleum Biomarkers Indicative of Source Rock Maturity (Table
2)
| Petroleum Fraction (Compound Class) |
Biomarker Parameter Measured in Petroleum Fraction |
Effect of Increasing Maturity |
Comments |
| Saturated Hydrocarbons |
C29 Steranes
[20S/(20S+20R)] |
Increase |
Useful in early to mid oil window.
Decreases at very high maturity levels. |
| C29 Steranes
[abb/(abb+aaa)] |
Increase |
Useful in early to mid oil window. |
| Moretane/Hopane |
Decrease |
Useful in early oil window. |
| C31 Hopane [22S/(22S+22R)] |
Increase |
Useful in immature rocks to onset of early oil window. |
| Ts/(Ts+Tm) |
Increase |
Also influenced by source lithology. |
| Tricyclic Terpanes/Hopanes |
Increase |
Useful in late oil window; also increases at high levels of
biodegradation. |
| Diasteranes/Steranes |
Increase |
Useful in late oil window; also affected by
source lithology (low in carbonates, high in shales); also
increases at high levels of biodegradation. |
| Aromatic Hydrocarbons |
Monoaromatic Steroids:
(C21+C22)/
[C21+C22+C27+C28+C29] |
Increase |
Useful in early to late oil window; resistant
to effects of biodegradation. |
| Triaromatic Steroids:
(C20+C21)/
[C20+C21+C26+C27+C28] |
Increase |
Useful in early to late oil window; resistant
to effects of biodegradation. |
| Monoaromatic /(Monoaromatic + Triaromatic Steroids) |
Increase |
Useful in early to late oil window; resistant to effects of
biodegradation. |
Several considerations must be kept in mind when using petroleum
biomarkers to assess source rock thermal maturity. For example:
- The exact relationship between a biomarker parameter and the
source maturity is a function of heating rate, source lithofacies,
and source organic facies (kerogen type). As a result, the exact
maturity (i.e., vitrinite reflectance equivalent) associated with a
given value for a biomarker parameter can change from basin to
basin. Furthermore, the relationship between a biomarker maturity
indicator and source rock maturity is generally non-linear.
- With increasing maturity, many biomarker maturity indicators
reach terminal values; hence, a given biomarker parameter is
applicable only over a specific maturity range.
- The concentrations of biomarkers in petroleum decrease with
thermal maturity.
Despite these limitations, biomarker indicators of source
maturity can be extremely useful. For example, biomarker maturity
parameters can be used to determine what the API gravity of a
biodegraded oil was prior to biodegradation. This is
accomplished by collecting a suite of non-degraded oils
from the same petroleum system as the degraded oils. Using the
non-degraded oils, the geochemist develops a correlation or
"transform" between a biomarker maturity parameter and API gravity.
The same biomarker parameter is then measured on a degraded oil,
and the original gravity is determined using the transform
developed from the non-degraded oil suite. Moldowan, et al. (1992)
provide an excellent example of this approach in which they
determine the original gravity of degraded Adriatic oils. For this
application, the most effective biomarker parameters are those
based on compounds that are highly resistant to biodegradation,
such as [Triaromatic/(Monaromatic +Triaromatic steroids)].
Source Rock descriptions and source rock maturity information
derived from oil biomarkers are often key input data for basin
modeling of a prospect or block.
Biomarkers in Petroleum are analyzed by gas chromatography mass
spectrometry (GC-MS) or gas chromatography - tandem mass
spectrometry (GC-MS-MS). Analyses are typically performed on the
saturated hydrocarbon fraction or the aromatic hydrocarbon
fractions. The oil fractions are prepared by liquid
chromatography.
For more information on the biomarker parameters described here,
or to discuss a specific project, e-mail us at info@oiltracers.com,
or call us at U.S. (214) 584-9169.
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