Geology Reference
In-Depth Information
apportionment of pollution, as well as in petroleum geochemistry to
indicate the origin and thermodynamic maturity of organic matter in
sediments. A commonly used index to broadly distinguish between ant-
hropogenic and biogenic sources of hydrocarbons, present in any envi-
ronmental compartment, is the Carbon Preference Index (CPI). CPI is
usually applied to the homologous series of semi-volatile n-alkanes with
carbon chain lengths ranging from
B
C
15
to C
35
, where the CPI is simply
the concentration ratio of the summed 'odd-numbered' chain lengths over
the 'even-numbered' chain lengths over a specified chain-length range.
The premise here is that recent biogenically derived n-alkanes (e.g.plant
waxes, biota exudates) tend to favour odd-carbon numbered chain
lengths (typically within the range C
25
to C
35
) whereas aged organic
material, such as petrogenic hydrocarbons that have undergone ageing
over geological timescales generally do not exhibit this odd-carbon
number preference. Therefore, environmental samples with a CPI 41,
suggest that the aliphatic hydrocarbons, have arisen through biogenic
input, whereas a CPI of
o
1, suggests petrogenic (e.g. fossil fuel) sources.
As an example, suspended particle matter (SPM) filtered from seawater
samples collected in the Black Sea, showed high CPI values of 3.81-13.1
in offshore locations, where the SPM was considered to be of algal origin
(where CPI
marine
ΒΌ
SC
15
25
/SC
14
24
).
10
Much lower CPI values were
observed closer to the Danube estuary, and were indicative of petrogenic
derived hydrocarbons, presumably due to river discharges of petroleum
products. There are other useful aliphatic indices or markers that are used
in conjunction with the CPI to provide evidence of oil 'weathering' or to
apportion carbon sources as either petrogenic or of recent biogenic origin.
The ubiquitous and rather persistent branched alkanes, pristane (C
19
)and
phytane (C
20
), are often ratioed to n-heptadecane (C
17
)andn-octadecane
(C
18
) respectively, as supporting evidence of either biogenically derived or
petrogenic hydrocarbons to sediments and aqueous systems (both pristine
and phytane closely elute with these two n-alkanes on a low-polarity GC-
column). For instance, a high n-C
17
/pristane ratio (410), would indicate
biogenically derived hydrocarbons, whereas a low ratio (
o
3) would
indicate a carbon source depleted in n-C
17
, perhaps indicative of pet-
rogenic carbon sources.
PAHs and their alkylated derivatives are also used to distinguish
between pyrolytic and petrogenic inputs to sediments and soils and
are often used in conjunction with aliphatic indices to discriminate
hydrocarbon sources. The lower molecular weight 2- to 4-ringed PAHs
and their alkylated versions comprise a small, but significant, component
of crude oils and liquid fuels. Ratios of parent PAHs (non-alkylated) as
well as parent to alkylated versions are frequently used to distinguish
