SOUVENIR COMMITTEE - Birbal Sahni Institute of Palaeobotany

Training Programme on “PALYNOLOGY IN FOSSIL FUEL EXPLORATION”
[ September 10-17, 2012 ]
GEOCHEMICAL CHARACTERIZATION OF HYDROCARBON CRUDES: ITS APPLICATION
IN EXPLORATION
UTTAM KUMAR BHUI
School of Petroleum Technology, Pandit Deendayal Petroleum University, Raisan, Gandhinagar,
E-mail: uttam.bhui@spt.pdpu.ac.in ; ukbhui@rediffmail.com
We have good understanding of what oil and gas are made up of and how they occur but the
precise processes and timing of oil generation, migration processes and pathways, their entrapment and
accumulations for commercial deposits, all are important pre-requisite for understanding the sequential
evolution of all the viable petroleum system of a basin. Thus we know how to find oil and gas but the
precise details of petroleum generation, migration and their trapping mechanism are still debatable even for
most of the producing basins and reservoirs.
The Organic Matter (OM or kerogen) in the sediments transform or cook step by step during burial
with increase in temperature of which petroleum is by-product and some stay back in the source
sedimentary rock as residual organic matter. Residual kerogen content in source rocks is insignificant
(between fraction of 1% to few %) but its significance is much much greater than its content because it
enables one to provide clues and to solve geochemical problems related to oil and gas genesis, oil-source
rock correlation, migration pathways and many more. Crude oil, which is a byproduct of kerogen cooking,
is a multicomponent mixture of hydrocarbons (saturated hydrocarbons: alkanes, cycloalkanes; aromatics:
mono, di and polyaromatics) and non-hydrocarbon compounds (resins, asphaltenes, metal-porphyrin
complexes and trace elements) and depends on the organic matter type i.e kerogen type(type I, II, III),
which is based on the elemental carbon, hydrogen and oxygen in the kerogen. Our knowledge about the
geochemical significance of the molecular composition of hydrocarbon fluids and rock extracts has been
growing exponentially and recent advances in geochemistry, especially in analytical techniques, have
resulted in rapid progress on this front.
Shales, generally considered as the source rocks of many crude oils, are highly enriched in trace
elements, PGE and Re, Os radioisotopes. Trace element abundances in crude oil, particularly Ni, Fe, V, Mo
are often present at ppm level and many researchers have identified their existence as part of porphyrin
structure. But no study had been undertaken yet towards the understanding of element fractionation during
oil generation. Sufficient amount of these radioisotopes are present in hydrocarbon to use the 187 Re- 187 Os
radioisotope systematic for constraining the source as well as the timing of separation from the source of
the migrated oil.
The knowledge regarding the geochemical fingerprinting and their significance of the molecular
composition of the crude oils and rock-extracts has been growing considerably during the last two decades.
Some compounds in oil lost structural features of parent organic matter, whereas some other molecules
preserved these features. They are called “chemical fossils”, “molecular fossils” or “bio-markers”. The
basic architecture of the biomarkers allows them to be undoubtedly related to parent compounds of the
biosphere. Among the numerous molecular structures present in oils, rock extracts, biological markers from
alkane and aromatic fractions have been playing a leading role in the present day analytical techniques and
accordingly, a significant part of complementary data is ignored or unavailable. The most significant points
to keep in mind are to consider the whole molecular fingerprint and to better assess the total biomarker
potential in the field of hydrocarbon exploration. Biomarkers have a variety of applications in petroleum
exploration: Source rock typing, oil-to oil and oil-to-source rock correlation, paleoenvironment of source
rock deposition, maturities of oil and source rocks.
Birbal Sahni Institute of Palaeobotany, Lucknow
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