SOUVENIR COMMITTEE - Birbal Sahni Institute of Palaeobotany

Training Programme on “PALYNOLOGY IN FOSSIL FUEL EXPLORATION”
[ September 10-17, 2012 ]
PRINCIPLES OF EVALUATION OF PETROLEUM SOURCE ROCK POTENTIALS OF
SEDIMENTARY ROCKS REVISITED
KULDEEP CHANDRA
Ex-Executive Director R&D ONGC Ltd.
Email: kuldeepchandra@rediffmail.com
All fine grained sedimentary rocks containing some organic matter are potential source
rocks of petroleum. The sedimentary rocks contain finely dispersed organic matter. It is present in the form
of sorbed gases, dispersed bitumen extractable by lipid solvents and kerogen which can be isolated from
rocks after dissolution of inorganic matrix by hydrochloric and hydrofluoric acids digestion under an inert
atmosphere to preserve as far as possible its native characteristics. The dispersed organic matters along
with the fine grained sediments are prime candidates to be considered as potential source rocks of
petroleum.
The aim of all methods of source rock potential evaluation is to assess how good or bad is the
potential and whether the potential is dominantly for oil or dominantly for gas.
All sedimented organic matter, depending upon its biogenic building blocks and early digenetic
transformations usually has hydrogen combined with carbon and minor amounts of oxygen and sulphur and
nitrogen and traces of metals. The typing of kerogen would be controlled by its organic matter facies
(petrological and palynological) and related magnitude of atomic H/C and O/C values.
So long there is abundant hydrogen it will source both oil and gas under various processes of
transformation of kerogen whether they are time temperature controlled or thermodynamically controlled
spontaneous processes at various temperatures based on phase equilibria amongst kerogen, water, carbon
dioxide plus methane and natural catalysts. The latter process involves series of hydrolytic oxidation and
reduction reactions resulting in hydrogen deficient and hydrogen richer kerogen and carbon dioxide and
incongruent melting of hydrogen richer kerogen to petroleum. The amount of liquid oil formed and relative
permeability to it would effectively decide dominantly oil vs dominantly gas potential. In case the relative
amount of oil formed is low enough to be not saturating the pore spaces and not expelled because f
unfavourable relative permeability considerations, it will eventually be converted to gas through the
oxidative reductive hydrolytic thermodynamic process into gas, dominantly methane. The temperature
programmed pyrolysis of source rocks under inert atmosphere in an open system mimics natural process of
generation of petroleum and is suitable for evaluation of total and remaining hydrocarbon generation
potential.
A comprehensive potential evaluation thus is best based on various parameters viz. a. Total
concentration of organic matter (TOC), b. Concentration and composition of sorbed gases, c. Concentration
and composition of lipid solvent extractable organic matter (EOM), d. Organic matter facies of dispersed
organic matter employing coal petrographic and palynological methods and temperature controlled
pyrolysis in open system under inert atmosphere; e. stage of maturation, f. clay content and nature of clay
minerals.
The maturation process under kinetically controlled concept measures the extent of thermal
transformation of organic matter and under the thermodynamic concept measures the extent of incipient
hydrolytic oxidation or depletion of hydrogen in kerogen. Both the concepts lead to measuring the effects of
aromatization on the residual kerogen or bitumen in the dispersed organic matter.
The currently used parameters are Tmax of Rock-Eval temperature programmed pyrolysis in open
system, vitrinite reflectance, ratio of tri and tetra ring cycloparffins to tri and tera aromatic ring hydrocarbon
ratios, the phenanthrene to alkylphenanthrene ratios still seem to be worthy of use. More research would be
required to better appreciate the role of epimerization and isomerization processes as a measure of stage of
Birbal Sahni Institute of Palaeobotany, Lucknow
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