Page 104 - The Petroleum System From Source to Trap

98 Peters and Cassa

sition and thermal maturity from microscopy can be
N = 100
Desmocollinite o/oR0= 0.61 (Telocollinitej used to estimate the atomic H/C ratio of a kerogen
(Figure 5.2). If the measured atomic H/C differs by more
than 0.1 from the estimated value, both analyses are
Fusinite
suspect and are repeated. These maturity and atomic
H/C results are commonly supported by T max and HI
I
I I I I I I I I I
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 data obtained from each whole rock sample using Rock­
% REFLECTANCE Eva! pyrolysis and TOC.
Coal
Figure 5.3. A complete reflectogram showing the
reflectance of all macerals in a kerogen sample. In cases Coal is a rock containing more than 50% organic
where selection of the '1rue" vitrinite population (telocolli­ matter by weight. Both coals and sedimentary rocks can
nite) is difficult, the trend of Ro versus depth established by contain any combination of macerals. Various classifica­
many samples is useful for selecting the correct popula­ tions of these organic-rich rocks are found in the litera­
tion. Here, telecollinite (hatchured) has a mean % Ro of ture (e.g., Cook and Sherwood, 1991). Not all coals are
0.61. This sample contains significant amounts of oxidized
vitrinite and semi-fusinite that could be mistaken for composed of humic organic matter (higher plant, type III
vitrinite. (Courtesy of S. C. Teerman.) kerogen). Humic and sapropelic coals contain less than 10%
and more than 10% liptinite, respectively. Humic coal
has long been recognized as a source for gas, primarily
Petrography alone is too imprecise to evaluate the methane and carbon dioxide. However, boghead and
petroleum potential of a source rock, prim rily because cannel coals are dominated by type I and II kerogens,
�
. .
hydrogen-rich and hydrogen-poor kerogen IS difficult to respectively, are oil prone, and thus show high oil
distinguish. "Amorphous" kerogen is commonly potential.
presumed to be hydrogen rich and oil prone, but not all Coals can generate oil, as exemplified by major accu­
amorphous kerogens can generate oil. Ultraviolet­ mulations in Indonesia and Australia. Two principal
induced fluorescence microscopy of samples of low limitations for coals as effective source rocks are (1)
thermal maturity distinguishes hydrogen-rich, oil-prone expulsion efficiency and (2) organic matter type (suffi­
amorphous (fluorescent) from hydrogen-poor, non­ cient hydrogen). Because of the physical properties of
generative amorphous (nonfluorescent) kerogen, thick coal seams, generated liquid products are usually
suggesting that petrographic methods might be further adsorbed and generally escape only when cracked to gas
refined to better predict generative potential (Senftle et and condensate (Snowdon, 1991; Teerman and Hwang,
al., 1987).
9
19 1 ) . Coals that can generate and release oil must
contain at least 15-20% by volume of liptinite macerals
Organic Facies
prior to catagenesis, corresponding to an HI of at least
Various workers have used the term organic f a cies as a 200 mg HC/g TOC and an atomic H/C ratio of 0.9
synonym for kerogen facies (based on chemical data) or (Hunt, 1991).
palynofacies or maceral assemblage facies (based n
?
petrographic data). Jones (1984, 1987) propose a conose Kerogen and Bitumen Composition
definition:
Detailed structural information on kerogen is limited
An organic f a cies is a mappable subdivision of a designated because of its heterogeneous composition and difficulties
stratigraphic unit, distinguished from the adjacent subdivisions
on the basis of the character of ils organic constituenls, without associated with the chemical analysis of solid organic
regard to the inorganic aspects of the sediment. matter. Kerogen has been described as a geopolymer,
which has been "polymerized" from a random mixture
Jones (1984, 1987) has defined organic facies using a of monomers. These monomers are derived from the
combination of three types of kerogen analyses: atomic diagenetic decomposition of biopolymers, including
H/C ratios, Rock-Eval pyrolysis and TOC, and trans­ proteins and polysaccharides (e.g., Tissot and Welte,
mitted-reflected light microscopy. He showed that all 1984). This view has led to many publications showing
organic facies can exist in either carbonates or shales and generalized chemical structures for kerogen, none of
that there is little evidence that TOC requirements are which are particularly informative.
lower for carbonate than for shale source rocks. Integra­ The discovery of insoluble biopolymers in living
tion of organic facies studies with the concepts of organisms, sediments, and sedimentary rocks has led to
sequence stratigraphy is a step toward improving our a reappraisal of the structure of kerogen (Rullkotter and
ability to predict the occurrence of a source rock (e.g., Michaelis, 1990). In the modified scheme, more emphasis
Pasley et al., 1991). is placed on selective preservation of biopolymers and
When used together, elemental analysis, Rock-Eval less on reconstitution of monomers. Progress has been
pyrolysis and TOC, and organic petrography are achieved by the application of specific chemical degrada­
powerful tools for describing the richness, type, and tion (Mycke et al., 1987), pyrolysis (Larter and Senftle,
thermal maturity of organic matter. Jones and Edison 1985), and spectroscopic techniques (Mann et al., 1991).
(1978) and Jones (1984) have shown how maceral campo-
Structural elucidation techniques are beyond the scope of