Organic metamorphism in the California petroleum basins; Chapter A, Rock-Eval and vitrinite reflectance

The results of ROCK-EVAL and vitrinite reflectance analyses
of a large sample base from more than 70 wells located in
three oil-rich California petroleum basins are reported. The
cores from these wells have a wide range of present-day burial
temperatures (40
?
to 220
?
C). The rocks in these basins were
deposited under highly variable conditions, sometimes resulting
in substantially different organic matter (OM) types in rocks tens
of meters vertically apart from each other in one well. The
kinetic response of these different OM types to equivalent wellknown
burial histories is a pivotal point of this study.
In the Los Angeles and Ventura Basins, rock organic-richness
significantly increased with depth, as did kerogen hydrogen
content, and the percentage of fine-grained versus coarsegrained
rocks. The shales in these basins are perceived as containing
primarily hydrogen-rich amorphous OM. In actuality,
the shallowest 2,000 to 3,000 m of rocks in the basins, and at
least the upper 6,000 m of rocks in parts of the Los Angeles
Basin central syncline, are dominated by type III/IV OM. In the
Los Angeles Basin, mainstage hydrocarbon (HC) generation
commences in the type III/IV OM at present-day burial temperatures
of 85
?
to 110
?
C, most likely around 100
?
C, and is largely
complete by 220
?
C. In the Southern San Joaquin Valley Basin,
mainstage HC generation commences in type III/IV OM at
150
?
C and is also largely complete by 220
?
C. In the Ventura
Basin, mainstage HC generation commences above 140
?
C in
type III/IV OM. The apparent lower temperatures for commencement
of HC generation in the Los Angeles Basin are
attributed to the fact that parts of the basin were cooled from
maximal burial temperatures by increased meteoric water flows
during the last glaciations.
All aspects of organic metamorphism, including mainstage
HC generation, are strongly suppressed in rocks with hydrogenrich
OM in these basins. For example, ROCK-EVAL data suggest
that mainstage HC generation has not commenced in rocks
with hydrogen-rich OM at present-day temperatures of 198?C.
This observation is attributed to much stronger bonds in hydrogen-
rich OM compared to types III and IV OM and, therefore,
significantly higher burial temperatures are required to break
these bonds. This difference in OM kinetics has profound ramifications
for petroleum-geochemical exploration models.
Organic-matter characteristics inherited from original depositional
conditions were overlaid on, and at times confused
interpretation of, characteristics from organic metamorphism in
all study areas. In all the basins examined in this study, immature
fine-grained rocks occasionally had high to very high
carbon-normalized concentrations of pre-generation indigenous
bitumen. This unusual characteristic may be due to unique depositional
conditions in these basins.