Organic metamorphism in the California petroleum basins; Chapter B, Insights from extractable bitumen and saturated hydrocarbons

Seventy-five shales from the Los Angeles, Ventura, and
Southern San Joaquin Valley Basins were extracted and analyzed.
Samples were chosen on the basis of ROCK-EVAL analyses
of a much larger sample base. The samples ranged in burial
temperatures from 40
?
to 220
?
C, and contained hydrogen-poor
to hydrogen-rich organic matter (OM), based on OM visual typing
and a correlation of elemental kerogen hydrogen to carbon
ratios with ROCK-EVAL hydrogen indices.
By extractable bitumen measurements, rocks with hydrogen-
poor OM in the Los Angeles Basin began mainstage hydrocarbon
(HC) generation by 90
?
C. The HC concentrations
maximized by 165
?
C, and beyond 165
?
C, HC and bitumen concentrations
and ROCK-EVAL hydrogen indices all began
decreasing to low values reached by 220
?
C, where HC generation
was largely complete. Rocks with hydrogen-poor OM in
the Southern San Joaquin Valley Basin commenced mainstage
HC generation at 135
?
C and HC concentrations maximized by
180
?
C. Above 180
?
C, HC and bitumen concentrations and
ROCK-EVAL hydrogen indices all decreased to low values
reached by 214
?
C, again the process of HC generation being
largely complete. In both cases, bell-shaped HC-generation
curves were present versus depth (burial temperature). Mainstage
HC generation had not yet begun in Ventura Basin rocks
with hydrogen-poor OM by 140
?
C. The apparent lower temperature
for initiation of mainstage generation in the Los Angeles
Basin is attributed to very recent cooling in that basin from
meteoric-water flow. Thus, HC generation there most probably
occurred at higher burial temperatures.
In contrast, mainstage HC generation, and all aspects of
organic metamorphism, were strongly suppressed in rocks with
hydrogen-rich OM at temperatures as high as 198
?
C. For example,
shales from the Wilmington field (Los Angeles Basin) from
180
?
to 198
?
C retained ROCK-EVAL hydrogen indices of 550-
700 and had saturated-HC coefficients of only 4-15 mg/g
organic carbon. The rocks with hydrogen-rich OM were subjected
to the same burial conditions as the rocks with hydrogenpoor
OM. We attribute this suppression of organic metamorphism
in this study primarily to much stronger bonds in the
hydrogen-rich OM compared to the bonds in hydrogen-poor
OM. Trends in bitumen compositions (qualitative characteristics)
versus burial temperature were also very different for rocks
with hydrogen-poor OM compared to that in rocks with hydrogen-
rich OM. This observation demonstrated that the two OM
types also had significantly different reaction pathways, in addition
to different reaction kinetics. Strong exploration implications
arise from these observations.
Above 40?C, but before mainstage HC generation, a lowtemperature
(pre-mainstage) HC generation occurred in all
rocks, and all OM types, studied. This low-temperature generation
resulted in significant qualitative changes in the bitumen
and HCS (hydrocarbons) from rocks of all OM types, especially
in rocks with hydrogen-rich OM, from 40
?
to 70
?
C. This, and
previous studies, document that very high carbon-normalized
concentrations of indigenous bitumen and HCS occur in late
Neogene immature rocks of any OM type in all southern California
basins. This characteristic is attributed to the low-temperature
generation occurring in both sulfur-poor and sulfur-rich
kerogens, which originally had unusually high concentrations of
weak (15-40 Kcal/mole) bonds. These observations and considerations
have marked relevance to exploration regarding the possible
formation of commercial oil deposits at immature ranks in
these basins. Other significant geochemical observations also
result from this study.