Figure 1. Comparison of transformation rates for 5 different kerogens.
(Also shown in black is the computed vitrinite reflectance)
Open System Kinetic Calculations
Introduction
Accurate description of the timing of decomposition of organic
matter (Kerogen) into oil and gas under geological conditions is
the goal for the highest quality basin modeling effort. When
evaluating a play or prospect using basin modeling, the conversion
of kerogen into gas and petroleum is dependent on the composition
of the kerogen. The composition of the kerogen and possibly the
surrounding rock matrix determines the thermal energy required to
cause cracking of bonds releasing gas and petroleum from the source
rock.
In the past it was common to model this decomposition using simple chemical rules-of-thumb which use the doubling of reaction (decomposition) rates for every 10oC increase in temperature. This was combined with vitrinite reflectance data and an empirical model for the decomposition of kerogen was devised (Lopatin, 1978; Waples, 1980). Numerous problems arise from this type of model. First, at geological temperatures of hydrocarbon formation, reaction rates do not behave according to this generalized chemical rule. This empirical rule evolved in the laboratory when estimating reaction times for simple chemical reactions. Second, the relationship of vitrinite reflectance to hydrocarbon generation varies depending on the composition of the source rock and its inherent kinetic decomposition rate. At a vitrinite reflectance of 0.60% given three petroleum source rocks of different chemical compositions, the generation of hydrocarbons could be 1%, 5%, or 20% of the total generation capability. Bear in mind, vitrinite reflectance is a good maturity parameter but it is not a direct measure of hydrocarbon generation.
Bulk Kinetic Determinations
Figure 1 shows the decomposition rates of five different petroleum
source rocks determined by bulk kinetic analysis. Three of the
samples are classical Type II, oil prone source rocks. However, they
have variable elemental compositions primarily in the amounts of
oxygen, sulfur, and nitrogen. Of the other two samples one is a
Type I (lacustrine) oil prone kerogen and the other a Type III gas
prone (coaly) kerogen. Note that at a given vitrinite reflectance
value each kerogen is at a different level of transformation, i.e.,
the conversion of kerogen to gas and petroleum has proceeded at
different rates. All begin to generate significant amounts of
hydrocarbons at different temperatures and generation proceeds at
different rates (note the onset temperatures and slopes of the
transformation rate curves in Figure 1). Likewise, there are
significant differences in the temperatures of peak generation.
In the laboratory we can now determine directly the bulk kinetic parameters using a technique known as open system nonisothermal pyrolysis. This technique is identical to the Rock-Eval pyrolysis process except the exact temperature inside the sample is directly measured and there is only a minute temperature gradient across the sample. The standard Rock-Eval apparatus has a 30-40° C gradient across the sample itself. The pyrolysis curves from experiments at multiple heating rates and measured temperature results are combined in a file which is processed using the Lawrence Livermore Kinetics© program. This program is capable of computing kinetic parameters using a variety of kinetic models. These include approximate, rigorous Gaussian and Discrete calculations, and a 3 Parameter Narrow Profile calculation for kerogens having a uniform composition that decompose over a narrow temperature range. All of the models calculate bulk kinetic parameters measuring the total decomposition rate without distinguishing gas from oil.
Figure 1. Comparison of transformation
rates for 5 different kerogens.
(Also shown in black is the computed vitrinite reflectance)
Compositional Kinetic Determinations
While bulk kinetic parameters describe the decomposition of organic
matter into volatile hydrocarbons, they do not provide any
information on when dry gas, wet gas, light oil, and normal crude
oil are formed. A technique to derive compositional kinetic parameters
was devised by and Stauffer (1994). This technique employs open system nonisothermal
pyrolysis but instead of just detecting the bulk decomposition
profile, this technique slices the pyrolysis peak into up to ten
fractions. These fractions are sequentially trapped during the
pyrolysis process and subsequently desorbed into a gas chromatography
where the products are separated and identified. Using this technique
it is possible to derive kinetic parameters for the rate of
formation of:
dry gas
wet gas
light oil
normal oil
These data can be directly utilized in current basin modeling programs such as BasinMod™ or Genex™. This permits accurate modeling by product class and maturity. For example, the amount of methane generated at 0.80% vitrinite reflectance or at a temperature of 130° C could be modeled from these results.
Determination of Hydrocarbon Yields
from Source Rocks
Introduction
The determination of either bulk kinetic parameters or compositional
kinetic parameters does not give an accurate measurement of the
amounts of gas and oil formed only its rate of formation. However,
closed system techniques such as hydrous pyrolysis and
MicroScale Sealed Vessel (MSSV) provide
information on product yields.
Hydrous Pyrolysis
In hydrous pyrolysis experiments a large aliquot of source rock
is heated with water isothermally to temperatures typically ranging
from 280° C to 370° C typically for 1-3 days. The organic
matter generates gas, a natural-like crude oil and bitumen under
these conditions. The amount of gas, oil, and bitumen generated can
be measured. In addition the cooked rock can be analyzed to determine
its present-state generation potential which can be used to calculate
the extent of transformation and the level of thermal maturity. This
can be compared to what has been predicted from kinetic experiments
on the original, extracted rock sample.
MicroScale Sealed Vessel
(MSSV) Pyrolysis
In the MSSV technique a very small aliquot of
extracted rock sample or kerogen is utilized, typically less than 10
milligrams of extracted rock sample or 1-2 milligrams of isolated
kerogen. The sample is loaded into a Pyrex or quartz tube which is
subsequently sealed under inert conditions. No water is added to the
sample tube. The tubes are heated to temperatures and times comparable
to those used for hydrous pyrolysis. The products formed resemble a
natural gas and crude oil sample similar to those seen in hydrous
pyrolysis experiments. However, the tube is broken directly into a
special gas chromatographic injector which permits detection of all
gases, hydrocarbons, and nonhydrocarbons. A patented internal standard
technique can be used to quantitate all gas and oil yields. In
addition tube's spent rock residues may be analyzed separately to
determine the extent of conversion and level of thermal maturity.
These data can be used to compute compositional kinetic results separately from the open system pyrolysis technique described above.
Optimization of Kinetic Data using Field
Data
Kinetic data must be extrapolated from laboratory conditions using
high temperatures and fast heating rates to the natural system where
much lower temperatures and heating rates occur. Since the
extrapolation is many orders of magnitude, there are inherent
limitations to these projections. To test these extrapolations kinetic
results should be compared to actual field data when possible.
These data may consist strictly of a database of geochemical data on
a field or basin to more detailed production information. For example,
immature Barnett shale formation kinetics and artificial maturation
data were compared to the results from a naturally matured series of
Barnett shale. The laboratory results, i.e., the kinetic modeling of
transformation, matched the natural series very accurately
( and , 1991). A
similar comparison in the Santa Maria basin showed that open system
kinetic results modeled the transformation of the Monterey shale
( and , 1994) much
more reasonably than reported hydrous pyrolysis kinetics (Hunt et al,
1992). In addition, the open system kinetics were able to accurately
predict the level of transformation in the hydrous pyrolysis
experiments. These cases illustrate that open system kinetic data
can be used to accurately model hydrocarbon generation in the natural
system as well as closed system experiments in the laboratory.
However, closed system experiments are necessary to measure the
yields of different petroleum products.
Secondary Cracking of Oil into Lighter
Hydrocarbons
In basin modeling schemes oil is generated and may be allowed to stay
resident at a given temperature or move to either a hotter or cooler
reservoir location. If oil stays in or nearby the source rock or moves
to a zone with higher temperatures, oil will begin to decompose into
lighter hydrocarbons. This is referred to as secondary cracking.
Experimental techniques for investigating oil-to-gas cracking include
heating oil samples in closed system reactors. The technique most
commonly used is the MicroScale sealed vessel system where a very
small aliquot of oil is heated and converted to lighter hydrocarbons.
It has been shown that petroleum source rocks decompose at different
rates depending on composition and possibly the rock matrix.
Similarly, oils have variable composition in hydrocarbons,
nonhydrocarbon (resins and asphaltene fractions), and elemental
compositions especially sulfur content. These differences as well
as reservoir rock composition could affect the thermal stability of
an oil. Determination of the rate of decomposition of oil permits
kinetic results to be used in basin modeling programs to model
the oil floor.
Oil decomposition can be reported as a single activation energy and Arrhenius factor or as a distribution of activation energies and a single Arrhenius factor.
Kinetic Services Available from
Humble Geochemical Services
Humble Geochemical Services Division provides all the techniques
necessary to accurately evaluate hydrocarbon generation and yields.
We offer the following services for kinetic calculations and yields
measurements:
Bulk oil generation kinetics providing
Compositional kinetics to describe the formation of hydrocarbon compound classes with activation energy(ies) and Arrhenius constants for:
Product yields from artificially matured samples using hydrous
pyrolysis and microscale sealed vessels
Kerogen transformation profiles using calculated and measured
conversion
Oil-to-gas cracking kinetics
References
, , John G. Reynolds, and J. E. Clarkson
Pyrolysis kinetics for lacustrine and marine source rocks by
programmed micropyrolysis, Energy & Fuels 5, 192-204, 1991.
, , T. T. Coburn, E. I. Sandvik,
, B. J. Schmidt, and R. A. Noble,
An appropriate model for well-preserved algal kerogens,
Energy & Fuels 10, 49-59, 1996.
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Services, Inc.. All rights reserved.
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