TSOP 1999 – Snow Bird Utah Meeting
Characterization of Solid
Insights to Formation
Mechanism, Timing, and Correlation
Wavrek1, David A., Daniel M.
Jarvie2, and Jack D. Burgess2.
1 = Petroleum Systems
International, Salt Lake City, UT
2 = Humble
Geochemical Services, Humble, TX
solid reservoir bitumen is an under-utilized technique, but one in which
organic petrographers and geochemists can make a significant impact to
petroleum companies. This is due to the enormous economic incentives that can
be attributed to the occurrence of these deposits; not only do the solid
reservoir bitumens occlude porosity and restrict permeability to prevent
hydrocarbons from being part of the production stream, these deposits lead to
an over-estimation of hydrocarbon reserves. This over-estimation occurs when
reserve calculations are based on routine log-based techniques that cannot
differentiate movable hydrocarbons from solid hydrocarbons; as a result, the
reservoir zones plugged by immovable solid hydrocarbons are erroneously
recorded as hydrocarbon-bearing reservoir pay zones. Likewise, the presence of
solid reservoir bitumen can explain a number of cases where drilling results
proved hydrocarbon shows, the logging results show hydrocarbon filled porosity
zones, but poor production test results lead to the abandonment of the well.
This paper will demonstrate the use of an integrated optical, isotopic, and
molecular analytical program to reveal insights as to the formation mechanism,
timing of emplacement, and respective petroleum systems assignment of the solid
reservoir bitumen populations that occur in a giant gas / gas-condensate field
in west-central Argentina (Loma La Lata field, Neuquen basin). The principles
outlined in this paper assist in reducing the drilling and evaluation costs on
the field-scale, as well as reducing exploration risk on the basin-scale.
The term solid
reservoir bitumen is preferred (Rogers et al., 1974; Lomando, 1992) since
it is: 1) descriptive without implication to a genetic origin, and 2) avoids
confusion with similar material associated with source rocks and kerogen. Solid
reservoir bitumens are usually formed by one of three processes: thermal
alteration, deasphalting, or biodegradation (Evans et al., 1971; Rogers et al.,
1974; Curiale, 1986); combinations of these processes are possible, as are
contributions from alternative sources such as the by-product of thermochemical
sulfate reduction (Sassen, 1988) and in association with CO2 miscible flooding
of oil reservoirs (Lomando, 1992). Thermal alteration of pre-existing liquid
hydrocarbons (e.g., essentially complete destruction of liquid fraction by
thermal cracking mechanism) to form a solid bitumen and associated gas is
probably the most frequently cited cause of solid hydrocarbons in a reservoir.
During this process, isotopically light methane is cracked off the solid
bitumen which leads to a solid hydrocarbon with a progressively heavy isotopic
composition. Deasphalting of an oil occurs where a large volume of gas
dissolves in a crude oil and causes the asphaltene fraction to be precipitated
in the reservoir interval. The deasphalting process may be driven by
increasing the depth of burial to the point that incipient in-situ crude oil cracking
occurs or it may be that the gas is injected into the reservoir from an
alternative source. Solid reservoir bitumens may also form as a by-product of
severe biodegradation and/or water washing; an extreme example of this process
would be the formation of the Athabasca tar sands in Canada. Each mechanism of
formation will impart molecular, isotopic, and/or optical traits to the
resultant deposit that can be used as clues during the interpretive process.
It is important
to consider the occurrence of solid reservoir bitumen on different scales:
small-scale pore systems, field-scale reservoir systems, and basin-scale
petroleum systems. Information derived from the field-scale study of reservoirs
include the volume of solid hydrocarbon present and how it is distributed
(e.g., trace amounts versus complete plugging; restriction of the occurrence to
isolated horizons or compartments; or the restriction to a particular fracture
orientation, grain boundaries, grain types, and/or pore throats). Likewise,
clues related to the physical setting can be important; examples include the
occurrence of solid reservoir bitumens being present at the top versus bottom
of the reservoir and/or the occurrence being tilted relative to the oil-water
contact (timing clue). These examples provide a few examples of the kinds of
information that needs to be taken into account to provide conclusive evidence
of the formation mechanism (Lomando, 1992).
The results of
the described study indicate that significant insight to solid reservoir
bitumen occurrence can be achieved with an integrated molecular, isotopic, and
optical analytical program. It is particularly important to interpret the
results in context of the geologic framework and petroleum systems synthesis of
the basin. When distinct reflectance populations among the solid reservoir
bitumens are converted into equivalent vitrinite reflectance populations (e.g.,
Jacob and Hiltman, 1985; Landis and Castano, 1995), the results may be
integrated into the regional maturity trends to assess the timing issue; the
results can be further verified by testing the hypothesis within context of
basin modeling exercises. The fluorescent light properties are also useful
with regard to differentiating the populations according to thermal maturity
and timing of emplacement issues. In contrast, analysis by thermal extract-gas
chromatography (TEGC), pyrolysis-gas chromatography (py-GC), and coupled mass
spectrometry methods (TEGC/MS and py-GC/MS) provides insight to the formation
mechanism and petroleum system assignment. Distinguished in this study are two
episodes of emplacement (time) and two mechanisms of formation. The early
episode of emplacement is associated with structural inversion and seal breach
(devolatilization and severe biodegradation as a formation mechanism), and a
less significant (in terms of volume) late episode of bitumen emplacement that
is described as a gas deasphalting event of original in-place oil. The study
also differentiates three different source rocks that contribute to the solid
reservoir bitumens, one of which provides the potential for a gas charge in the
deep portions of the basin which is consistent with other lines of evidence
(Wavrek and Lara, 1999; Wavrek et al., 1999).
pitfalls in the solid bitumen characterization process are recognized and
circumvented during the interpretation process. For example, caution must be
exercised when interpreting results from samples with mixed populations (as
defined by reflectance study) as the molecular signal from the lower reflecting
population will normally obscure that from the higher reflecting population;
this underscores the importance of including the optical aspects into the
molecular interpretation. Likewise, textural traits noted during the optical
analysis can provide additional clues to the formation mechanism (Minamidate et
al., 1995). The use of polished whole rock mounts are advocated in this type
of analysis as the relationship of the solid reservoir bitumen should be
interpreted in context of the rock matrix and pore type. Overall, the results
indicate that substantial progress is possible with existing geochemical tools
which can increase the impact of organic petrology in the petroleum industry.
The most important message to be gleaned from this presentation is the
importance of integrating the optical and molecular characterization techniques
to provide an increased understanding of the solid reservoir bitumen
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Origin of solid bitumens, with emphasis on the biomarker results. Org.
Geochem. 10, 559-580.
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Jacob H. and Hiltmann W.
(1985) Dispersed bitumen solids as an indicator for migration and maturity
within the scope of prospecting for petroleum and natural gas: a model for NW
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Kohlechem, Project 267, Hamburg, 54.
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J.R. (1995) Maturation and bulk chemical properties of a suite of solid
hydrocarbons. Org. Geochem. 22, 137-150.
Lomando A.J. (1992) The
influence of solid reservoir bitumen on reservoir quality. AAPG Bull., 76,
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M., Kajiwara Y. and Machihara T. (1995) Pyrobitumen from hydrous pyrolysis of
oil and bitumen: implication for the interpretation of amorphous carbonaceous
materials. AAPG Abstract with Program, Denver, CO.
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(1974) Significance of reservoir bitumens to thermal-maturation studies, West Canada basin. AAPG Bull., 58, 1806-1824.
Sassen R. (1988)
Geochemical and carbon isotopic studies of crude oil destruction, bitumen
precipitation, and sulfate reduction in the deep Smackover Formaiton. Org.
Geochem. 12, 351-361.
Wavrek D.A., Lara M.E., and Laffitte G.A. (1999) Gas
systems of the Loma La Lata region, Neuquen basin, Argentina. AAPG Hedberg
Conference on Natural Gas Formation, Migration, and Occurrence, Durango,
Wavrek D.A. and Lara M.E. (1999) Risk reduction in
gas exploration: Application of compositional kinetic analysis to the deep
Neuquen basin, Argentina. AAPG Abstract with Program, San Antonio, TX.