TSOP 1999 – Snow Bird Utah Meeting

Characterization of Solid Reservoir Bitumen:

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

           Characterization of 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).

           Potential 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 occurrences.

REFERENCES:

Curiale J.A. (1986) Origin of solid bitumens, with emphasis on the biomarker results. Org. Geochem. 10, 559-580.

Evans C.R., Rogers M.A. and Bailey N.J.L. (1971) Evolution and alteration of petroleum in western Canada. Chem. Geol. 8, 147-170.

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 Germany. Final report, Deutsche Gesellschaft Fur Mineralolwissenschaft und Kohlechem, Project 267, Hamburg, 54.

Landis C.R. and Castaño 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, 1137-1152.

Minamidate T. Takeda N., Tsuzuki N., Suzuki 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.

Rogers M.A., McAlary J.D. and Bailey N.J.L. (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, CO.

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.