Oil–source rock correlations – Limitations and recommendations
Introduction
Correlation of a crude oil to one or more source rocks is a common industrial application of petroleum geochemistry. Confirmation that oil has been generated in the target sedimentary basin is the most critical piece of knowledge a petroleum explorationist can derive; second in importance is the determination of the source(s) of that oil. For this reason, an extensive arsenal of analytical methods is utilized to collect primary data on the organic matter in crude oils and possible source rocks, and various components of these data are used to relate oils causally to their prospective sources. Oil–source rock correlations at various confidence levels have been established for the petroleum systems of all major sedimentary basins.
Published case studies establishing causal correlations between oil and source rock pairs began to appear in the third quarter of the 20th century (e.g., Hunt et al., 1954, Welte, 1965, Dow, 1974, Williams, 1974) and summaries of oil–source rock correlation studies and methods are now available (Curiale, 1993, Curiale, 1994a, Waples and Curiale, 1999). Although correlations are fundamentally subjective exercises, the results are rarely challenged either conceptually or on their merits. Numerous published examples correlating one or two oil samples to one or two source rocks purport to establish relationships which are commonly carried basinwide. Other examples utilize a decidedly limited suite of geochemical analytical types and data, often establishing strong chemical correlations but no convincing causality. Still others attempt to enforce a chemical correlation on sample sets whose geologic circumstances make such a correlation quite unlikely.
Concerns of this type have been expressed since oil–source correlations were first attempted, although often only as passing remarks in published literature and industry reports. Interestingly, although these concerns have been duly noted by various authors, most of these authors proceeded with their correlation efforts anyway, without any serious effort to address the noted concerns. This has led to unnecessary uncertainty in published results, and the impression that any correlation is better than no correlation. My objectives here are to gather into a single publication the reasons for the subjectivity of oil–source correlations and to recommend conceptual and practical improvements for the use of these correlations in solving industrial problems. In addition, I present some criteria for judging the success of practical oil–source rock correlations, in the hope that these criteria will assist in defining the risk associated with the charge component of exploration efforts in both frontier and mature basins.
Any successful oil–source rock correlation must include three attributes: (a) requirement of causality; (b) comparable chemical data for all samples; and (c) geological support. For the purposes of this paper, I will use the definition of Curiale (1993), derived from previous authors and cited in that publication as: an oil–source rock correlation is a causal relationship, established between a crude oil and an oil-prone petroleum source rock, which is consistent with all known chemical, geochemical and geological information. The three key points of this definition are listed below.
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The relationship must be causal. That is, the oil must arise (at least in part) from the specified source rock.
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Chemical data used in the correlation must be comparable. That is, the elemental, molecular and isotopic data derived from the source rock must be of the same type as that derived from the oil.
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All available geological data must be supportive. That is, clear geological evidence must exist which allows the proposed source rock to have sourced the oil.
The absence of any of these three key points necessarily negates the validity of a proposed oil–source rock correlation. That is: the presence of all three points is required, at a minimum, before declaring a correlation to be successful.
The importance of both the chemical and geological character of these three definitional points cannot be overemphasized. Establishing chemical similarities between the organic matter in a source rock and that in an oil, even if these similarities involve ‘genetic’ (i.e., source-derived) molecular and isotopic characteristics, is necessary but insufficient. Such a result must also be supplemented by supporting geological data establishing that the source was capable – in all spatial and temporal dimensions – of having generated a specific oil. These geological data, including the details of depositional history and structural configuration through time, are provided as input to a robust basin model which is used to support the correlation conclusion in the spatial (i.e., fluid flow configuration) and temporal (i.e., timing of generation and expulsion) dimensions. Only when this is confirmed can a bona fide oil–source rock correlation be concluded with confidence.
Several published, successful oil–source rock correlations have appeared since the first was presented by John Hunt and colleagues over 50 years ago. Hunt et al. (1954) provided clear chemical and geological evidence in a causal framework for the source of oils and solid bitumens of the Uinta Basin (USA). Two decades passed before publication of the seminal work by Williams (1974) and Dow (1974) on the Williston Basin (USA), in which elemental, isotopic and molecular data were used in successful correlations among three source rocks and oil families. Although later studies became progressively more analytically sophisticated, the concepts established by Hunt, Williams and Dow, from which the definition given above was derived, still dominate oil–source rock correlation approaches.
All analytical methods used by petroleum geochemists to characterize oils and the organic matter in source rocks have also been used to correlate oils to source rocks, and the confidence we have in oil–source correlations parallels the confidence we have in petroleum geochemical methods in general. Indeed, the ease and rapidity with which precise analytical data can now be obtained has tended to de-emphasize the importance of geological data, because the precision of chemical data tends to be far greater than that of geological data. In part, this is because rapid temporal changes in the depositional conditions of many sedimentary systems make it difficult to sample these systems representatively. Thus, the fundamental non-analytical obstacle to successful oil–source rock correlations is the natural variability of the geological system, and much of this paper focuses on coping with this variability when attempting oil–source rock correlations.
Our knowledge of variability within depositional systems at all scales has not kept pace with our ability to generate highly precise analytical data. Indeed, chemical correlations are now easy to establish – almost automatic when using some statistical approaches – whereas knowledge of natural, fine-scale variation in source rock deposition (e.g., Curiale, 1994b, Keller and Macquaker, 2001, Barker et al., 2001) is substantially more difficult to acquire. Therefore, although a few aspects of analytical uncertainty remain (mostly based on the type of rock extraction method used – see below), the success of an oil–source rock correlation depends largely on our understanding of sedimentary organic variability in the natural setting.
This problem – making certain that the natural variability of depositional processes is fully considered when establishing oil–source rock correlations – requires that lateral and vertical differences in organic matter distribution in both source and reservoir rocks are accounted for properly. This variability extends from organic differences in source shales at the laminae scale to the potential for multiple source units to drain toward a single focal point (trap). The remainder of this paper discusses these factors using examples. Individual sections briefly address correlation problems arising from the occurrence of extraction technique differences, multiple source units, lateral/vertical source organic variability and lateral/vertical reservoir organic variability. A closing section proposes recommendations for more accurate and consistent correlations, in an effort to minimize the uncertainty in oil–source correlations. In this manner, more accurate correlations can be used in an industrial context, thus reducing exploration risk.
Section snippets
Extraction differences
Most correlation techniques in common use involve molecular and isotopic comparison of components in crude oil with components extracted from a candidate source rock. Natural extraction of crude oil from its source rock – usually denoted as “expulsion” – differs chemically, mechanically and temporally from laboratory extraction, leading to a correlation problem referred to here as “extraction difference”. Laboratory extraction usually utilizes an organic solvent at its boiling point, occurs
Mixing from multiple source units and multiple maturity levels
Increasing use among petroleum geologists and geochemists of thermal and flow modeling at the basin scale has resulted in an increased awareness of the occurrence and importance of multiple source rock units in a sedimentary basin, and the post-source mixing of oil expelled from these units en route to the trap. In addition, maturity-induced compositional variations in crude oil and in the soluble organic matter remaining in a source rock after explusion can be significant, and efforts to
Lateral and vertical organic variability in source deposition
Inspection of modern depositional settings indicates that organic matter accumulation in fine-grained sediments can show extensive compositional variability, both laterally and temporally (Tyson, 1995). As modern sediments become part of the lithified stratigraphic section, this variability is often preserved as compositional variations at the bulk (organic petrographic) and molecular levels. Lithologically homogenous sections often show molecular differences over relatively small vertical
Organic variability in carrier bed and reservoir rock
Although organic facies variations in oil-prone source units provide a first-order control on the composition of expelled oil, so-called non-genetic or post-sourcing events can also change an oil’s composition. In-trap biodegradation, for example, results in several well-recognized compositional changes in crude oil (Larter et al., 2006, and references therein) which, if unrecognized, can result in a failed oil–source correlation effort. Less well-recognized compositional changes that often go
Recommended approach
The previous sections have provided examples of four situations where oil–source rock correlations can be compromised: where laboratory rock extraction techniques differ dramatically from natural oil expulsion; where multiple source units and maturity levels occur in the basin, resulting in oil mixing within the fetch area; where lateral and temporal organic variability within a source unit (i.e., organic facies difference) is substantial; and where migration-induced compositional changes in an
Parting comment – oil–source rock correlations using only oil
Traditional oil–source rock correlations, as discussed throughout this paper, establish compositional and geological comparisons between oil samples and rock samples. However, increased understanding of petroleum systems and, in particular, detailed knowledge of the molecular and isotopic composition of crude oils and the organic matter in their source rocks has led to a non-traditional approach to oil–source rock correlation which I will refer to here as the inversion approach.
Inversion
Acknowledgements
Numerous Unocal and Chevron co-workers have provided ideas and correlation examples which have affected my thinking about oil–source rock correlations, and their help and advice are appreciated. My thanks go to both of these organizations for providing the regional petroleum systems overviews and extensive databases and analytical capabilities necessary to evaluate petroleum source rocks and their associated oils and gases in detail, and for providing approval to present and publish this work.
References (43)
- et al.
Application of pyrolysate carbon isotope and biomarker technology to organofacies definition and oil correlation problems in North Sea basins
Organic Geochemistry
(1990) - et al.
Mixed oils derived from multiple source rocks in the Cainan oilfield, Junggar Basin, Northwest China; Part I: genetic potential of source rocks, features of biomarkers and oil sources of typical crude oils
Organic Geochemistry
(2003) - et al.
Mixed oils derived from multiple source rocks in the Cainan oilfield, Junggar Basin, Northwest China; Part II, Artificial mixing experiments on typical crude oils and quantitative oil–source correlation
Organic Geochemistry
(2003) High-resolution organic record of Bridge Creek deposition, northwest New Mexico
Organic Geochemistry
(1994)A review of the occurrences and causes of migration–contamination in crude oil
Organic Geochemistry
(2002)- et al.
The Migration of petroleum into Vermilion 14 Field, Gulf Coast, USA – molecular evidence
Organic Geochemistry
(1996) - et al.
Migration-induced compositional changes in oils and condensates of a single field
Organic Geochemistry
(1996) - et al.
Application of new diterpane biomarkers to source, biodegradation and mixing effects on Central Llanos Basin oils, Colombia
Organic Geochemistry
(1999) - et al.
Reservoir overprinting of crude oils
Organic Geochemistry
(1995) - et al.
Organic matter and trace element rich sapropels and black shales: a geochemical comparison
Earth and Planetary Science Letters
(1999)
Geochemistry of petroleum systems in the Niuzhuang South Slope of Bohai Bay Basin. Part 2: evidence for significant contribution of mature source rocks to “immature oils” in the Bamianhe field
Organic Geochemistry
Mixed marine and lacustrine input to an oil-cemented sandstone breccia from Brora, Scotland
Organic Geochemistry
Compound-specific isotope analysis as a tool for characterizing mixed oils: an example from the West Shetlands area
Organic Geochemistry
Gas-condensate migration and oil fractionation in deltaic systems
Marine and Petroleum Geology
Deposition of sedimentary organic matter in blacks shale facies indicated by the geochemistry and petrography of high-resolution samples, Blake Nose, western North Atlantic
Petroleum geochemical characterization of the Lower Congo Coastal Basin Bucomazi formation
Oil to source rock correlation -- concepts and case studies
Application of oil-correlation and source-rock data to exploration in Williston Basin
American Association of Petroleum Geologists Bulletin
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