Magoon, L. B. and W. G. Dow, 1994, The petroleum system, in L. B. Magoon, and W. G. Dow, eds., The petroleum system - from source to trap: AAPG Memoir 60, p. 3-24.
Magoon and Dow (1994) formalized the petroleum system, shifting the focus from solely the reservoir to the genetic relationship between the generative pod (effective source rock) and the resulting accumulations. They note that in order for a petroleum system to exist, all elements (source rock, reservoir, seal, and overburden) need to be present, as do the processes of trap formation, generation, migration, and accumulation. The simple presence of the noted elements and processes is insufficient to establish a petroleum system. The elements must share the appropriate temporal and spatial relationships. A series of standard petroleum system diagrams were developed. These diagrams included: 1-a map that highlights the source rock distribution, the area where the source rock is effective and the maximum geographic extent of the system (essentially the migration limit); 2-a burial history plot, which includes the noting of the critical moment, i.e., the time when the current petroleum system was established; 3-a cross-section that displays the stratigraphic extent of the petroleum system, clearly identifying the system elements; and 4-the events chart showing the temporal relationships between the system elements and processes. The authors also incorporate into their assessment the preservation of accumulations.
This work fundamentally changed how basins and exploration opportunities are examined. The establishment of a viable petroleum system is now considered foundational by most explorationists prior to maturing a prospect.
This manuscript has been cited in more than 800 papers. Versions of the diagrams are routinely included when describing exploration opportunities. The "events chart" is often utilized as part of a prospect's risk assessment. Petroleum systems have been the focus of multiple books, conferences, and technical sessions.
Espitalie, J., M. Madec, B. Tissot, J. J. Mennig, and P. Leplat, 1977, Source rock characterization method for petroleum exploration: OTC Paper 2935.
Espitalie et al. (1977) introduced to the English-speaking world the Rock-Eval pyrolysis instrument as a means to characterize petroleum source rocks. A more complete version of the paper was published in French in the Rev. Inst. Fr. Petrol. The Rock-Eval has become the workhorse for the geochemist. It provides information on several key source rock characteristics including: 1-source rock potential as measured by the free (S1) and generatable (S2) hydrocarbons; 2-organic matter type as reflected in the hydrogen index (S2mg x100/g TOC) and oxygen index (S3mg x 100/g TOC); and 3-thermal maturity as inferred by Tmax (the temperature of peak pyrolysis yield), the production index (the ratio of S1/[S1+ S2]), and the relationship between the hydrogen and oxygen indices. In addition, the instrument provided information on hydrocarbon shows as manifested by elevated absolute and relative free hydrocarbon contents.
The simplicity of the Rock-Eval opened basic source rock geochemistry to the general exploration community. The instrument also sped the analytical process. Prior to the introduction of the Rock-Eval, source rock assessment was largely dependent on the extraction and characterization of bitumen and the isolation and analysis kerogen. These analyses took days to weeks as opposed to ~30 minutes for the Rock-Eval.
The OTC paper has been cited by about 350 papers, with the expanded French version being cited by more than 1050 papers. The significance of this work extends well beyond the cited papers, with the data as outlined in the paper being collected on most exploration wells.
Seifert, W. K., and J. M. Moldowan, 1978, Applications of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils: Geochimica et Cosmochimica Acta, v. 42, p. 77-95.
Seifert and Moldowan (1978) provided the foundation for the application of biomarkers to exploration problems, moving the focus away from the identification of individual natural products. The authors shifted from assessments based solely on absolute concentrations to a suite of ratios, many of which remain commonly used when interpreting the history of a crude oil. The authors provided a summary of sixteen biomarker indices that could be used to assess source input, the thermal maturity of the source rock, migration distance, and thermal alteration within the reservoir. Importantly, the authors note that many of the proposed ratios are impacted by multiple parameters, such as both source character and thermal maturity. The sterane distribution as a source parameter was, however, highlighted because it is largely insensitive to maturity or migration. Seifert and Moldowan suggest that, by utilizing multiple parameters, solutions for maturation or migration distance can be defined. For example, given a common source changes in sterane and terpane abundance may reflect migration and/or maturity. They further note that the utility of these compounds exists even though their specific precursor and/or formation mechanisms may be poorly understood.
These molecular data were considered more informative than bulk parameters, but are mutually supportive. The analysis of biological markers has become a routine part of any oil study or examination of extracted bitumen.
This publication has been cited by over 900 papers. Although the number of citations to this specific paper has decreased through time, the ratios discussed are routinely reported and the concept of biomarker geochemistry continues to grow with numerous papers defining new biomarker indices being regularly published.
Tissot, B., B. Durand, J. Espitalie, and A. Combaz, 1974, Influence of nature and diagenesis of organic matter in formation of petroleum: AAPG Bulletin, v. 58, p. 499-506.
Tissot et al. (1974) introduced geochemical concepts developed during the study of coals to the petroleum industry. This included the introduction of the van Krevelen diagram for the typing of organic matter contained within petroleum source rocks and the associated kerogen evolution pathways for the three primary kerogen types. The nature of the products generated (carbon dioxide and water, oil, and gas) during maturation (thermal evolution) are described as are the relative yields from each of the kerogen types. The thermal evolutionary pathway were defined using both a suite of natural samples from multiple basins and a suite of laboratory heating experiments. The paper concluded that the petroleum potential of a sedimentary sequence could be determined through a suite of chemical and physical analyses.
This work is considered foundational and is among the early papers of modern petroleum geochemistry. Not only did the paper define the three kerogen types in terms of the atomic H/C and O/C ratios and their evolution through generation, it described the nature of the organic matter that was deposited to form the kerogen and the general depositional setting for each of the kerogen types. Kerogen type as defined in this paper is cited in nearly every discussion on source rocks.
The paper has been cited more than 800 times. Although the paper is more than 40 years old, it continues to be cited in the geochemical literature and the material contained within it is included in nearly every petroleum geochemistry class presented.
Philippi, G. T., 1965, On the depth, time and mechanism of petroleum generation: Geochimica et Cosmochimica Acta, v. 29, p. 1021-1049.
Philippi (1965) provided a foundation for several of the concepts associated with hydrocarbon generation. It was observed that the relative increase in the abundance of hydrocarbons increased with depth as a result of increasing temperature and that the nature of the extractable became chemically more similar to oils with depth. He concluded that generation was a chemical reaction driven by thermal processes. Differences in the depth to the top of the oil-window were explained by differences in geothermal gradient, indicating that there is no fixed depth to the window and that the complete thermal history had to be considered when assessing the intensity of hydrocarbon generation, as did the abundance and character of the organic matter. It was also noted that older potential source rocks would need less thermal stress than a younger source sequence. Philippi observed that producing horizons were often shallower than the effective source rock demonstrating the importance of vertical migration along faults and fractures.
This work was among the first papers to indicate that three factors influence that amount of petroleum formed in the subsurface:1-the amount and nature of organic matter present in the source rock; 2-the temperature history of the source; and 3-possibly the presence of catalysts, although the importance of catalysts in the generation process remains ambiguous.
This work has been cited by over 440 papers. The concepts introduced in the manuscript concerning time, temperature, and the need for organic-rich sediments have been accepted by most exploration geologists and geochemists and provide part of the foundation for basin modeling software.
Pepper, A. S., and P. J. Corvi, 1995, Simple kinetic models of petroleum formation, part I: oil and gas generation from kerogen: Marine and Petroleum Geology, v. 12, p. 291-319.
Pepper and Corvi (1995) presented a suite of kinetic data for hydrocarbon generation that have become the foundation for various basin modeling software packages. Data were presented based on the depositional setting of the source rock, so an appropriate analogy can be used in areas lacking geochemical data. Five organofacies were defined: 1-aquatic, marine non-clastic (age independent); 2-aquatic, marine siliciclastic (age independent); 3-aquatic, lacustrine (Phanerozoic); 4-terrigenous, waxy, non-marine, ever-wet (swampy), coastal (Mesozoic and younger); and 5-terrigenous, wax-poor, non-marine, coastal (late Paleozoic and younger). The kinetics were developed using a combination of field and laboratory data. Separate kinetic parameters were provided for the oil and gas fractions so that changes in product composition can be tracked. The work also describes the differences that might be expected in the positions (temperatures) of both the oil and gas generation windows as well as the width of each of these windows, noting that the lacustrine kerogen displayed the narrowest window. The authors further note that the generation model is only a partial explanation of a region's resource potential and that expulsion and oil cracking need to be considered. Pepper and Corvi also discussed the differences between their kinetic model and others as well as the natural variability within the data.
In addition to a detailed explanation as to how the kinetic parameters (activation energy and pre-exponential factor) were derived, the authors discuss how these data can be utilized, including the application of the parameters in frontier settings. This work provides a bridge between geochemists, basin modelers, and exploration geologist.
This work has been cited by more than 250 papers. The kinetic parameters have been incorporated into the kinetic libraries of the various basin modeling software packages and are routinely used in basin modeling studies independent of the degree of exploratory maturity.
Demaison G. J., and G. T. Moore, 1980, Anoxic environments and oil source bed genesis: AAPG Bulletin, v. 64, p. 1179-1209.
Demaison and Moore (1980) described the importance of anoxic environments on the development of petroleum source rocks. They reported that the amount and quality of organic matter is greater when deposition occurred under anoxic conditions compared with oxygenated settings. Under anoxic conditions, organic carbon contents in excess of 1% may be expected. Demaison and Moore suggested that this was largely a result of the absence of benthonic scavenging in anoxic settings. Four specific depositional settings were highlighted that favored the development of anoxia and hence petroleum source rock formation; these included: 1-large anoxic lakes that lack seasonal overturn because of their tropical setting (e.g., Lake Tanganyika); 2-anoxic silled basins in regions with a positive water balance or where the sill depth is properly positioned relative to the oxygen minimum layer (e.g., Black Sea); 3-anoxic layers associated with oceanic upwelling caused by elevated biological oxygen demand resulting from elevated primary productivity (e.g., coastal Peru); and 4-open ocean anoxic layers that develop because of their remoteness relative to the source of the water mass (e.g., northern Indian Ocean).
This work suggested that with an understanding of paleogeography and paleoclimate, potential source rocks and oil shales could be mapped in time and space, thus providing guidance on one of the key elements of a petroleum system. It also established that petroleum source rocks are not ubiquitous, but form within a limited number of settings and during limited stratigraphic intervals.
This work has been cited in more than 1300 papers, although not all of these papers agreed with the proposed source rock model. Some authors argued that elevated productivity rather than elevated organic preservation was the primary mechanism for source rock development. The fact that this work spurred a significant discussion in the literature on source rock development is yet another reason for its overall significance.
Dow, W. G., 1977, Kerogen studies and geological interpretations: Journal of Geochemical Exploration, v. 7, p. 79-99.
Dow (1977) introduced several key concepts to the geochemical community. Key among them was the introduction of vitrinite reflectance as a thermal maturity indicator to the petroleum industry. The work noted differences in methodology between the coal and petroleum industries and established the use of a semi-log depth verses reflectance plot as a standard presentation format. Potential problems with vitrinite data such as caving and recycling were noted as were geologic causes for non-linear profiles such as intrusives and faulting. Profiles associated unconformities were also discussed, in which the "gap" in reflectance profile was used to estimate the amount of stratigraphic section lost. Dow related vitrinite reflectance to several maturity indices and the generation and destruction of hydrocarbons. This included estimates of the amount of hydrocarbons generated by different kerogen types at different levels of thermal maturity. Dow also noted a relationship between depositional setting and the level of organic enrichment and that these differences were persistent through geologic time.
The vitrinite reflectance profile as defined by Dow remains the standard means to establish the position of the oil- and gas-window and to calibrate basin models. Even in those cases where vitrinite is lacking, a reflectance equivalent profile is often created and reported. The importance of the reflectance profile has increased with the growth of shale plays and the need to establish the oil versus gas potential.
This paper has been cited by more than 750 papers and continues to be cited today. Its main points are routinely presented in geochemistry classes and can be observed in most geochemical well studies and many regional studies.
Welte, D. H., and M. A. Yukler, 1981, Petroleum origin and accumulation in basin evolution-a quantitative model: AAPG Bulletin, v. 65, p. 1387-1396.
Welte and Yukler (1981) established a foundation for the integrated basin models that have become key in the assessment of hydrocarbon charge during exploration. The 3-D model introduced in the paper integrated geologic, geophysical, geochemical, hydrodynamic, and thermodynamic data to yield a quantified prediction of hydrocarbons available. The model resolves the physical and thermal changes in the basin, which are then integrated with established geochemical principles of hydrocarbon generation. The simulation helps to ensure mass and energy balances and calculates changes in organic matter as a function of burial temperature. The model computes pressure, temperature, physical properties of the sediments, maturity of organic matter, and hydrocarbons as a function of time and space. The simulation results can be presented as a series of maps representing any geologic time slice of interest. It was further noted that these computer models can be applied during any exploration stage and updated as information becomes available. The authors also describe the use of sensitivity analysis and their assessment of allowable errors. The differences between the "real system", "conceptual model", and "mathematical model" are explained.
With this paper and the introduction of basin modeling, three questions became resolvable: 1-where might the hydrocarbons be positioned; 2- how much was generated, expelled, and trapped; and 3- when were the hydrocarbons generated.
This work has been cited by more than 220 papers. Similar works were also published by the authors in 1980 and 1981. The longevity of this work is clearly manifested by the role that basin modeling plays in exploration, the commercialization of basin modeling software, and the proliferation of publications and presentations that have occurred over the past quarter of a century.
Schoell, M., 1983, Genetic classification of natural gases: AAPG Bulletin, v. 67, p. 2225-2238.
Schoell (1983) provided the framework for the interpretation of gas geochemical data by combining molecular and stable isotope (carbon and hydrogen) compositions. The interpretive schemes described the mode of formation, the nature of the source, and secondary processes that may alter gas composition such as oxidation, mixing, and migration. The work included a suite of interpretive diagrams that provided for the classification as biogenic, thermogenic associated, and thermogenic not associated. The work further differentiated between marine and continental biogenic gas, and humid and sapropelic/liptinic thermogenic gas. The interpretive scheme also provided estimates on source maturity when vitrinite reflectance is greater than 1.2%. These diagrams were empirically derived based on a large dataset. Examples of the application of the interpretive diagrams were presented using gases from the Gulf of California, the South German Molasse basin, the Vienna basin, and the Po basin. Schoell noted that this study did not effectively address coal gas.
This paper provided the first consistent description of natural gases in terms of their origin and provided guidance on their potentially complex histories. Much of the published work prior to this publication focused on methane using stable carbon and to a lesser degree hydrogen isotopes. This work incorporated the wet gas components and the isotopic composition of ethane as well as methane.
This work has been cited by more than 750 papers. The interpretative schemes proposed are still in common use when examining gases from both conventional and unconventional resource plays.