Jonathan Evenick
BP America Inc.
Houston
Improved understanding of the predominant organofacies through the past ~500 million years and the number of source rocks preserved in the geologic record will lead to better estimates of paleo-organic matter storage. These higher-resolution observations will help climate modelers better understand the historical geologic links between oceanic anoxia and source rock deposition.
Most literature proposes 6-10 key source rock intervals in the geologic past that are tied to the majority of known hydrocarbon resources and that these intervals are associated with various causal mechanisms, such as marine transgressions, oceanic anoxic events, large igneous provinces, tectonic plate positioning, and productivity.1-9
This article used a global dataset and detailed paleogeographic mapping of 90 geologic time slices to focus on source rock distribution and organofacies while also investigating potential linkages. The mapping revealed clear global trends among marine, terrestrial, and lacustrine-dominated organofacies through the Phanerozoic and that widespread source rock deposition is likely not tied to one causal mechanism, but more likely the interplay of various factors. These results will reshape our views of when and how much organic carbon has been stored in the rock record, helping recalibrate calculations of the Earth's organic carbon budget.
Organic carbon distribution
The distribution of rocks containing high levels of total organic carbon (TOC) through the geologic record has been a topic of interest in the petroleum industry for many decades, as it is a primary risk in frontier exploration. The type of organofacies or kerogen also has been important since it plays a major role in determining the type of product expelled from, and retained within, a thermally mature source rock.10
The advent of the concept of a petroleum system in the early 1980s tied to prior key geochemical correlations and the recent emergence of unconventional resources has prompted a wealth of published literature on source rock geochemistry.11 Fig. 1 contains roughly 40,000 unique sites with organofacies (yellow points) and other geochemistry data (red points) from literature covering most of the world's 750 sedimentary basins (shaded black polygons). These geochemical data not only quantify the amount of TOC in a sample or interval, but also indicate the dominant organofacies. Organofacies types are often associated with general depositional setting, hydrocarbon generative capacity, biomarker ratios, kerogen type, and bulk-rock compositions.10 12-14
Source rock deposition
Many source rocks represent condensed stratigraphic sections that can span millions of years and multiple geologic stages.9 Further complicating proper reconstruction of their paleogeographic locations are the various formal and informal names these intervals have within a given region. With many researchers focused on the same regions with proven petroleum reserves, and in time intervals with proven or suspected oceanic anoxic events (OAE), it was imperative to account for these data clusters as completely as possible to get a clear global signal.1-3 5-7 16
Source rock studies have also mainly focused on the stratigraphic intervals that have sourced the majority of conventional fields in the world and therefore have limited their chronostratigraphic and geographic reach. This article has incorporated as broad a global dataset as possible from literature and was neither limited to thermally mature source rocks that have expulsed hydrocarbons nor tied to any particular time period.
The results are revealing and support the general correlations regarding the interplay of sea level, volcanic activity, oceanic geochemistry, paleoclimate and paleolatitude, and tectonic plate configurations on source rock deposition. Previous publications have focused mainly on prolific hydrocarbon basins and fields, and these studies have highlighted variable source rock trends through time.1-4 9 16 The trends, however, in both organofacies and total number of source rocks are not irregular.
Fig. 2 displays a geologic time scale representing the Phanerozoic Eon with relative changes in source rock (wt %), large igneous provinces (LIP), and main OAEs.5 7 17-18 20-22 The source rock trends look relatively straightforward and very similar to sea level curves.17 18 They also appear somewhat similar to the inverse of carbonate deposition through time.19 Compositional source rock data derived from X-ray diffraction analyses show that higher percentages of carbonate typically have lower amounts of TOC.
Diverse and potentially interconnected variables appear to control the deposition of different types of source rocks. This study found that in periods dominated by marine source rock deposition (Organofacies A and B), the relative trends track sea level, but not perfectly. Definitive perturbations can be explained by other mechanisms, such as significant mountain building events, that arguably overrode sea level controls and supplied large amounts of clastic material, diluting organic matter deposition over a large geographic footprint.
The most prominent decoupling of the curves occurred during the formation of Pangea in the Pennsylvanian through Early Permian. During this period there was a global spike in source rock deposition overall and these source rocks are predominantly coals and terrestrial-related shales (Organofacies D, E, and F). The second significant time for Organofacies D, E, and F deposition was the Paleogene, when the broad Himalayan-Alpine orogeny was occurring in Asia, along with the Sevier-Laramide orogeny in western North America, and the ongoing Andean orogeny in South America.
Lacustrine-related source rocks (Organofacies C) are not very common in the geologic record and rarely account for >10% of the source rocks identified. Organofacies C source rocks are mainly found at rifted continental margins or along passive margins. Many diachronous lacustrine source rocks were deposited through the majority of the Mesozoic, associated with the breakup of Pangea. Another less common setting for lacustrine source rocks is in intramontane settings that have experienced differential uplift due to compression.23
The total number of source rocks deposited and preserved through the Phanerozoic generally increased and roughly mimicked sea level, supporting the concept that there are general periods of time with widespread, marine-dominated source rock intervals within the Phanerozoic. These periods roughly correspond to the Middle Devonian through Late Mississippian, the Late Jurassic, and the Late Cretaceous.
The correlation of sea level and high TOC source rocks is compelling for the last 400 million years. The correlation appears to break down before the Devonian. Undersampling due to drilling depth, limited surface outcrops, lack of preservation due to orogenic reworking of paleobasins, changes in the amount of biomass over time, the lack of land-based plants, and potential oceanic chemistry inhibitors with widespread deposition of inorganic carbon in the form of carbonates in the Early Paleozoic could all contribute to this breakdown.9 19
The very noticeable peak in the overall increasing trend is controlled by mechanisms beyond global marine transgressions alone. The prominent two-fold increase in source rock deposition occurred during the Cretaceous (Aptian-Turonian) OAEs. Within these events, many but not all of the global sedimentary basins that have preserved marine intervals show some source rock potential, further displaying the widespread, but not necessarily global, nature of OAEs.24
No such spike appears in other proposed OAEs such as the Toarcian (Lower Jurassic).25 This neither rules out nor condones the idea that there are other OAEs within the Phanerozoic, but suggests they may just be more geographically restricted. During proven OAEs, widespread anoxia led to the deposition of many geographically clusted source rocks. But the average TOC per source rock is not particularly high when compared to other similar marine-dominant intervals such as the Late Devonian through Early Mississippian.
The Lower Jurassic and possible Toarcian OAE are not generally too disparate from the values and trends seen in the Paleogene. During these two time periods there are many source rocks preserved that indicate fairly terrestrial-dominated depositional systems with high TOC averages per source rock.
Large igneous provinces
This study also looked at possible spatial and temporal correlations between LIPs and source rock deposition (Fig. 3). The crossplot in Fig. 3 represents the average sea level vs. the number of marine source rocks per geologic stage from the Devonian to present. The color coding shows the presence or absence of an LIP during the corresponding stage. Time periods with LIPs tend to have more source rocks than time periods without, and the highest amount of marine source rocks are also associated with the Cretaceous OAEs. The correlation among time periods with LIPs has an R2 value of 0.82, whereas the trend-line fit through all of these data is 0.65.
There were 52 LIPs paleoreconstructed to look at their spatial correlation during their proposed most prominent eruptive phases.7 20-22 This study observed a strong temporal connection between LIPs and the relatively higher amount of source rock deposition. A bit surprisingly, there were no strong spatial correlations between good source rocks and the locations of the LIPs in the paleoreconstructions. This may show that the LIPs' effects are more atmospheric in nature than oceanic on a regional-to-global level.
This study cursorily looked at LIP volumetric estimates relative to source rock numbers and found a large scatter.21-22 26 Some time periods had large amounts of source rocks associated with quite small LIPs and vice-versa, pointing to more important factors such as sea level or perhaps showing that some of the LIPs' effects may be more cumulative. This study didn't capture the duration of events, but future work could be done in this area.
Paleolatitude, upwelling
The last two variables this research investigated were the role of paleolatitude and upwelling on source rock deposition in some of the paleogeographic reconstructions. Paleogeography allows for quick climate estimates based on approximate distance from the paleo-equator. The models also provide some idea of plate margins that may have been more prone to large-scale upwelling. The source rock data with TOC over 2.0 wt % were deposited within 60° of the paleo-equator within restricted shelfal areas, epicontinental seaways, and restricted intracratonic or intramontane basins 85-90% of the time. The only exception to this robust correlation was in the Silurian, with a significant amount of source-rock deposition near the paleo South Pole.
The proprietary reconstructed maps also showed that inferred areas of upwelling based on paleoplate positions did not strongly align with the sites of preserved high-TOC source rocks. This may be related to the limited preservation potential in these areas absent the accommodation space increasing via tectonic or eustatic changes rather than a lack of deposition.8
The paleogeographic reconstructions also show that when conjugate margins are rifted apart, the recessed margin has a higher likelihood of containing high-TOC source rocks of lacustrine origin (Organofacies C). If the embayment is coeval with a large clastic influx the organofacies may be more D, E, and F prone. These robust correlations remained even within different plate models.
Variables
Periods of predominant terrestrial source rock deposition don't appear to be driven by sea level, but are arguably more important to organic carbon flux estimates since they commonly have higher average TOC content due to the presence of coals.27 Organic carbon budget calculations need to properly take into account not only these important time periods, but also the large amount of marine and lacustrine dominated organofacies (A, B, and C), along with their geographic extent and thickness throughout the geologic record.
Estimates purely focused on coals will underestimate the amount of organic carbon currently buried and stored within the long-term carbon cycle. There is a large amount of work to be done on this subject in further describing the interrelation of various oceanic and atmospheric variables and applying these to forecasting models.
Acknowledgment
The author would like to thank BP for permission to publish this research and Lee McRae in particular for initial support of this work. Uyen Nguyen provided technical assistance with various plate reconstruction software packages and models. Additional thanks go to Richard Corfield, Rebecca Wiles, Howard Leach, and Bruce Abbuhl for their continued support. Zhiyong He granted the license to use the HotSpot analytical tool within his Trinity software package.
References
1. Grunau, H.R., "Abundance of source rocks for oil and gas worldwide," Journal of Petroleum Geology, Vol. 6, No. 1, 1983, pp. 39-54.
2. Klemme, H.D. and Ulmishek, G.F., "Depositional controls, distribution, and effectiveness of World's petroleum source rocks," USGS Bull. 1931, 1990, pp. 59.
3. North, F.K., "Episodes of source-sediment deposition," Journal of Petroleum Geology, Vol. 2, No. 2, October 1979, pp. 199-218.
4. Tissot, B., "Effects on prolific petroleum source rocks and major coal deposits caused by sea-level changes," Nature 277, Feb. 8, 1979, pp. 463-465.
5. Schlanger, S.O. and Jenkyns, H.C., "Cretaceous oceanic anoxic events: Causes and consequences," Geologie en MIjnbouw, Vol. 55, No. 3, November 1976, pp. 179-184.
6. Demaison, G.J. and Moore, G.T., "Anoxic environments and oil source bed genesis," AAPG Bull. 64, No. 8, Aug. 1, 1980, pp. 1179-1209.
7. Kuroda, J., Ogawaa, N.O., Tanimizub, M., Coffinc, M.F., Tokuyamac, H., Kitazatoa, H., and Ohkouchia, N., "Contemporaneous massive subaerial volcanism and late cretaceous Oceanic Anoxic Event 2," Earth and Planetary Science Letters, Vol. 256, No. 1-2, April 2007, pp. 211-223.
8. Bohacs, K.M., Norton, I.O., Gilbert, D., Neal, J.E., Kennedy, M., Borkowski, W., Rottman, M., and Burke, T., "The accumulation of organic-matter-rich rocks within an earth systems framework: The integrated roles of plate tectonics, atmosphere, ocean, and biota through the Phanerozoic," Regional Geology and Tectonics: Principles of Geologic Analysis, February 2012, pp. 647-678.
9. Huc, A.Y., van Buchem, F.S., and Colletta, B., "Straigraphic control on source-rock distribution," Society for Sedimentary Geology (SEPM) Special Publication No. 82, 2005, pp. 225-242.
10. Peters, K.E., "Guidelines for evaluating petroleum source rock using programmed pyrolysis," AAPG Bull., Vol. 70, No. 3, March 1986, pp. 318-329.
11. Magoon, L.B. and Dow, W.G., "The petroleum system-from source to trap," AAPG Memoir 60, 1994, pp. 3-24.
12. Pepper, A.S. and Corvi, P.J., "Simple kinetic models of petroleum formation - part 1: Oil and gas generation from kerogen," Marine and Petroleum Geology, Vol. 12, No. 3, 1995, pp. 291-319.
13. Peters, K.E., Walters, C.C., and Moldowan, J.M., "The biomarker guide: Biomarkers and isotopes in the environment and human history," Vol. 1, 2005, pp. 451.
14. Evenick, J.C. and McClain, T., "Method for characterizing source rock organofacies using bulk rock composition," AAPG Memoir 103, 2013, pp. 71-80.
15. Amante, C. and Eakins, B.W. "ETOPO1 1 arc-minute global relief model: Procedures, data sources and analysis," National Oceanic and Atmospheric Administration (NOAA) Technical Memorandum NESDIS NGDC-24, National Geophysical Data Center, 2009.
16. Bois, C., Bouche, P. and Pelet, R., "Global geologic history and distribution of hydrocarbon reserves," AAPG Bull. 66, No. 9, September 1982, pp. 1248-1270.
17. Haq, B.U., Hardenbol, J., and Vail, P.R., "Chronology of fluctuating sea levels since the Triassic," Science, Vol. 235, No. 4793, Mar. 6, 1987, pp. 1156-1167.
18. Haq, B.U. and Schutter, S.R., "A chronology of Paleozoic sea-level changes," Science, Vol. 322, No. 5898, Oct. 3, 2008, pp. 64-68.
19. Walker, L.J., Wilkinson, B.H., and Ivany, L.C., "Continental drift and Phanerozoic carbonate accumulation in shallow-shelf and deep-marine settings," Journal of Geology, Vol. 110, No. 1, January 2002, pp. 75-87.
20. Bryan, S.E. and Ernst, R.E., "Revised definition of large igneous provinces (LIPs)," Earth-Science Reviews, Vol. 86, No. 1-4, January 2008, pp. 175-202.
21. Coffin, M.F., and Eldholm, O., "Large igneous provinces: crustal structure, dimensions, and external consequences," Reviews of Geophysics, Vol. 32, No. 1, February 1994, pp. 1-36.
22. Kravchinsky, V.A., "Paleozoic large igneous provinces of Northern Eurasia: Correlation with mass extinction events," Global and Planetary Change, Vol. 86, April 2012, pp. 31-36.
23. Dickenson, W.R., Klute, M.A., Hayes, M.J., Janecke, S.U., Lundin, E.R., McKittrick, M.A., and Olivares, M.D., "Paleogeographic and paleotectonic setting of Laramide sedimentary basins in the central Rocky Mountain region," Geological Society of American (GSA) Bull., Vol. 100, No. 7, July 1988, pp. 1023-1039.
24. Takashima, R., Nishi, H., Huber, B.T. and Leckie, R.M., "Greenhouse world and the Mesozoic ocean," Oceanography, Vol. 19, No. 4, December 2006, pp. 64-74.
25. Jenkyns, H.C., "The early Toarcian (Jurassic) anoxic event: stratigraphic, sedimentary, and geochemical evidence," American Journal of Science, Vol. 288, No. 2, February 1988, pp. 101-151.
26. Bond, D.G. and Wignall, P.B., "Large igneous provinces and mass extinctions: An update," GSA Special Papers, Vol. 505, June 10, 2014, pp. 27-55.
27 Berner, R.A., "The long-term carbon cycle, fossil fuels and atmospheric composition," Nature 426, Nov. 20, 2003, pp. 323-326.
EXPLORATION &
DEVELOPMENT
The author
Jonathan C. Evenick ([email protected]) is a senior basin analyst-structural geologist working at BP America Inc. in Houston. He has more than 10-years' experience in global exploration and access projects and 7 years of experience in unconventional reservoirs. He has a PhD and post-doctoral degree from the University of Tennessee, Knoxville.