Source Rock Modeling with OF-Mod

Over the last few years, the process-based source-rock modeling software OF-Mod (Mann and Zweigel, in press) has been applied for quantitative prediction of source-rock potential and -type away from well control in many frontier exploration areas all around the world. OF-Mod simulates processes relevant for organic matter accumulation in sedimentary basin areas and interactions between these processes (Figure 1).

Modern Analogues of Ancient Source Rock Systems

High-productive continental margin systems are of major importance for today’s marine carbon cycle, but are also considered modern analogues of ancient source rock systems (Figure 2) (e.g., Calvert et al., 1992; Parrish, 1995).

Modeling Modern High-Productive Marine Settings with OF-Mod

OF-Mod can also be applied to simulate organic carbon burial in modern marine settings. In a first approach we compiled bathymetric data, satellite-based ocean primary productivity models (SeaWIFS, Behrenfeld and Falkowski, 1997; Antoine et al., 1996), published well data (TOC and HI values, lithology, sedimentation rates and age datings). Based on these, we constructed simple OF-Mod models of present-day production, degradation, sedimentation and accumulation of organic matter at three of the most productive areas of the ocean (numbers according to Carr, 2002):

• The Benguela upwelling area off southwestern Africa (Namibia, 0.37 Gt C/yr) (Figure 3).
• The upwelling area off northwestern Africa (Mauritania, Senegal, 0.33 Gt C/yr) (Figure 4).
• The Peruvian upwelling area off northwestern South-America (0.20 Gt C/yr) (Figure 5).

These ocean areas show features that are important for the accumulation of high amounts of organic carbon and represented in the OFMod software:

• Lenses of high marine productivity in a certain distance from the coast related to the upwelling.
• Oxygen minimum zones on the continental shelf and slope improving preservation of the organic matter.

Calculating Organic Carbon Budgets

The results from the scientifically well-understood modern systems allow on the one hand for evaluation and calibration of the processes represented in OF-Mod and improvement and extension of the software.

On the other hand the software can be used to quantify organic carbon accumulation in the investigation areas at present-day as well as over geological time scales.

The permanent burial of organic carbon in marine sediments corresponds to carbon dioxide sequestration from the atmosphere, which is continuously gaining importance with respect to increasing consumption of fossil energy resources and the buffer capacity of the ocean against anthropogenic carbon dioxide increase. Therefore, quantification of the deep ocean carbon budget is crucial to understand the relevance of marine systems for the global carbon cycle. Preliminary results for organic matter accumulation in the

modeled areas are:

Area	Benguela (400 000 km2)	Northwest Africa (550 000 km2)	Peru (470 000 km2)
Carbon accumulation (Megatons/year)	2.2	3.7	3.0

Research has shown that lateral redistribution with oceanic currents can play a crucial role on organic matter accumulation in the sediments (e.g., Inthorn et al., 2006). The implementation of a simple ocean current model in OF-Mod or a coupling to existing ocean current models is therefore aspired.

The modeling projects for the three upwelling areas will be continuously improved by collecting and quality control of more well data. Other modeling areas with major significance for the oceanic carbon cycle will be approached.

The quantification of the organic carbon burial for key areas and time slices can then be used as input in climate models to investigate the effect of the carbon sequestration on carbon dioxide concentrations in the ocean-atmosphere system.

Outlook

Research has shown that lateral redistribution with oceanic currents can play a crucial role on organic matter accumulation in the sediments (e.g., Inthorn et al., 2006). The implementation of a simple ocean current model in OF-Mod or a coupling to existing ocean current models is therefore aspired.

The modelling projects for the three upwelling areas will be continuously improved by collecting and quality control of more well data. Other modelling areas with major significance for the oceanic carbon cycle will be approached.

The quantification of the organic carbon burial for key areas and time slices can then be used as input in climate models to investigate the effect of the carbon sequestration on carbon dioxide concentrations in the ocean-atmosphere system.

Selected References

Antoine, D., J.M. André, and A. Morel, 1996, Oceanic primary production; 2. Estimation at global scale from satellite (coastal zone color scanner) chlorophyll: Global Biogeochemical Cycles, v. 10/1, p. 57-69.

Behrenfeld, M. J. and P.G. Falkowski, 1997, Photosynthetic rates derived from satellite-based chlorophyll concentration: Limnology and Oceanography, v. 42/1, p. 1-20.

Calvert, S.E., T.F. Pedersen, 1992, Organic carbon accumulation and preservation in marine sediments: how important is anoxia? in: J.K. Whelan, J.W. Farrington (editors), Organic Matter: Productivity, Accumulation and Preservation in Recent and Ancient Sediments: Columbia University Press, New York, p. 231-263.

Inthorn, M., T. Wagner, G. Scheeder, M. Zabel, 2006, Lateral transport controls distribution, quality and burial of organic matter along continental slopes in high-productivity areas: Geology, v. 34/3, p. 205-208.

Mann, U. and J. Zweigel, in press, Modelling source rock distribution and quality variations: The OF-Mod approach, in P. de Boer (editor), Special Publication of the International Association of Sedimentologists: Blackwell Science.

Parrish, J.T., 1995, Paleogeography of Corg-rich rocks and the preservation versus production controversy, in A.Y. Huc (editor), Paleogeography, paleoclimate, and source rocks, AAPG Studies in Geology, v. 40, p. 1-20.