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Climate Modeling – a Tool for the Assessment of the Paleodistribution of Source and Reservoir Rocks M DGMK/ÖGEW-Frühjahrstagung 2008, Fachbereich Aufsuchung und Gewinnung Celle, 10./11. April 2008 Climate modeling – a tool for the assessment of the paleodistribution of source and reservoir rocks M. Roscher*, U. Berner**, J.W. Schneider*, *Institut für Geologie, TU Bergakademie Freiberg, **Referat Organische Geochemie/Kohlenwasserstoff-Forschung, BGR, Hannover Abstract In an on-going project of BGR and TU Bergakademie Freiberg, numeric paleo-climate model- ing is used as a tool for the assessment of the paleo-distribution of organic rich deposits as well as of reservoir rocks. This modeling approach is based on new ideas concerning the formation of the Pangea supercontinent. The new plate tectonic concept is supported by pa- leo-magnetic data as it fits the 95% confidence interval of published data. Six Permocarboniferous time slices (340, 320, 300, 290, 270, 255 Ma) were chosen within a first paleo-climate modeling approach as they represent the most important changes of the Late Paleozoic climate development. The digital maps have a resolution of 2.8°x2.8° (T42), suitable for high-resolution climate modeling, using the PLASIM model. CO2 concentrations of the paleo-atmosphere and paleo-insolation values have been estimated by published me- thods. For the purpose of validation, quantitative model output, had to be transformed into qualita- tive parameters in order to be able to compare digital data with qualitative data of geologic indicators. The model output of surface temperatures and precipitation was therefore con- verted into climate zones. The reconstructed occurrences of geological indicators like aeolian sands, evaporites, reefs, coals, oil source rocks, tillites, phosphorites and cherts were then compared to the computed paleo-climate zones. Examples of the Permian Pangea show a very good agreement between model results and geological indicators. From the modeling approach we are able to identify climatic processes which lead to the deposition of hydrocarbon source and reservoir rocks. The regional assessment of such at- mospheric processes may be used for the identification of the paleo-distribution of organic rich deposits or rock types suitable to form hydrocarbon reservoirs. Introduction As climatic processes and climatic changes have a significant imprint on the distribution of petroleum reservoir and source rocks (Parrish 1993), we have chosen the Late Paleozoic as a test of a new modeling approach, as Carboniferous and Permian sediments contain a sig- nificant number of petroleum source and reservoir rocks. The climate of the Late Paleozoic was marked by a shift from icehouse conditions during Late Carboniferous to the Permian Warmhouse phase (Chumakov & Zharkov, 2002). The Carboniferous climate is dominated by the glaciation of Gondwana which began during Mid-Carboniferous times (Dickins, 1996; Wright & Vanstone, 2001; Bruckschen et al., 1999; Saltzman, 2003). Saltzman (2003) suggests that the closure of the equatorial seaway be- tween Laurussia and Gondwana caused the onset of the glaciation as a re-organization of the global ocean currents with enhanced moisture transport to the southern continent as a major consequence. The maximum extent of the Gondwana Glaciation has occurred accord- ing to DiMichele et al. (1996) and Sano et al. (2003) during the late Westphalian. However, for the South American Paraná Basin dos Santos et al. (1996) suggested the maximum ice extent to occur close to the boundary between Carboniferous and Permian which is sup- ported by findings of Broutin et al. (1995) from Oman. The end of this glaciation in the As- selian also marks the end of the Paleozoic icehouse. After the Asselian, only regional gla- ciers are reported (Dickins, 1996; Henderson, 2003). The Late Carboniferous and Permian climates of Europe and North American are unquestionably dominated by an aridization trend (Chumakov & Zharkov, 2002). This trend was at least in the tropics (Roscher & Schneider, 2006; Schneider et al., 2006) interrupted by several wet phases (Fig. 1). The DGMK-Tagungsbericht 2008-1, ISBN 978-3-936418-79-8 329 DGMK/ÖGEW-Frühjahrstagung 2008, Fachbereich Aufsuchung und Gewinnung, Celle wetness fluctuations of the tropics correlate well with the Carboniferous deglaciation events of the southern African Dwyka Group. In the present paper numeric climate modeling is used as a tool for the assessment of the paleo-distribution of organic rich de- posits and reservoir rocks. This modeling approach is based on new ideas on the formation of the Pangea supercontinent (Schneider et al., 2006; Roscher & Schneider, 2006) and their observations on climatic variability. The results presented here, show the great potential of climate modeling for the prediction of potential regions for hydrocarbon source and reservoir rocks. Paleogeographic reconstructions The paleogeography of the Late Paleozoic is commonly based on paleo-magnetic data (Irving, 1977; Scotese, 2002; Golonka, 2000). However, existing paleogeographic reconstructions of Pangea are still debated with respect to their plausibility (Dob- las et al., 1998; Muttoni et al., 2003; Fluteau et al. 2001). New ideas on the formation of Pangea have been published by Roscher & Schneider (2006), Schendel (2004) and Schneider et al. (2006). Their paleogeographic model depends on the Wegnerian Pangea A concept and on relative plate motions only. Using observed structural data of the initial phase of the orogeneses the continental plates were rotated back to their former position. The resulting maps were converted to paleo- geographic maps by using climatic data of Scotese (2002). Paleo-magnetic data of Grunow (1999) fit very well to the new paleo-geographic concept (Fig. 2). Because of the geometric constraints on the sphere the closure of the Rheic Ocean occurs later than previously thought (Sco- tese 2002, Golonka 2000). The post-Caledonian orogens along the suture between Gondwana and Laurussia are evidence of the collision processes taking place from the Early Devonian to the Late Permian. The collision starts in the eastern parts of the Hercynides and moves continuously to the west. We suggest that there were no contemporaneous orogenic processes along the entire plate boundary which is supported by observational evidence for a Figure 1. Climatic chan- diachronous development (Schneider et al. 2006). Uplift and ges during Permocarbon- exhumation processes show large differences between the iferous times (Roscher & Variscides, the Appalachians and the Ouachita Mountains. Schneider, 2006) Therefore, it is highly unlikely that a continuous mountain belt with high altitudes reaching from the Variscides to the Ouachita Mountains (Keller & Hatcher, 1999) has existed. Further support to this hypothesis, the maximum of high pressure metamorphism occurred in the Central European Variscan Orogen during the Early Carboniferous. Enhanced erosion and differential uplift equilibrated the orogenic crust of Europe at the end of the Carbonifer- ous (Kroner et al., 2004). During the Permian, dextral transpression occurred locally during small scale tectonothermal events (Capuzzo & Wetzel, 2004; Gaitzsch, 1998; Kroner & Hahn, 2003). On the western side of the suture the rocks of the Ouachita Mountains have been affected only moderately by an arc-continent collision during Late Carboniferous times (Thomas, 1989). The closure of the Rheic Ocean is caused by the clockwise rotation of Gondwana, and was completed during the Late Permian (Fig. 3). The conversion of relative to absolute plate motions is based on the reconstruction of climatic belts. The distribution of climatic belts is derived from published data of Scotese (2002). The suggested position of the paleo-equator is based on the assumption that arid and temperate climates were symmetri- DGMK-Tagungsbericht 2008-1, ISBN 978-3-936418-79-8 330 DGMK/ÖGEW-Frühjahrstagung 2008, Fachbereich Aufsuchung und Gewinnung, Celle cally distributed around it. The main difference between the commonly used maps of Golonka (2000), Scotese, (2002) and Ziegler et al. (1997) and the new Pangea concept relates to the timing of the closure of the Rheic Ocean flanked by Laurussia and Gondwana, and the absence of a Hercynian mountain chain traversing the Pangean conti- nent. This changed position of Gondwana leads also to a different plate configuration in the Asian region. The South China Block is situated in the northern tropics during Carboniferous and Permian times. Because of this feature we suggest that the Tethys was not separated from the Panthalassic Ocean by smaller terranes (comp. Fig. 3). Numerical Climate Modeling Model set up For modeling of paleo-climate we used PLASIM (Fraedrich et al., 2005), an atmospheric circulation model of medium complexity with slap-ocean, sea-ice and biome mod- ules. Numerical paleo-climate model- ing with PLASIM requires basic input data which comprise the distribution of land and oceans and the paleo- topography as geopotential, the CO2 concentration of the atmosphere and the solar constant as boundary condi- tions. From the new paleogeographic re- construction six time slices which mark important paleo-climatic and/or plate tectonic phases were chosen as the foundation for the different model- ing runs. We started our modeling approach with a reconstruction of the world at 340 Ma where the paleo- geography is marked by an open equatorial seaway. The 320 Ma re- construction shows the closure of the Mid-Pangean seaway and the onset of the Gondwana glaciation. The Figure 2. Left column: position of the
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