Climate Modeling – a Tool for the Assessment of the Paleodistribution of Source and Reservoir Rocks M

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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 modeling 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 paleo-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 methods. For the purpose of validation, quantitative model output, had to be transformed into qualitative 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 converted 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 atmospheric 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 significant 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 between 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 according 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 supported 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 glaciers 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

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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 deposits 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 paleogeographic 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-

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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 continent. 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 modules. Numerical paleo-climate modeling 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 conditions. From the new paleogeographic reconstruction six time slices which mark important paleo-climatic and/or plate tectonic phases were chosen as the foundation for the different modeling 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 reconstruction shows the closure of the Mid-Pangean seaway and the onset of the Gondwana glaciation. The Figure 2. Left column: position of the South Pole maximum of this ice age occurs at according to paleo-magnetic data of Grunow (1999); about 300 Ma. The shift from the Pa- right column: position of the South Pole based on leozoic icehouse to the following the new paleo-geographic concept. warmhouse stage started at about 290 Ma ago. The maximum of aridity on central Pangea was reached during the Middle Permian (270 Ma) and the influence of the

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large Zechstein ingression is simulated in the 255 Ma reconstruction. All paleogeographic reconstructions were transformed into digital grids which have a 2.8°x2.8° (T42) resolution, suitable for high-resolution climate modeling. The land-sea mask and the topography were digitized from the maps of Fig. 3. Values of paleo-insolation (Table 1) were calculated from the equation given in Caldeira & Kasting (1992). Greenhouse forcing of PLASIM only relates to prescribed values of atmospheric carbon dioxide concentrations (Tab. 1) estimated by the method of Freeman & Hayes (1992) using isotopic data of Veizer et al. (1999) and Veizer et al. (2000). For sensitivity studies we not only doubled the CO2 concentration but have as well run the model with half and a quarter of the reconstructed concentrations (Tab. 1).

Table 1: Insolation and CO2 during Late Paleo- zoic

The elevation of the Hercynian orogen is of major importance for global climate during the Permocarboniferous. All previously published climate modeling approaches of Pangea that include topography (Kutzbach & Ziegler, 1994; Fluteau et al., 2001; Gibbs et al., 2002), assume an equatorial mountain chain (according to Kel- ler & Hatcher, 1999) with an average elevation of about 2000 to 2500 m, contrary to Gibbs et al. (2002) who suggest a maximum height of 1800 m. The estimates range from 1800 m to more than 5000 m for the Variscian mountain chain (Becq-Giraudon et al., 1996; Fluteau et al., 2001; Gibbs et al., 2002; Kutzbach & Ziegler, 1994). However, following the ideas of Roscher & Schneider (2006) and Schneider et al. (2006) the Hercynian mountain range never acted as an equatorial barrier traversing the Pangea continent. Topographic values for climate modeling have been estimated from the above reconstruction and are related to the hypothesis of Rosch- er & Schneider (2006) and Schneider et al. (2006). In all of our model runs orbital parameters with the exception of obliquity were zero. Obliquity was kept at the present day value of 23.441°. Figure 3. Paleogeographic maps of six

Permocarboniferous time slices.

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CO2 sensitivity Studies of CO2 sensitivity of the atmospheric model with ice, ocean and biome modules switched off, show that a rise of the atmospheric carbon dioxide concentration by a factor of 2 (Tab. 1, Fig. 4) leads to a rise in the global annual mean temperature of 0.2 K (constant insolation). Runs with sea-ice and ocean modules switched on reveal a much higher sensitivity of 4 to 5 K for a doubling of CO2. The highest sensitivity of 6 to 9 K per doubling of carbon dioxide was observed in runs with the additional biome module switched on (Fig. 3). Figure 4: CO2sensitivity of the atmosphere model All sensitivity analyses relate to a (Mid Permian, 270 Ma, paleogegraphy) Mid-Permian (270 Ma) paleo- geography. Climatic zones The numerical model produces a variety of hypothetical numeric data related to different climatic parameters which need to be validated by paleo-climatic observations. Geological data of paleo-climatic studies (e.g. Roscher & Schneider, 2006) on Pangea opens the opportunity to evaluate the model output with respect to its relevance. Due to their origin paleo-climatic data are normally integrated over long periods of time as sediments as well as ecosystems build up over thousands of years. Moreover, geological data as climatic indicative sediments, paleo-botanical as well as faunal paleo-data only contain qualitative information. The comparison of observed climate indicators and hypothetical numeric model output is only possible if model data are transformed to qualitative data. This transformation has been carried out using a simplified method after Köppen and Geiger (cf. Heyer 1993) which produces descriptive and effective information on climatic zones. The Köppen-Geiger classification consists of five main climates (A to E) with further sub- indices describing the precipitation and temperature characteristics of a region. For the purpose of our project we have restricted the climate sub-division to the basic climatic features necessary for a comparison with climate indicators. We subdivide the tropical/ equatorial A climates into ever-wet and seasonal tropics, and the arid B climates into hot and cold desert as well Figure 5. Simplified Köppen-Geiger climate classification of Re- as into hot and cold cent Earth, based on model output. steppe based on precipitation and annual temperature. The temperate C and boreal D climates were subdivided in ever-wet and sea-

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sonal biomes, and the polar E regions were sub-classified into areas of tundra and polar frost. This simplified Köppen-Geiger approach with twelve climate zone was tested with model data of a present-day world (sea-ice and ocean module switched on, but no biome module). The results (Fig. 5) are close to the original classification based on observational data (Kottek et al. 2006) giving hope that modeling a Late Paleozoic world might as well pro- vide realistic results.

Climatic indicators In order to validate the modeled climatic data of our Late Paleozoic reconstructions a comparison between model data and indicator sediments was performed. Eight climate indicators have been chosen which comprise coals/peat, oil-source rocks, aeolian sands, evaporites, reefs, phosphorites, cherts and tillites. Information on their paleo-distribution can be ex- tracted from the database of the GEON PaleoIntegration Project (2005) at www.geongrid.org. Coals and peat are restricted to regions with a high bio-productivity in humid or semi-humid environments. These conditions are accomplished in the ever-wet tropics and the boreal climatic belt. The seasonality under boreal climatic conditions does not affect the accumulation of coaly biomass because the low temperatures, especially during winter slow down biologi- cal processes, and the decomposition of organic carbon does not overexceed burial. So the occurrence of coals indicates a rather humid climate with only short dry seasons. Terrestrial oil-source rocks i.e. lake deposits indicate very similar humid climate conditions like coals and peat. Additionally, marine oil-source rocks indicate ocean regions of high organic production and burial which are more or less restricted to upwelling regions of continental shelfs where cold deep-water, rich in nutrients, emerges to the photic zone. Coastal upwelling due to alongshore winds is caused by the Coriolis Effect. In the Northern Hemi- sphere wind-driven currents tend to be driven to the right of the winds and to the left of the winds in the Southern Hemisphere. Therefore surface waters are driven away from the coasts by Ekman transport and replaced by denser waters from below. The present-day occurrence of coral reefs is restricted to the tropics, they also indicate regions with a low clastic input from the continents. The limiting factors of coral growth are min- imum temperatures and annual temperature variability. Aeolian sands are not as unambiguous as the other paleo-climatic indicative sediments. Their occurrence points to regions which are at least temporarily dry enough to allow winds to transport sand. This is possible in nearly every coastal environment, but large dune fields are commonly associated with desert environments. Aeolian sediments lack temperature information, although, they normally occur in hot environments. Evaporites indicate an overall lack of water. The annual potential evaporation is higher than the yearly amount of precipitation but for the transport of dissolved salts water is essential. These conditions are restricted to semi-arid and arid regions whereas in hyperarid areas the availability of water as the medium of transport is too low. Chert deposits of the marine environment indicate cold water. In these settings the primary production of carbonates is reduced and in relation to that the siliceous producers prevail. Upwelling regions and cold climates can be discriminated by the co-occurrence of phosphorites. The occurrence of tillites is restricted to glacial regions, and comprises alpine type glaciers as well as icecaps. A distinction between both settings is not possible by the occurrence tillites alone, but in the context with other geological data.

Model validation To validate the model output a transformation of the computational data into the simplified Köppen-Geiger climate classification was performed. The resulting paleo-climatic maps were blended with available information on the regional occurrence of eight types of geological climate indicators (database of the GEON PaleoIntegration Project, 2005 at www.geongrid.org). In the following, we present three validation examples. The first describes Late Permian

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(Lopingian, 255 Ma) times (Fig. 6), and the second depicts the Mid-Permian (270 Ma) with aggregated paleo-data of Guadalupian and Late Cisuralian times (Fig. 7). The database information on Early to Middle Cisuralian deposits were used for the validation of the Early Permian (290 Ma) model output (Fig. 8). The model experiments are based on the reconstructed values of atmospheric CO2 concentration and paleo-insolation values given in Tab. 1). The Late Permian (Lopingian, 255 Ma, Fig. 6) coal/peat occurrences of the Siberian plate and North China block are located in the calculated northern boreal belt, and the East Afri- can, Indian and Australian coal/peat occurrences are situated in the southern boreal belt. Oil- source rocks match with seasonal boreal climate conditions, as well as tropical conditions on the South China block. The Permian maverick source rocks in the Southern Permian Basin are linked to the special conditions of the huge epicontinental sea of the Zechstein whereas the sediments on the Tarim block (western China, see also Fig. 10c) are assigned to upwelling regions because they are accompanied by phosphorites and chert deposits in a subtropi- cal climate. A second upwelling location can be assumed on the Indian plate. Tillites are reported only at a single location in southern India suggesting local mountain glaciation. The distribution of evaporites and aeolian sands match the calculated arid zones.

Figure 6. A comparison of the distribution of Late Permian (255 Ma) climate indicative deposits and modeled climate classification (plate tectonic boundaries is also shown). Arrows indicate direction and strength of mean wind.

The Mid-Permian (270 Ma, Fig. 7) coal deposits in the north are situated in the modeled boreal zone. On Gondwana, the calculated tundra like biome covers areas where coal deposits have been observed. This indicates that the Mid-Permian model experiment produced lower than expected temperatures on southern Gondwana, and suggests that further adjustments of the model are needed. All other indicators, reefs and tropical coals in the equatorial belt, the evaporites and aeolian sands in the arid regions fit to the calculated climate conditions. The position of the upwelling locations differs slightly from the Late Permian setting. At the western border of Laurentia a coast parallel occurrence of cherts and phosphorites indicate upwelling with associated oil-source rocks. Upwelling occurred also on the South China

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block, and might indicate a major reorganization of the global ocean current during the Latest Permian where upwelling was restricted to the Tarim block.

Figure 7. A comparison of the distribution of Mid-Permian (270 Ma) climate indicative deposits and modeled climate classification (plate tectonic boundaries is also shown). Arrows indicate direction and strength of mean wind.

Figure 8. A comparison of the distribution of Mid-Permian (270 Ma) climate indicative depos-

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its and modeled climate classification (plate tectonic boundaries is also shown). Arrows indicate direction and strength of mean wind. The climatic conditions during Early Permian (290 Ma, Fig. 8) are very similar to those of Mid-Permian times. In addition, the occurrence of coals in the equatorial belt matches the calculated paleo-tropics. Calculated cold climates in the southern part of Gondwana are supported by the distribution of tillites. The interfingering with coal deposits furthermore suggests that these tilites have been formed by montanotype glaciers as proposed by Isbell et al. (2003).

Paleo-wind systems The modeled circulations of the lowermost atmospheric level have been used to de- scribe the paleo-directions of mean and maximum winds. For the Southern Permian Basin the calculated mean wind directions fit well to a measured and published NE paleo-wind direction during Mid-Permian (270 Ma) times (Ellenberg et al. 1976, Glennie 1983). Field observations often show a bipolar distribution of paleo-wind indicators which can be attributed to chang- ing wind directions. The bipolar state is interpreted to be associated with long lasting mean winds and short lasting but in- tense maximum winds. For the Southern Permian Basin, transcribed to a present- day orientation of the continents, a SSE wind direction is modeled for the maximum wind (comp. Fig. 9) which could partially overprint mean wind indicators of a modeled and measured NE direction. For marine environments the direction of the mean wind has a significant imprint on oceanic surface currents. This is shown in Fig. 10. We are able to show that the model results fit well to geologic observations of climate indicators. Therefore, model results could as well be used to identify regions of paleo-upwelling potential and/or regions with hydrocarbon potential. Fig. 10 shows upwelling associated oil source rocks of western North America (a) and of the Chi- nese Tarim block (c) as well as potential upwelling regions in western South Ameri- ca (b) and western North Australia (d). The Figure 9. Wind directions of the South Per- oil source rocks in southern North America mian Basin during Mid-Permian (270 Ma) demonstrate the hydrocarbon potential times. upper Figure: arrows indicate direction around the remnant of the Rheic Ocean of mean wind. lower Figure: arrows indicate and we therefore suggest that under similar direction of maximum wind. climatic conditions one can as well expect the deposition of similar sediments in the northwest of South America (Fig. 10e).

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Figure 10. Examples of regions with known oil source rocks (dark drops) and regions with suggested hydrocarbon potential (white drops). Left: Mid-Permian upwelling areas due to coast parallel winds of western North America (a) and regions of suggested hydrocarbon potential in western South America (b). Middle: Late-Permian upwelling areas on the Chi- nese Tarim block due to offshore winds (c) and Early Permian regions of suggested hydrocarbon potential in eastern North Australia (d). Right: proven (north) and suggested (south) hydrocarbon potentials around the Early Permian Rheic Ocean.

Conclusions Numeric climate modeling is used as a tool for the assessment of the paleo-distribution of organic rich deposits and reservoir rocks. This modeling approach is based on new ideas concerning the formation of the Pangea supercontinent. Our new approach is based on the reconstruction of relative plate motions. The resulting plate arrangements have been opti- mized on the basis of paleo-climatic data. Published paleo-magnetic data support the new paleo-geographic concept. Numeric climate modeling was used to simulate the distribution of paleo-climatic belts which refer to the paleo-geography. Input data of PLASIM, a medium complexity model, requires a reconstructed paleo-topography which differs significantly from former approaches as our Pangaean paleo-geography does not contain a high mountain chain across the paleo- continent (Roscher & Schneider, 2006). Six Permocarboniferous (340, 320, 300, 290, 270, 255 Ma) paleo-geographic maps were transformed into 2.8°x2.8° (T42) digital grids and used as the base for the modeling. The model was driven by predefined CO2 concentrations of the atmosphere and insolation values, derived from published methods and measurements. The model was validated by a comparison of modeled numeric values with qualitative geological indicators. For this purpose the model output was converted into a descriptive and effective climate classification, according to Köppen and Geiger. The originally detailed sub- classification was simplified resulting in twelve climate zones. As geological indicators coals/peat, oil-source rocks, aeolian sands, evaporites, reefs, phosphorites, cherts and tillites were chosen. Their distribution fit well to the modeled paleo-environments. The spatial ar- rangement of the paleo-climatic belts as well as the climatic evolution for several regions matches the geologic record. The results presented here, show the potential of climate modeling to predict regions of hydrocarbon source rocks as well as reservoir rocks. The present findings encourage us to proceed with paleo-climate modeling using either re- fined versions of the PLASIM and/or experiment with climate models of higher complexity and resolution which also allow for detailed ocean circulation modeling in order to improve the quality of the predictions.

DGMK-Tagungsbericht 2008-1, ISBN 978-3-936418-79-8 338 DGMK/ÖGEW-Frühjahrstagung 2008, Fachbereich Aufsuchung und Gewinnung, Celle

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DGMK-Tagungsbericht 2008-1, ISBN 978-3-936418-79-8 339