TSK 11 Göttingen 2006 Davaa et al. Evolution of the Tamtsag Basin / NE-Mongolia — part II: structure and hydrocar- bon potential Poster Buyan Davaa1 Peter Geerdts1 Marc Vogler1 Andreas Henk1 Introduction The Tamtsag basin in NE Mongolia is part of a widespread basin system which formed during Late Jurassic and Cre- taceous times (Graham et al. 2001, Qing-Ren et al. 2003). It is filled with continental sediments and vol- canics which can reach up to 4 km in thickness. Rifting and subsequent basin inversion led to a complex basin ge- ometry characterized by several horst and graben structures. The geody- namic causes for regional basin forma- tion are discussed controversially and several hypothesis ranging from oro- genic collapse via subduction rollback to collision-induced rifting have been put forward. Scientific research on the Mesozoic basins in Mongolia has so far concen- trated on the East Gobi basin to the southwest (Graham et al. 2001, Prost 2004, Johnson et al. 2004) and some work has also been published on the Hailar Basin (Qing-Ren et al. 2003), the northeastward continuation of the Tamtsag Basin into China. Fundamen- tal data on the fill and tectonics of the Tamtsag Basin in between is still miss- ing. This is partly due to poor expo- sure as most of the basin fill is cov- ered by Cenozoic sediments and only locally, near the borders faults, rocks are accessible for surface investigations. 1 Geologisches Institut, Universität Freiburg Albertstraße 23b, D-79104 Freiburg However, recent discoveries of oil in the Tamtsag and Hailar Basins have resulted in intense exploration activity and a strong interest in the area. This contribution describes the results of a field campaign in Fall 2005 fo- cusing on the structure and hydrocar- bon potential while a companion paper (Geerdts et al. this volume) deals with the fill of the Tamtsag Basin. Tectonics The Tamtsag Basin is an approximately 300 km long and 80 km wide ENE- trending fault-controlled structure. The basin architecture is characterized by uplifted fault blocks in the central parts and graben-like structures to the North and South, close to main basin bounding faults. In the nearby East Gobi Basin, Prost (2004) distinguished five struc- tural episodes: (1) pre-Jurassic north- east shortening, (2) Middle Jurassic to Early Cretaceous north-east directed extension, including rifting, (3) late Early Cretaceous north-south shorten- ing, that led to the basin inversion on already existing normal faults, (4) Middle-Cretaceous uplift and erosion, and finally (5) east-west directed short- ening and dextral movement on north- east trending faults. Field work concentrated on the west- ern margin of the basin where suffi- cient outcrops for structural analysis are available. The large-scale structural el- ements, i.e. the major faults, follow the regional ENE trend. Of particular in- terest are ridges composed of Mesozoic magmatic rocks which are uplifted rela- tive to the surrounding Cretaceous sedi- mentary basin fill and strongly resemble positive flower structures. For example, one of the ridges is crosscut by several faults striking 20°, thus forming an angle 1 Davaa et al. TSK 11 Göttingen 2006 Figure 1: Schematic petroleum system chart of the Tamtsag Basin of 40° to the principle basin axis. Most are normal or reverse faults with an ad- ditional strike-slip component. Hydrocarbon potential Oil has recently been found in block XIX of the Tamtsag Basin. Reservoirs are lo- cated in sandstones of the Lower Creta- ceous Tsagaantsav and Zuunbayan For- mations. Source rocks are situated in the same stratigraphic units and total organic carbon contents in the corre- sponding shales reach up to 3%. Trans- pression at the end of the Lower Creta- ceous formed structural traps, e.g. ti- tled fault blocks and anticlinal folding, as well as stratigraphic traps beneath the related unconformity. In order to get a quantitative under- standing of petroleum formation in the Tamtsag Basin the subsidence and ther- mal histories of various wells are mod- elled. This requires a complete recon- struction of the sedimentation and up- lift history, i.e. the thickness and age of the eroded, and hence missing, strata has to be estimated. Using shale com- paction data derived from sonic logs and the method of Magara (1976), respec- tively, it can be found that the rocks of the Tamtsag basin were once buried 350–800m deeper than today. Compari- son with the regional evolution indicates that this maximum burial occurred dur- ing late Lower Cretaceous time prior to deposition of Upper Cretaceous Sayn- shand formation. This burial history forms the basis for maturity modelling using the software Petromod1D of IES, Jülich. The thermal history model is calibrated against observed vitrinite re- flectance data. Modelling results show heating of the source rocks, the criti- cal moment, i.e. the onset of oil mi- gration as well as the time of peak oil generation. For example, the oldest source rocks of the Tsagaantsav forma- tion reach maximum temperatures of up to 136°C during the late Lower Creta- ceous at about 118Mabp. Petroleum systems modelling will be extended to 2D and basin-wide sections to provide quantitative information for future ex- ploration in blocks XX, XXI and XXII. Acknowledgements Financial sup- port by Deutsche Forschungsgemein- schaft is gratefully acknowledged. ‘Min- 2 TSK 11 Göttingen 2006 Davaa et al. eral and Petroleum Authorities of Mon- golia’ kindly supported our work in the Tamtsag Basin. 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