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Accueil du site → Doctorat → Allemagne → 2010 → Geodynamic development and hydrocarbon potential of the Tamtsag Basin, Eastern Mongolia

Albert-Ludwigs-Universität Freiburg im Breisgau (2010)

Geodynamic development and hydrocarbon potential of the Tamtsag Basin, Eastern Mongolia

Davaa, Buyan-Arivjikh

Titre : Geodynamic development and hydrocarbon potential of the Tamtsag Basin, Eastern Mongolia

Geodynamische Entwicklung und Kohlenwasserstoffpotenzial des Tamtsag Beckens in der östlichen Mongolei

Auteur : Davaa, Buyan-Arivjikh

Université de soutenance : Albert-Ludwigs-Universität Freiburg im Breisgau

Grade : Doktorgrades der Fakultät für Chemie, Pharmazie und Geowissenschafte 2010

Résumé
The Tamtsag Basin in easternmost Mongolia was formed by rift extension occurring during the Late Jurassic and Early Cretaceous. It is filled with continental sediments and volcanics that can reach up to 4 km in thickness. Rifting and subsequent basin inversion led to a complex basin geometry characterized by several horst and graben structures. Based on published works on the subject of geological research in eastern and southeastern Mongolia, the chrono- and lithostratigraphy of the Tamtsag Basin has been described in this study. The results were verified by means of detailed geological mapping carried out on the northwestern edge of the basin. Outcrops of Early Palaeozoic to Late Jurassic magmatic rocks are distributed on the Baruun-Urt, Matad, Numrug and Nukhet Davaa highs that surround the basin in the north, east and south. Pre-rift Palaeozoic basement rock consists of marine siliciclastics, carbonates, volcanics, volcanoclastics and slightly to highly metamorphosed rocks. They are exposed widely at higher levels. Seven chronostratigraphic units have been distinguished for the Mesozoic basin fillings : the Khamarkhoovor, Sharalyn, Tsagaantsav, Lower and Upper Zuunbayan, Sainshand and Bayanshiree. The largest part of the basin is covered by thin unconsolidated sediments of recent age. Basalt flows originating from the Dariganga volcanic plateau which restricts the basin in the southwest partially covers the southwestern section of the basin. Moreover, there are widespread crops of volcanic rocks of the Upper Triassic to Valanginian of the Early Cretaceous on the Baruun-Urt and Matad highs. In addition, the basin floor is covered by large volumes of those rocks, as evidenced by well data. Sediments of the Early Cretaceous Tsagaantsav Formation were deposited in alluvial fan, fluvial and shallow lake depositional environments. Although, the reservoir rocks in the Tsagaantsav Formation are qualitatively poor, due to large amount of brittle volcanic materials in their lithology, reservoir rock of higher quality was found locally, where porosity was preserved. The Tsagaantsav Formation is non-conformly covered by the Lower Cretaceous Lower Zuunbayan Formation. It consists of low energy clay sediments deposited in a deep lake with anoxic conditions at the bottom. This organic rich shale contains kerogen type II. Furthermore, it can be observed in the rock column that the deep lake depositional environment subsequently changed to shallow lake and fluvial, in which the sediments of the Upper Zuunbayan Formation were deposited. They contain mainly kerogen prone to gas generation. Further up in the profile, the Upper Zuunbayan Formation is non-conformly covered by high energy sediments deposited in an alluvial fan and braided river. Relatively thin, up to 200 meter thick sandstones and conglomerates determine the Sainshand and Bayanshiree formations. Early and Middle Triassic strata and sediments of the second half of the Upper Cretaceous are missing completely in the Tamtsag Basin. The Tamtsag Basin consists of a total of eight sub-basins located in two parallel rows. Their extension passes with structural grain of a Palaeozoic basement. The basin is divided into eastern and western parts by an optional line drawn on the intra-basinal high between the Sangiin Dalai Lake sub-basin and the Zurkh Khamar-Bayangobi sub-basin on the country’s border. This study is focused on the geometric structure of the western rim, which includes the Ar Bulag, Shar Bogi, Mogoit, Sangiin Dalai Lake and Zagiin Khovoo Lake sub-basins. Rotated blocks indicative of extensional tectonic regime, flower structures caused by strike slip movement and faulted and folded structures related to deformation occur abundantly in those sub-basins. Each sub-basin represents its own specific structure reflecting its geodynamic history. In general, those basins developed in an extensional regime of the rifting, although the Sangiin Dalai Lake and the Shar Bogi sub-basins would appear to be a pull-apart basin developed on the transitional zone of a strike slip fault.

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