TECTONIC AND STRATIGRAPHIC EVOLUTION OF THE MONGOLIAN PEOPLES-REPUBLIC AND ITS INFLUENCE ON HYDROCARBON GEOLOGY AND POTENTIAL

The stratigraphy of Mongolia can be divided into five 'megasequences', each characterized by a common tectonic and stratigraphic history. The earliest, megasequence 1 (Precambrian-Silurian), records the amalgamation of Precambrian-Early Palaeozoic passive and active margin terranes and the resulting growth of a 'Caledonian' fold belt in western Mongolia. Megasequence 2 (Devonian-Permian) appears to be the most complex, with a variety of active margin collisions in southern and eastern Mongolia, and the continued deformation of the Caledonian fold belt in the west. A substantial fold belt, underpinned by a variety of granitic plutons, existed across Mongolia by the Triassic. There was widespread erosion and limited deposition in small foreland and intermontane basins: megasequence 3 (Triassic-Early Jurassic). Megasequence 4 (Mid-Jurassic-Cretaceous) records extension across the partially eroded fold belt, with deposition confined to a series of alluvial and lacustrine rift basins. These basins later experienced transpression, inversion and erosion followed by onlap and overstep, giving more widespread sedimentation from large post-rift alluvial systems during the Late Cretaceous and Tertiary (megasequence 5). Megasequences 1-3 are usually extensively folded. Megasequences 4 and 5 have been involved in transpressive inversion, and Mongolia is an area of present day compressive and transpressive tectonics, erosion and general uplift. The Late Jurassic-Early Cretaceous sequences (megasequence 4) are the most prospective for finding extractable hydrocarbons. Oil has been successfully produced from megasequence 4 alluvial and lacustrine rift sequences with rich, oil-prone lacustrine mudrocks which can form both source and seal facies. Interbedded sandstones, siltstones and mudstones form various combinations of reservoirs and seals. A large number of hydrocarbon trap types are yet to be tested. Past drilling has been concentrated around surface seeps and on simple structures that can be mapped at the surface, with usually only shallow tests (< 1200 m). Mongolia remains an underexplored frontier area and there can be no doubt that Mongolia contains yet to be developed and yet to be discovered petroleum.