Sedimentary Basins and Petroleum Occurrences, Mongolia

NEMEC, M. C., M. M. PAGE, and W. C. PENTTILA, Exploration Associates International of Texas, Inc., Houston, TX

Mongolia is situated on the southern margin of the Siberian craton. Successive tectonostratigraphic terrain accreted to the Laurasian, Siberian craton during the proto-Tethys, paleo-Tethys, and neo-Tethys. These terrains have been described as micro-plates made up of continental, ocean basin and volcanic-arc fragments docking onto the Siberian craton. The first accretionary terrain north of Mongolia is the Late Proterozoic, Baykalian. In Northern Mongolia, the terrain involves Caledonian accretion of Devonian and younger sediments. In Southern Mongolia, the terrain is made up of the Hercynian accretion of Permian and younger sediments.

Extensive, post-accretionary rifting developed during extensional periods of opening and closing of paleo-Tethys and neo-Tethys. Rift and graben deformation was initiated in the Upper Jurassic. Rift-fill included conglomerates, breccias, and volcanics. During the Lower Cretaceous, continental deposits with lacustrine clastics continued as the rift-fill sequence. The Upper Cretaceous continental sediments are related to the wrench-rifting, compressive phase.

Geophysical data (gravity, magnetics, and seismic) are used with surface, regional geology, and subsurface well data to illustrate the Mesozoic rift basins' geometry, the thickness of sediments, and the existing type of hydrocarbon traps.

The Upper Jurassic-Lower Cretaceous shale-sandstone sequence contains the known source rocks and reservoir rocks in the southeastern and eastern part of Mongolia. These rocks are described and their structural and stratigraphic relationship are illustrated.

The known oil occurrences are surface oil seeps, impregnated outcrops, oil shows in core holes and in samples, and cores in exploration wells. The two oil fields that have been discovered in Southeastern Mongolia, Tsagaan-Els and Zuunbayan, are described.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)