Subsurface Evidence for Late Mesozoic Extension in Western Mongolia: Tectonic and Petroleum Systems Implications

The late Mesozoic extensional province of eastern China and Mongolia continued west into the Gobi Altai region of western Mongolia, based on new seismic data, core evaluation, and recent outcrop studies. New seismic reflection profiles from the Tugrug basin of western Mongolia (petroleum block 5) demonstrate the existence of preserved late Mesozoic extensional basins by imaging listric normal faults, syn-rift growth strata, and partially inverted grabens. Several new core holes from this region include one core of ~1600 continuous meters of Upper Jurassic - Lower Cretaceous (Kimmeridgian-Berriasian) syn-rift and Tertiary post-rift strata overlying Late Triassic volcanic basement. The cored stratigraphic section is dominated by lacustrine and marginal lacustrine deposition ranging from stratified lacustrine to subaqueous fan and delta to subaerial alluvial-fluvial environments. These units are located on the flexural margin of a partially inverted graben with a strongly folded and reactivated master listric normal fault. Strike-slip faulting certainly postdates extension as documented by late Cenozoic transpression, however late Mesozoic basins may also have experienced a transtensional component. Prospective petroleum source and reservoir intervals are documented, as well as multiple unconformities representing pre-, syn- and post-rift basin evolution. Ties to correlative outcrop sections underscore that in general, this basin appears to share a similar tectono-stratigraphic evolution with other petroliferous basins in eastern Mongolia and China. Nevertheless, some interesting apparent contrasts to these other basins are noted, including comparatively high source rock maturity, distinct sandstone provenance, and younger (Cenozoic) inversion structures. The Tugrug basin and related areas in the Gobi Altai occupy an important paleogeographic position between late Mesozoic contractile and extensional provinces, and could represent an accommodation zone, perhaps with a Mesozoic strike-slip component, and/or a rapid temporal shift from orogenic thickening to gravitational collapse in the Late Jurassic. Key conventional petroleum systems components are documented, particularly source and reservoir facies and the likelihood of thermal maturity, with implications for future exploration in the region.