New Core-Based Evidence for Mesozoic Petroleum Systems in Southwest Mongolia
Active petroleum systems are relatively well known and developed in eastern Mongolia (e.g., Hailar-Tamsag and East Gobi basins), where late Mesozoic rift basins preserve mainly synrift petroleum systems components including: Lower Cretaceous lacustrine source rocks (and intraformational seals) at generally early oil window maturity levels, and Upper Jurassic-Lower Cretaceous perilacustrine fluvial and deltaic reservoirs. Late synrift generation and migration is largely assumed, with structural traps associated with Late Cretaceous-Recent basin inversion and strike slip faulting. These known accumulations are quite similar to those of other well known basins in eastern China, including Songliao (Daqing oil field) and Erlian basins. To date, comparatively little is known about how far west into Mongolia analogous hydrocarbon accumulations may exist. New seismic reflection profiles from the Gobi-Altai region prove the existence of Mesozoic rifts by imaging listric normal faults and horst and graben features. A remarkable core hole from this region recently recovered ~1600 continuous meters of synrift to postrift strata and 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. Six main lithofacies were documented: 1) Stratified lacustrine, possibly with anoxic bottom water conditions; 2) Poorly stratified (oxygenated) lacustrine; 3) Sublacustrine fan (proximal?): mass transport deposits; 4) Sublacustrine fan (distal?): turbidites; 5) Subaerial environments including possibly delta and alluvial fans: distributary (fluvial) channels; 6) Subaerial floodplain and interdistributary deposits: paleosols (generally poorly developed). Very promising source and potential reservoir intervals are documented. Ties to correlative outcrop sections underscore that overall, this basin appears to share a similar stratigraphic evolution to known petroleum basins in eastern Mongolia and China. Ongoing studies investigate some interesting apparent contrasts to these other basins, including a possible older age of the synrift section (Jurassic?), comparatively high maturity, and younger (Cenozoic) inversion structures.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California