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ABSTRACT: Mongolia, Tamtsag Basin, evidence for widespread, high quality, mature Lower Cretaceous Source Rock

Neves, Roger1, Duncan McLean1, Raymond H. Bate2, William C. Penttila3 , (1) Sheffield University, N/A, United Kingdom (2) Lacustrine Basin Research Limited, Ansty, West Sussex, United Kingdom (3) Exploration Associates, Inc, N/A

The Tamtsag Basin in eastern Mongolia contains a Lower Cretaceous continental clastic rift-fill. The Berriasian to Valanginian early rifting sequence consists of immature fluvio-deltaic conglomerates and sandstones of the Tsagaantsav Formation. This fines upwards into deep water lacustrine mudstones and oil shales of the Hauterivian to Barremian Lower Zuunbayan Formation. These source rocks onlap fault-bounded basement highs and are widespread across the basin. The final stage of infill consists of the Upper Zuunbayan Formation, an Aptian to Albian fluvial and lacustrine mudstone-sandstone facies sequence in which emergent coal mire facies are developed. Late Aptian intra-basin fault reactivation is evidenced by local development of fanglomerates in the Upper Zuunbayan.

High TOC values in the Upper Tsagaantsav and Lower Zuunbayan are correlated with algal concentrations in sapropelic lacustrine mudstones. Peaks in concentrations of the euryhaline dinoflagellate Vesperopsis fragilis are correlated with gamma-log signatures allowing basin-wide source rock correlation and providing an indicator of source rock thickness and quality. TAI and reflectance values indicate the Lower Cretaceous source rocks are mature, lying within the peak oil generation window. Gas-prone clastics and coals in the Upper Zuunbayan Series lie within the early oil-early gas generation phase.

Detailed correlation of source rocks and potential reservoir sequences in 12 recent exploration wells in the Tamtsag Basin is achieved by the recognition of a sequence of palynostratigraphic events. Terrestrial microfloras are comparable to those from northern China (Songliao Basin), Siberia, Europe and western Canada.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia