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 Late Jurassic-Early Cretaceous Rifting in the Tugrug and Taatsiin Tsagaan Nuur Basins, Gobi-Altai Region of SW Mongolia — Implications for Petroleum Exploration

Constenius, Kurt N.*1; Coogan, James C.1; Erdenebat, Bolor 1; Tully, Justin 1; Johnson, Cari 3; Graham, Stephan 2; Cunningham, Dickson 4
(1) Petro Matad LLC, Ulaanbaatar, Mongolia.
(2) School of Earth Sciences, Stanford University, Stanford, CA.
(3) Department of Geology and Geophysics, University of Utah, Salt Lake City, UT.
(4) Department of Geology, University of Leicester, Leicester, United Kingdom.

Exploration and drilling in eastern Mongolia’s Tamtsag basin and the Songlio and Hailar basins in adjoining parts of northeast China have established viable petroleum systems in Late Jurassic-Early Cretaceous age rift basins. The tectonic and burial history of these basins is complex in that they have sustained episodes of extension and subsequent contraction. The history of tectonism in the Tamtsag basin is relatively well understood based on an extensive industry database of seismic and borehole data. In contrast, prior to Petro Matad’s 2010-2011 exploration program of seismic, gravity and stratigraphic core drilling, no subsurface data existed in the Gobi-Altai region of southwestern Mongolia. These data combined with field investigations reveal that the Gobi-Altai region sustained considerable rift related extension and magmatism. The Tugurg and Taatsiin Tsagaan Nuur half grabens imaged on seismic reflection data contain up to four kilometers of folded and faulted basin fill. Thicknesses of basin fill were determined using depth conversion velocities derived from long offset refraction shooting and derived from PSDM based reflection tomography. The source-receiver offsets for the refraction spread ranged up to 16 kilometers and the velocity field for the first 1000-1500m of the subsurface was calculated using tomographic inversion processing of first arrivals. Accurate basin fill thickness estimates are critical because vitrinite reflectance and RockEval data from the core and exhumed parts of the Late Jurassic-Early Cretecous rift basins indicate that the source rocks along basin flanks are thermally immature or just at the threshold of oil generation (Average Ro ~ 0.5-0.6). Field studies and core drilling document thick successions of lacustrine and peri-lacustrine strata. Petroleum system elements found in these sequences include: organic rich shales with TOC’s averaging 3-4 percent, with some as high as 6 percent, lacustrine and deltaic sandstone reservoir rocks, and stratigraphic and structural traps. Two discrete structural trap styles are imaged on the seismic reflection data: Late Jurassic-Early Cretaceous fault traps created by rift related normal faulting and Miocene-Recent transpressional folds formed during structural inversion of the half grabens.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California