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