Print this pageUppermost Triassic-Lowermost Jurassic Lacustrine Shale as a Possible Source Rock Interval for Southern Mongolian Sedimentary Basins
HENDRIX, M. S., T. J. GREENE, A. R. CARROLL, J. M. MOLDOWAN, and S. A. GRAHAM
Recent attempts to evaluate the petroleum potential of Mongolia have been
hampered by a sparsity of data regarding the distribution, facies, and
geochemistry of potential source rook sequences. During the 1992-1995 field
seasons, we studied and sampled outcropping organic-rich strata of various ages
in order to assess their petroleum source rock potential. Pre-Mesozoic
(principally Carboniferous-Permian) source intervals have proven overmature,
with average Ro = 5.11 %. Though widely distributed across Mongolia, Lower and
Middle Jurassic coal and associated organic-rich shale is generally dominated by
vitrinitic and inertinitic macerals and Type III kerogen and is likely
gas-prone. Lower Cretaceous rift-related lacustrine strata have been postulated
to be a significant source interval in southern Mongolia, largely by analogy
with adjacent basins in north central China. In this study, we present
sedimentary and geochemical data regarding a previously unrecognized, important
source rock of latest Triassic - earliest Jurassic age. Specifically, approx.
200 m of uppermost Triassic to lowermost Jurassic laminated lacustrine shale
crops out in the core of the No-yon Uul syncline in south central Mongolia.
Noyon Uul shale is characterized by abundant soft-sediment deformation, dolomite
concretions, and local siltstone inter-beds. TOC values average 3.09% (range
2.68-3.48%), H I values average 580 (range 497-649), and Ro values average
0.46%. Visual kerogen analysis indicates a dominance of algal macerals with
subsidiary input from terrestrial sources. Importantly, the Noyon Uul shale
displays a striking geochemical similarity with oils from Mongolia's only
producing field at Zuumbayan. Saturate fractions both from extracted Noyon Uul
shale and from Zuumbayan oils are characterized by Pr/Ph approx. 1, slight odd
over-even n-alkane preference, dominance of C[19]-C[22] alkanes, rapid fall off
of n-alkane peak heights >n-C[22], and presence of gamma-cerane and
beta
-carotane. Independent regional basin studies conducted by our group suggest
that Noyon Uul shales are part of a widespread, if discontinuous, collisional
foreland basin system which stretched across much of southern Mongolia and
adjacent north central China in the Triassic and Early Jurassic. Thicknesses of
Jurassic and younger strata estimated from subsurface petroleum industry data
from late Mesozoic extensional basins of southeastern Mongolia appear sufficient
to render strata of early Mesozoic age mature or overmature, thereby
significantly raising the prospectivity of these basins.