--> ABSTRACT: Uppermost Triassic-Lowermost Jurassic Lacustrine Shale as a Possible Source Rock Interval for Southern Mongolian Sedimentary Basins, by M. S. Hendrix, T. J. Greene, A. R. Carroll, J. M. Moldowan, and S. A. Graham; #91021 (2010)

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Uppermost 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.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.