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Organic Geochemical and Petrographical Characterization of Continental Triassic Source Rocks (Madygen Basin, Southern Tien Shan, Kyrgyzstan)

Berner, Ulrich 1; Scheeder, Georg 1; Kus, Jolanta 1; Voigt, Sebastian 2; Schneider, Joerg 2
1 Petroleum and Coal Geochemistry, Fed. Inst. of Geosciences and Natural Resources, Hannover, Germany.
2 Palaeontology, Geological Institute, TU Bergakademie Freiberg, Freiberg, Germany.

Along the northern foothills of the Turkestan-Alai Range (SW Kyrgyzstan) outcrops of a 1000 to 1500 m thick succession of Mesozoic sediments document regional changes of the paleo-landscape from small upland basins during Triassic times to near-shore marine environments of the Cretaceous. Due to the paleo-geographic position of Central Asia during the Early Mesozoic the investigated area has to be considered as a point of intersection of the North American-European, Sibirian, and SE Asian faunal and floral provinces. As an all-time tectonically active region tectonic movements have changed the sedimentary properties through subsidence and uplift and have likely led to a thermal alteration of the sedimentary organic matter.

Organic petrographical and organic geochemical methods (the latter including RockEval, biomarker and stable isotope analyses) were applied to a suite of terrestrial sedimentary rocks from the Triassic Madygen Basin with the intention to identify the depositional environment. Special attention was paid to the assessment of the hydrocarbon potential and source rock maturity and to the identification of secondary alteration processes of the sedimentary organic matter.

Microscopic analyses indicate that the sedimentary organic matter is dominated by land plant material (inertodetrinite, detrovitrinite, vitrinite, cutinite, sporinite), but also contains smaller amounts of aquatic material (alginite) which suggests a deposition in swamp-like and/or lacustrine environments. Vitrinite reflectance data show that the sediments have undergone thermal alteration processes, which, however, are limited to a narrow maturation spectrum from immature to the early oil window.

The inspection of organic geochemical parameters supports the microscopic analyses and hydrocarbon extracts indicate mixtures related to varying contributions of higher land plants and aquatic organic matter. Biomarker investigations suggest that the sediments where deposited in swamps or lacustrine environments. Molecular maturity parameters indicate that the hydrocarbons come from a narrow maturity range, which corresponds to the initial stages of hydrocarbon generation, and compares well with the organic petrographic observations.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009