--> Abstract: Molecular Organic Geochemical Identification of Eight Different Genetic Groups of Oils/Condensates from the Tarim Basin, NW China, by A. D. Hanson; #90937 (1998)

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Abstract: Molecular Organic Geochemical Identification of Eight Different Genetic Groups of Oils/Condensates from the Tarim Basin, NW China

HANSON, ANDREW D., Stanford University

A suite of 40 oils, condensates, and extracts of purported source rocks collected from throughout the Tarim basin, NW China, were analyzed using standard and molecular organic geochemical methods. Analytical methods included whole oil carbon isotopes, sulfur content, gas chromatography (both nC5-nC11 and nC12-nC40+ ranges), gas chromatography/mass spectrometry of saturate and aromatic fractions, and gas chromatography/mass spectrometry/mass spectrometry of selected samples. The resulting data set allows separation of the suite of oils and condensates into two large sets which can be subdivided into at least eight different genetic groups (Groups 1a-d and 2a-d).

Group 1a samples (from the Tazhong and southern Tabei areas) have light Carbon isotopic values (-31.1 to -34.5), pristane/phytane (Pr/Ph) ratios of 0.85-1.4, high tricyclic terpane/hopane ratios, contain gammacerane, and have low abundances of higher n-alkanes. Other Group 1 samples generally share these traits but have the following characteristics that distinguish them: Group 1b (from Tazhong) has distinctly low C27 steranes; Group 1c (from Tazhong) contains beta-carotane; and Group 1d (from the Bachu area) has high diasterane/steranes. All Group 1 oils are believed to be sourced from Ordovician marine strata.

Group 2a samples (from northern Tabei) have heavy Carbon isotopic values (-25.3 to -30.2), have Pr/Ph >2, have greater preservation of the higher n-alkanes, have low tricyclic terpane/hopane, and have poor preservation of C35 homohopanes. Other Group 2 samples generally share these traits but have the following characteristics that distinguish them: Group 2b (from Bachu area) has distinctly low C27 steranes and contains beta-carotane; Group 2c (from northern Kuqa Depression) has Pr/Ph=4.4, low C27 steranes, and high C19 tricyclic and C24 tetracyclic terpanes; and Group 2d (from the Kekeya area) has high diahopane/norhopane. Group 2 oils potentially are sourced from Jurassic nonmarine strata.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah