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Liquid hydrocarbon characterization of the lacustrine Yanchang Formation, Ordos Basin, China; organic-matter source variation and thermal maturity

Xun Sun, Liang Quansheng, Jiang Chengfu, Daniel Enriquez, Tongwei Zhang and Paul C. Hackley
Liquid hydrocarbon characterization of the lacustrine Yanchang Formation, Ordos Basin, China; organic-matter source variation and thermal maturity
Interpretation (Tulsa) (May 2017) 5 (2): SF225-SF242

Abstract

Source-rock samples from the Upper Triassic Yanchang Formation in the Ordos Basin of China were geochemically characterized to determine variations in depositional environments, organic-matter (OM) source, and thermal maturity. Total organic carbon (TOC) content varies from 4 wt% to 10 wt% in the Chang 7, Chang 8, and Chang 9 members - the three OM-rich shale intervals. The Chang 7 has the highest TOC and hydrogen index values, and it is considered the best source rock in the formation. Geochemical evidence indicates that the main sources of OM in the Yanchang Formation are freshwater lacustrine phytoplanktons, aquatic macrophytes, aquatic organisms, and land plants deposited under a weakly reducing to suboxic depositional environment. The elevated C (sub 29) sterane concentration and depleted delta (super 13) C values of OM in the middle of the Chang 7 may indicate the presence of freshwater cyanobacteria blooms that corresponds to a period of maximum lake expansion. The OM deposited in deeper parts of the lake is dominated by oil-prone type I or type II kerogen or a mixture of both. The OM deposited in shallower settings is characterized by increased terrestrial input with a mixture of types II and III kerogen. These source rocks are in the oil window, with maturity increasing with burial depth. The measured solid-bitumen reflectance and calculated vitrinite reflectance from the temperature at maximum release of hydrocarbons occurs during Rock-Eval pyrolysis (T (sub max) ) and the methylphenanthrene index (MPI-1) chemical maturity parameters range from 0.8 to 1.05%/R (sub o) Because the thermal labilities of OM are associated with the kerogen type, the required thermal stress for oil generation from types I and II mixed kerogen has a higher and narrower range of temperature for hydrocarbon generation than that of OM dominated by type II kerogen or types II and III mixed kerogen deposited in the prodelta and delta front.


ISSN: 2324-8858
EISSN: 2324-8866
Serial Title: Interpretation (Tulsa)
Serial Volume: 5
Serial Issue: 2
Title: Liquid hydrocarbon characterization of the lacustrine Yanchang Formation, Ordos Basin, China; organic-matter source variation and thermal maturity
Affiliation: University of Texas at Austin, Bureau of Economic Geology, Austin, TX, United States
Pages: SF225-SF242
Published: 201705
Text Language: English
Publisher: Society of Exploration Geophysicists, Tulsa, OK, United States
References: 70
Accession Number: 2017-051942
Categories: Economic geology, geology of energy sourcesIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. cols., 3 tables, sketch map
Secondary Affiliation: Shaanxi Yanchang Petroleum Company, Research Institute, CHN, ChinaU. S. Geological Survey, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by Society of Exploration Geophysicists, Tulsa, OK, United States
Update Code: 201728
Program Name: USGSOPNon-USGS publications with USGS authors
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