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Geochemical evidence for oil and gas expulsion in Triassic lacustrine organic-rich mudstone, Ordos Basin, China

Tongwei Zhang, Wang Xiangzeng, Zhang Jianfeng, Xun Sun, Kitty L. Milliken, Stephen C. Ruppel and Daniel Enriquez
Geochemical evidence for oil and gas expulsion in Triassic lacustrine organic-rich mudstone, Ordos Basin, China
Interpretation (Tulsa) (May 2017) 5 (2): SF41-SF61

Abstract

Forty-six core samples were collected from a deep well that penetrated organic-rich layers of the Chang 7, 8, and 9 members of the Yanchang Formation (Fm) in the Ordos Basin. Tests for total organic content (TOC), Rock-Eval pyrolysis, X-ray diffraction (XRD) mineralogy, and molecular composition of gases released from rock crushing were conducted. Analytical results indicate that TOC and clay contents are elevated. The organic matter (OM)-rich mudstone in the Triassic Yanchang Fm suggests good-to-excellent source potential for oil generation. Its thermal maturity is in the oil window. Strong petroleum expulsion occurred from the upper part of the approximately 13 m (42.6 ft) thick Chang 7 member, and for the Chang 8 and Chang 9 members, resulting in low free oil and low methane (CH (sub 4) ) concentration in these OM-rich intervals. A combination of sandstone and thin organic-rich mudstone layers is a perfect hybrid lithology stacking pattern for petroleum expulsion. The thickness for effective source rock, approximately 10-12 m (32.8-39.3 ft), varied with sandstone/mudstone lithology stacking pattern. In contrast, limited or no oil expulsion occurred in the lower part of Chang 7 member, a 25 m (82 ft) thick organic-rich interval, which is indicated by high free oil and high CH (sub 4) concentration. A CH (sub 4) -TOC plot can be used to differentiate generated gas, retained gas in OM-rich mudstones, and migrated gas in permeable sandstone beds. We have developed a conceptual model for petroleum expulsion from OM-rich thin versus OM-rich thick layers. Compaction and thermal volume expansion of oil generated from OM may play an important role in petroleum expulsion in OM-rich mudstones. The estimated petroleum expulsion efficiency is approximately 70% and 35% for thin and thick OM-rich mudstone layers, respectively. The redistributed OM in clay-dominated rock assemblage likely forms the preferred migration path to petroleum expulsion.


ISSN: 2324-8858
EISSN: 2324-8866
Serial Title: Interpretation (Tulsa)
Serial Volume: 5
Serial Issue: 2
Title: Geochemical evidence for oil and gas expulsion in Triassic lacustrine organic-rich mudstone, Ordos Basin, China
Affiliation: Bureau of Economic Geology, Austin, TX, United States
Pages: SF41-SF61
Published: 201705
Text Language: English
Publisher: Society of Exploration Geophysicists, Tulsa, OK, United States
References: 75
Accession Number: 2017-069982
Categories: Economic geology, geology of energy sourcesIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. chart, 5 tables, sketch maps
Secondary Affiliation: Shaanxi Yanchang Petroleum Company, CHN, China
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: 201737
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