Cause of shale gas geochemical anomalies and mechanisms for gas.pdf

Marine and Petroleum Geology 44 (2013) 1e12
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Marine and Petroleum Geology
journal homepage: ate/marpetgeo
Review article
Cause of shale gas geochemical anomalies and mechanisms for gas
enrichment and depletion in high-maturity shales
Fang Hao a,b,*, Huayao Zou b
a Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China
b State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Changping, Beijing 102249, China
article info abstract
Article history: This article reviews the abnormal characteristics of shale gases (natural gases produced anic-
Received 3 July 2012 rich shales) and discusses the cause of the anomalies and mechanisms for gas enrichment and deple-
Received in revised form tion in high-anic-rich shales. The reported shale gas geochemical anomalies include rollover
24 February 2013
of iso-alkane/normal alkane ratios, rollover of ethane and propane positions, abnormally
Accepted 12 March 2013
light ethane and propane d13C values as well as isotope reversals among methane, ethane and propane.
Available online 26 March 2013
These anomalies reflect plex histories of gas generation and associated isotopic fractionation as
well as in-situ “mixing and accumulation” of gases generated from different precursors at different
Keywords:
Shale gas thermal maturities. A model was proposed to explain the observed geochemical anomalies. Gas gener-
Geochemical anomalies ation from kerogen cracking at relatively low thermal maturity accounted for the increase of iso-alkane/
13
Closed systems normal alkane ratios and ethane and propane d C values (normal trend). Simultaneous cracking of
Kerogen cracking kerogen, retained oil and wet gas and associated isotopic fractionation at higher maturity caused
Oil-cracking decreasing iso-alkane/normal alkane ratios, lighter ethane and propane d13C