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TitleRevelation of organic matter sources and sedimentary environment characteristics for shale gas formation by petrographic analysis of Middle Jurassic Dameigou Formation, northern Qaidam Basin, China
AuthorQin, J; Wang, S; Sanei, H; Jiang, C; Chen, Z; Ren, S; Xu, X; Yang, J; Zhong, N
SourceInternational Journal of Coal Geology vol. 195, 2018 p. 373-385,
Alt SeriesEarth Sciences Sector, Contribution Series 20160011
Mediapaper; on-line; digital
File formatpdf; html
AreaGolmud; Qilian Mountain; Aljin Mountain; Kunlun Mountain; China
Lat/Long WENS 90.0000 100.0000 40.0000 35.0000
Subjectsfossil fuels; sedimentology; geochemistry; petroleum resources; hydrocarbon potential; hydrocarbons; gas; source rocks; sedimentary environments; depositional environment; petrographic analyses; geochemical analyses; organic geochemistry; vitrinite reflectance; organic materials; bedrock geology; lithology; oil shales; mudstones; sandstones; siltstones; thermal maturation; maceral analyses; Middle Jurassic; Qaidam Basin; Dameigou Formation; shale gas; organic matter; rock-eval analyses; biomarkers; total organic carbon; pristane-phytane ratios; sterane; gammacerane index; Phanerozoic; Mesozoic; Jurassic
Illustrationslocation maps; geoscientific sketch maps; columnar sections; tables; photomicrographs; histograms; plots; ternary diagrams; profiles
ProgramShale Reservoir Characterization, Geoscience for New Energy Supply (GNES)
AbstractThe 7th member of Middle Jurassic Dameigou Formation (J2d7) in northern Qaidam Basin, China was proposed to have good hydrocarbon generating potential in previous studies. Here we apply an integrated petrographic and geochemical analysis to investigate the depositional environment, real thermal maturity and shale gas potential of organic-matter (OM)-rich assemblage. The relatively high content of C29 regular sterane (56.8%-62.8%) and vitrinite predominance in maceral composition suggesting plenty of higher-plant input, combined with the high pristane/phytane ratio (Pr/Ph)(2.8-8.2) and low gammacerane index (0.2-0.4) reveals oxic water column with in-situ depositional condition of carbonaceous-mudstone when the lower J2d7 was deposited. The maceral composition of oil shale at the upper part of J2d7 is mainly amorphous OM with small proportion of reworked vitrinite and inertinite, consistent with a mainly algea and small amount of exogenous OM input. When associated with low Pr/Ph (0.8) and high gammacerane index (4.3), the oil shale was deposited in an anoxic saline water column with mainly zooplankton OM source. The geochemical and petrographic result implies a transitional environment ranging from suboxic semi-saline to oxic fresh water environment with varying proportion of higher-plant and algae input by in-situ deposition and distant transportation for the mudstone at the middle part of J2d7. By use of random reflectance (Ro) distribution, two sedimentary environment microcycles which are confirmed by maceral composition, from the depth of 1983m to 1962m and 1954m to 1931.9 m, are differentiated within the homogenous mudstone section. The variation of OM input condition and alternation of depositional environment led to accumulation and deposition of various kinds of abundant OM when J2d7 was deposited. Although the OM has actually low maturity at oil window with vitrinite reflectance around 0.6% according to the petrographic results, the liptinite inherited from higher-plants especially suberinite is the main maceral to generate gaseous hydrocarbons at the low maturity, which implies that the Dameigou formation (J2d7) of Qaidam Basin is of gas potential.
Summary(Plain Language Summary, not published)
Organic petrographic observation and organic geochemical analysis have been carried out on core samples from the Dameigou Formation of the Middle Jurassic in northern Qaidam Basin, NW China. The lithology of the samples includes oil shale, mudstone, sandstone and coal. Organic macerals including vitrinite, huminite, liptinite and inertinite have been identified in various amounts in different lithological intervals. Four populations of reflectance measurements have also been defined, which, together with the maceral compositions have been related to the depositional environments of studied samples. The interpretation of depositional environments based petrographic observation is supported by Rock-Eval results and the hydrocarbon composition of solvent extracts from the samples.