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The Geochemical Characteristics and Its Implication of Organic Matter Enrichment Conditions in the Upper Cretaceous Oil Shale Sequences of the Songliao Basin (NE China)
Author(s) -
SONG Yu,
LIU Zhaojun,
MENG Qingtao,
XU Jinjun
Publication year - 2015
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/1755-6724.12304_22
Subject(s) - oil shale , cretaceous , structural basin , geology , china , organic matter , geochemistry , paleontology , shale oil , petroleum engineering , geography , archaeology , chemistry , organic chemistry
rift basin located in northeastern China. It has attracted widespread attention due to the development of two oil shale sequences in the upper Cretaceous K2qn1 and K2n2 (Liu et al., 2011; Bechtel et al., 2012; Jia et al., 2013). The oil shale provides valuable information about organic matter (OM) enrichment due to its enriched OM. The northern and southern parts of the Songliao basin are influenced by the different sedimentary system, so the sedimentations present distinct differences. Previous studies about upper Cretaceous oil shale sequences are mainly concentrated on the southern part of the basin (Bechtel et al., 2013; Jia et al., 2013), whereas the research about the northern part of the basin is relatively few. In this study, based on the abundant organic and inorganic geochemical data from two sampled core profiles located in the northern part of Songliao basin, influencing factors of input, preservation and dilution of OM in K2qn1 and K2n2 oil shale sequences are discussed and the OM enrichment models are established. According to the organic geochemical data, the K2qn1 oil shale with high TOC contents (avg. 8.79 wt. %) is dominated by Type-I kerogen (aquatic organisms); the K2qn1 mudstone with low TOC contents (avg. 2.56 wt. %) and Type-II kerogen (mixed type). Whereas the K2n2 oil shale with moderate TOC contents (avg. 6.66 wt. %) is dominated by Type-I kerogen (aquatic organisms), and the TOC contents of mudstone is low (avg. 1.44 wt. %) and with Type-III kerogen (terrigenous plants). The Sr/Cu ratio is used to reflect the paleoclimate, it indicates the K2qn1 was deposited in warm-humid paleoclimate, whereas the paleoclimate became semi warm-humid during deposition of the K2n2, and it is confirmed by the sporo-pollen assemblages and oxygen isotope data (Wang et al., 2013). According to the C-S-Fe relationships, HI, EF (Mo/Al), V/Cr, Ce-anomaly and Sr/ Ba, Ca/ (Ca+Fe) ratio, strong water salinity stratification and dysoxic conditions are prevailing during the deposition of K2qn1 oil shale. In contrast, the K2n2 oil shale was deposited in weak water salinity stratification and partially oxic conditions. Moreover, high bioproductivity and low detrital matter input within the K2qn1, whereas moderate bio-productivity and detrital matter input within the K2n2 are confirmed by Ti/Al, Si/Al ratio and P, U contents. In addition, the marine transgression event may improve the OM enrichment of K2qn1 (Huang et al., 2013). The comprehensive study of bio-productivity, preservation and dilution of OM indicates there are significant differences between the OM enrichment models within the K2qn1 (Fig. 1a) and K2n2 (Fig. 1b). In summary, high bio-productivity and strong water salinity stratification were responsible for OM enrichment in K2qn1, whereas the origin of OM and dysoxic conditions are the major controlling factor for OM enrichment in the K2n2.