Molecular Fossils and Sedimentary Environment of the Cretaceous Lower Qingshankou Formation in Eastern Songliao Basin, Northeast China

INTRODUCTION

The Songliao basin in Northeast China is one of the largest Cretaceous continental rift basins in the world. The black shale and oil-bearing shale in the lower Qingshankou Formation were considered to be the record of a lacustrine anoxic event (Huang et al., 1998), forming a primary source for the basin's hy-drocarbon reserves (Li et al., 1995). More studies were conducted on the central zone of Songliao basin (Feng et al., 2007; Yang et al., 1985), leaving the east-ern zone largely unknown. The maturity index of the lower Qingshankou Formation is commonly very high in the central zone (Zhong et al., 2009), which is not conducive to the preservation of molecular fossils. The study area is located in the Qingshankou Village, southeastern Songliao basin (Fig. 1), where the lower Qingshankou Formation crops out along the southern bank of Second Songhua River (Wang et al., 2001). In this article, we conducted a preliminary organic geochemistry investigation on the surface outcrop samples in the southeastern zone of Songliao basin.

Figure  1.  Sketch map showing the study area in the Songliao basin.

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MATERIALS AND METHODS

A black shale sample selected from the lower Qingshankou Formation was subject to the analyses of pyrolysis, GC and GC-MS. Sample preparation and analyses were performed at the Research Institute of Petroleum Exploration and Development, PetroChina. Pyrolysis analysis was performed with a Rock-Eval 2plus. GC analysis was conducted on an HP6890 chromatograph with a column (30 m×0.22 mm×0.25 μm film thickness), and using H₂ as carrier gas. The GC oven temperature was held initially at 100 ℃ (5 min), and increased from 100 to 310 ℃ at 8 ℃/min (held 20 min). GC-MS analysis was performed with an Agilent 5973N spectrometer. A BPX5 column (30 m×0.22 mm×0.25 μm film thickness) was used with split injection and He as carrier gas. The oven tem-perature was programmed from 60 (held 1 min) to 220 ℃ at 4 ℃/min, and 220 to 300 ℃ at 3 ℃/min (held 20 min).

RESULTS AND DISCUSSION

Total organic carbon (TOC) concentration of the black shale is 5.51%, implying a high paleo-productivity and/or an anoxic bottom water environ-ment. High HI value (649 mgHC/gTOC) and low OI value (32 mgHC/gTOC) indicate a dominant organic origin of algae and microbial biomass, as well as a few wax coatings of higher plants (Meyers, 2003). T_max value of 436 ℃ indicates the immature level, which is different from those of SK-1 (mature levels) (Zhong et al., 2009).

n-alkanes of the black shale range in carbon number from C₁₂ to C₄₀, with maxima at C₂₁ and C₂₃ (Fig. 2), reflecting a mix organic input of algae, bacte-ria and higher plants. The terrigenous/aquatic ratio (TAR=(C₂₇+C₂₉+C₃₁)/(C₁₅+C₁₇+C₁₉)) of 0.79 might suggest the dominant contribution of algae and bacteria. Low Pr/Ph ratio (0.67) implies an anoxic source-rock deposition (Peters et al., 2005). Ts < Tm and low ratio of C₂₉ ααα20S/(20S+20R) also show immature to low mature source rocks. Large amounts of gammacerane found in the black shale might indi-cate a water column stratification (Sinninghe Damste et al., 1995).

Figure  2.  GC-MS mass chromatograms for the black shale from the lower Qingshankou Forma-tion.

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The ancient Songliao Lake was a large, deepwa-ter and eutrophic lake during the sedimentation of the lower Qingshankou Formation (Yang et al., 1985). A stratified lake occurred during the sedimentation of the black shales, with the high plankton productivity in the epilimnion and the anoxic condition in the bot-tom water. Though algae were proposed to be the main source of organic matter in the central lake (Zhong et al., 2009; Feng et al., 2007), the study area, located not far from shoreline, is featured by the addi-tional input of higher plant organics. Not only abundant dinoflagellates and other algae were found in the black shales of the lower Qingshankou Formation (Gao et al., 1992), but also relatively abundant pollen and spore fossils were identified. On the basis of our molecular data and previous reports, we suggest that algae and bacteria were the main source of shale or-ganic matter, along with the additional contribution of higher plants.

CONCLUSIONS

The black shale of the lower Qingshankou For-mation is characteristic by high TOC concentration and great HI value, implying a great plankton produc-tivity. T_max value of 436 ℃, low Ts/Tm and low C₂₉ ααα20S/(20S+20R) ratios indicate a low mature source rocks. The great HI value and the dominance of small-chained n-alkanes indicate a dominant organic origin of algae and microbial biomass, as along with an additional input from higher plants. During sedi-mentation of the lower Qingshankou Formation, the ancient Songliao Lake was deep, eutrophic and strati-fied, with great plankton productivity in the epilimnion and the anoxic condition in the bottom water.

ACKNOWLEDGMENTS

Authors acknowledge help of Prof. Xie Shucheng for editing English and making suggestions to improve the manuscript.