A Comparative Geochemistry Study of Several Oil Shale-Bearing Intervals in the Paleogene Huadian Formation, Huadian Basin, Northeast China

The Huadian Basin is an oil shale-bearing basin located in northeastern China. Thirteen oil shale layers deposited in this basin, and the characteristics of oil shale are different among these oil shale layers. Based on the core observation and microscope identification, using the organic and inorganic data from borehole HD3 and outcrops, the formation conditions of different grade oil shale have been evaluated. Based on measuring oil yield (OY in short) of an oil shale to determine its grade, this paper classifies the oil shale as high grade (OY≥10%), medium grade (10% > OY≥5%) and low grade (5% > OY≥3.5%). The high grade oil shale is mainly in brown or dark brown, and the bulk density ranges from 1.59to 1.81 g/cm³. The results of X-ray diffraction indicate the content of carbonate minerals is 28.0%. The HI (hydrogen index)-T_max, HI-OI (oxygen index) and S₂-TOC (total organic carbon) diagrams indicate the kerogen types are Ⅰ and Ⅱ₁. The high grade oil shale generally formed in the relatively arid paleoclimate, deposited in the brackish water, dysoxic environment, when the bioprodctivity is extremely high, and the information is mainly from the inorganic parameters diagrams of chemical index of alteration (CIA), Sr/Ba and V/(V+Ni). The medium grade oil shale mainly shows grey-black or black-brown color and the bulk density ranges from 1.87to 2.08 g/cm³. The average content of carbonate minerals is 16.4%, far less than high grade oil shale and the kerogen type is mainly Ⅱ₁. The inorganic parameters diagrams reflect the medium grade oil shale generally formed in the less humid paleoclimate, deposited in the brackish water, dysoxic to anoxic environment, when the bioproductivity is medium. The color of low grade oil shale is major in grey-black or dark grey and the bulk density ranges from 2.00to 2.15 g/cm³. The average content of carbonate minerals is sharply decreased and the kerogen type is mainly Ⅱ₁. The inorganic parameters diagrams indicate the low grade oil shale generally formed in the relatively humid paleoclimate, deposited in the freshwater to brackish water, anoxic environment, when the bioproductivity is relatively low. Comprehensive study suggest the increasing precipitation caused by a relatively humid paleoclimate resulted in decreasing oxygen concentration and salinity in the bottom water, restrained the salinity stratification, and tended to form the low grade oil shale. The strong evaporation leading to relatively arid paleoclimate resulted in increasing oxygen concentration and salinity in the bottom water, and promoted the salinity stratification. The area with well preservation conditions caused by the relative high sedimentary rate and salinity stratification are favorable to high grade oil shale deposit. In summary, the bioproductivity and the paleoclimate are the main factors controlling the oil shale grade in the Huadian Basin, when the bioproductivity is relatively high and the paleoclimate is relatively arid, it is prone to form the high grade oil shale. However the low grade oil shale developed in the low bioproductivity and relatively humid paleoclimate.