Sediments of northeastern shelf of the sea of Okhotsk in the area of South Kirin hydrocarbon deposits: microstructure, mineral, chemical and trace element composition

The study results of the microstructure, mineral, chemical and trace element composition of the northeastern shelf sediments in the area of the South Kirin hydrocarbon deposits of the Sea of Okhotsk in the Sakhalin region are considered. Sediment samples were taken during well drilling from the «Trias» special vessel at the depth of 25—30 m from the bottom surface (by the sea depth of 200—220 m). Interval coring (every one meter) was carried out by the pressing-in method. Analytical studies were performed for 30 air-dry samples (firmly cemented cylinders with a diameter of 4 cm, a height of 3 cm, and a weight of 50—70 g) of well No. 49 (30 m). According to lithological characteristics, three intervals are distinguished in the section which are represented by clay sands (0,0—4,0 m), sandy, silty clays (4,0—21,0) and clays (21,0—30,0). REM images of the sediment microstructure were obtained using a scanning microscope JEOL JIB-Z4500 (xlOOO-6000) for each interval. The scientific imaging was performed for ten thin undisturbed plates from the top part of air-dry sample-cylinder. Fauna residues and micropores were found. The content of aggregates, primary (free) particles and the volume of real clay content were determined by the «Microstructure» method. On the basis of these data, grouping of samples was carried out using the Q-type cluster analysis program which confirmed the picked intervals in the studied section. Changes in the mineral composition were determined by powder diffraction in the sediment thickness. The quartz content decreases with depth, and the clay phase increases in which hydromica dominates, and the almost complete absence of kaolinite is noted. The content of rock-forming oxides (method of silicate analysis) and statistical data processing showed a homogeneous distribution of silicon, aluminum and potassium oxides in the section, the coefficient of variation (V, %) was only 1—3 %. Augmentation of variability was found for БегОз (P12). Values of geochemical coefficients and statistical data processing revealed their insignificant variability in the section (V3—8). Noticeable changes are characteristic for the ratio of calcium and magnesium oxides (V16). A lesser degree of chemical transformations of sediments was established. The content of 23 trace elements was determined by the X-ray fluorescence method using an S8 TIGER spectrometer. The sediment thickness has the critical level of pollution by values of the pollution index (Zc) calculated for a group of toxic elements (V, Cr, Co, Ni, Cu, Zn, Pb, As). Based on the obtained information, the conclusions were drawn about the zonal structure of the sediment thickness, the features of their microstructure, mineral and chemical composition, as well as the degree of contamination by toxic trace elements.