A LACUSTRINE BLACK SHALE DEPOSITIONAL MODEL IN THE SONGLIAO BASIN, NORTHEASTERN CHINA

The Songliao Basin is a large-scale petroliferous basin in China. Cretaceous Qingshankou Formation has favorable geological conditions for the deposition of black shale-the most important oil shale in the basin. A total of 120m of Qingshankou sublayer conventional cores from 12 wells located in the Qijia depression were described and cuttings sampled in this study. Estimated total organic carbon (TOC) values averaged 2.67 wt%. X-ray diffraction (XRD) analysis of 39 shale samples indicate the formation is clay-rich and quartz-poor, with the clay content averaging 45.8 % and the quartz content averaging 30.9%. According to the sediment texture and grain size, there are four types black shale in Qingshankou Formation: ①lamellar structure shale,② blocky structure sale, ③silty mudstone,④ carbonaceous mudstone. Organic C and values and composition vary through the Qingshankou Formation with the most organic-rich lithology occurring at the base. A set of black shale deposed at the base of Qingshankou stage with 18 meters thickness and the maximum TOC value 5.47 wt%.

From lake center to lake margin, four facies were distributed with different total organic carbon (TOC) values: ①deep lake facies with high TOC value averaged 3.48 wt% ; ②semi-deep lake facies with high TOC value averaged 2.21 wt%; ③coastal shallow-lake facies with low TOC value averaged 1.62 wt%; ④arm of lake facies with low TOC value averaged 1.84 wt%. On the basis of well H14 core data analysis, from up and down, the TOC value showed an increased tendency gradually and some trace elements levels increased such as uranium, manganese, zinc and aluminium. Just like the black sea, the Qingshankou lake of Songliao basin receives large volumes of fresh water from rivers and is also supplied with marine water from Eastern Nankai trough. The definitive euxinic condition is one important factor for black shale formation in fresh water lake. It is well known organic matter-rich black shale were deposed deep lake. In Songliao basin, the semi-deep lake slop is still a very important area for black shale deposition where organic matter were transported by turbidity currents and buried immediately.