Thermal alteration of spores and pollen and maturity of organic matter of the Cretaceous system,Songliao Basin,Northeast China

The relationship between sporo-pollen color and the degree of maturation of organic matter is discussed with regard to oil generation and evolution, as typified by the Cretaceous system in the Daqing Oil Field, central Songliao Basin, Northeast China. Color variation of spores and pollen is considered as a function of sedimentary environment and thermal alteration. Sporo-pollen color is classified into seven grades, and the degree of thermal alteration is studied in terms of color index. Results show that the sporo-pollen color index for the strata at the depth of 1,000–3,000 m (stratigraphically from the first member of the Liangjiang Formation to the upper Quantou Formation) ranges from 2.5–5.0, corresponding to a palaeotemperature range of 60°–140°C. These are the optimum oil-generating strata. The strata underlying the lower Quantou Formation below 3,000 m with the color index in excess of 5 and the palaeotemperature over 140°C may be favorable for gas accumulation. As for the strata at the depth of less than 1,000 m, i.e., stratigraphically overlying the second member of the Liangjiang Formation, which are characterized by a color index of 1.0–1.5 and a palaeotemperature of less than 60°C, the degree of maturation of organic matter is lower than that in the oil-generating strata.