Research Method on Organic Sequence Characteristics of Qingshankou Formation in the Northern Songliao Basin

Previous researchers have conducted multiple studies on the sequence stratigraphy of the Songliao Basin. Generally, the study of sequence stratigraphy at the basin level is mainly guided by classical sequence stratigraphy, while the study of sequence stratigraphy at the zone level is mainly guided by high-resolution sequence stratigraphy. The Qingshankou Formation in the northern part of the Songliao Basin has a set of organic rich lacustrine sedimentary rock layers with high clay content and thin interbedded combinations of shale and dolomite. The stratigraphic units of various levels of the Qingshankou Formation are particularly developed, especially for the shale layers in terrestrial basins. It is difficult to identify the lithology and compare the sequence stratigraphy in the fine-grained sedimentary area of the lacustrine facies. The Qingshankou Formation is composed of dense sandstone formations of river and shallow water delta facies beneath the Quantou Formation, and overlaid with a large delta facies sandstone formation of the Yaojia Formation. Both formations exhibit abrupt lithological contact and show obvious step like changes in the logging series curve characteristics. The natural gamma value and acoustic wave time difference sharply decrease, and the density and deep lateral curves show a positive shift and form a stable trend. The organic matter content also shows a sudden change trend. Within the shale of the Qingshankou Formation, it was found that the distribution curve of shale organic matter (total organic carbon TOC, free hydrocarbons S1) exhibits obvious cyclicality and identifiable interfaces, indicating the development of organic matter sequence stratigraphy under the control of lake flooding events in the Qingshankou Formation shale. Therefore, the study area is suitable for using the method of organic sequence stratigraphy division: based on organic matter abundance and free hydrocarbon content, and assisted by environmental parameters to carry out cyclic analysis of sequence stratigraphy division. Unlike conventional cycle analysis techniques based on lithological changes or vertical distribution curve characteristics, this technique does not rely on logging curve analysis. However, due to the correlation between organic matter abundance and free hydrocarbons with resistivity and gamma curves, certain cycle characteristics can be reflected to some extent from resistivity or gamma curves. Capable of conducting research on the distribution characteristics of geological strata in the entire or large area based on well logging predictions.