Distribution of organic matter in the Qingshankou Formation Shale, Gulong Sag,Songliao Basin observed within an isochronous sequence stratigraphic framework

The Qingshankou Formation shale, Gulong Sag, Songliao Basin, has been gushing out oil in a rate stunning the Chinese oil industry. However, further prediction of sweet spots in the formation remains a challenge due to the highly uneven distribution of organic matter and confusion in the understanding of controlling factors. This study adopted the concepts of “hierarchy in sequence stratigraphy” and “transgressive-regressive (T-R) sequences”, with updated astronomical cycle research results to determine the accurate duration of sequences and re-establish an isochronous sequence stratigraphic framework specifically for lacustrine deep-water shale based on core, outcrop, and thin section observation, as well as seismic profile, well-logging, geochemical, and paleontological data analyses from a microcosmic to macroscopic scale. Subsequently, using modern lakes sedimentation as an analogy made it possible to propose qualitative and quantitative indexes for identifying paleo-water environment, and probe into the origin of the heterogeneity of organic matter enrichment under the sequence stratigraphic framework on the basis of the coupling relation among paleoproductivity, redox conditions, and sedimentation rate. The main conclusions are as follows: 1)There are four third-order sequences in Qingshankou Formation Shale, Gulong Sag, Songliao Basin. Among them,SQ1 and SQ2 consist of two T-R sequences of 13 parasequence sets (made up of 52 parasequences). The durations of parasequence and parasequence set are approximately 40 kyr and 170 kyr, respectively. 2) Three types of lithofacies, four types of laminae, five laminae combinations, eleven laminae combination patterns, and three sedimentary microfacies have been recognized under the sequence stratigraphic framework. T-R cycles control the vertical distribution of laminae combination, lithofacies and sedimentary microfacies. The argillaceous shales deposited in deep-lake stagnant water and mud flow are prospective lithofacies. 3) T-R cycles control the enrichment of organic matter and different orders of flooding surfaces are the favorable locations for organic matter enrichment. Parasequence set 2 in Gulong Sag and parasequence sets 1 to 4 in Sanzhao Sag have been evaluated to be the most promising exploration targets. This study aims to provide sedimentological evidence for shale oil target area and “sweet spots”prediction.