Lithology identification and reservoir characteristics of the mixed siliciclastic-carbonate rocks of the lower third member of the Shahejie formation in the south of the Laizhouwan Sag, Bohai Bay Basin, China

In the Shahejie Formation in the D oilfield area of the Laizhouwan Sag, Bohai Bay Basin, a mixed sedimentary structure of terrigenous clastic and carbonate rocks has developed. Scholars at home and abroad have performed much research on clastic and carbonate lithology identification. However, studies on lithology identification and reservoir characteristics of mixed siliciclastic-carbonate rocks in the study area have not been conducted thus far. Due to the complex logging response characteristics of mixed sedimentary rocks, it is difficult for conventional lithology identification methods to clearly reflect the lithological characteristics of mixed siliciclastic-carbonate rocks. In this study, the sensitivity of acoustic time difference, natural gamma ray, density, deep lateral resistivity and compensated neutron logging curves to each lithology was analysed using the conventional cross-plot method in the D oilfield. On this basis, a decision tree method is used to further screen and identify the lithology layer by layer, and the process and results of identifying a mixed lithology with logging curves are clearly reported. In addition, imaging logging and core data are used to verify the identified lithology, and a lithology identification template and technical method consisting of conventional logging cross plots, decision trees, imaging logging and core calibration are established to improve the accuracy of lithology identification of mixed rocks. Moreover, the influence of different lithologies on the reservoir characteristics in the study area is analysed.