Abstract: The lithological complexity and strong heterogeneity of the mixed siliciclastic and carbonate reservoirs result in difficulties in lithofacies identification and insufficient understanding of depositional patterns in the upper Shahejie 4 Member and lower Shahejie 3 Member in the western Dalu Lake Oilfield, Bohai Bay Basin. This study integrates drilling, logging, and seismic data. On the basis of a chronostratigraphic framework, Fisher discriminant analysis was applied for quantitative characterization of mixed siliciclastic and carbonate lithofacies from well logs, and the depositional evolution of mixed sediments was comprehensively revealed. The results show that: (1) Based on Fisher discriminant analysis, quantitative lithology identification models for sand–mudstone and limestone–mudstone systems were established within subsequences using optimally selected natural gamma ray (GR), the sensitivity of acoustic time difference (AC), density (DEN) and compensated neutron (CNL) logging curves, effectively addressing the problem of overlapping log responses between limestone and sandstone and significantly improving lithology identification accuracy. For the sand–mudstone system, the lithology identification accuracy in the validation set was 79.5%, with a siltstone prediction accuracy of 95%; for the limestone–mudstone system, the overall back-prediction accuracy reached 84.5%. (2) The highstand systems tract (HST) of the upper Shahejie 4 Member is dominated by shallow-lake to semi-deep-lake facies, consisting of interbedded calcareous shale and mudstone. Controlled by paleo-water depth and salinity, organic matter is enriched in the central–western and locally in the southeastern parts, with contents mainly between 0.8%-1.9% The lowstand and lake-expansion systems tracts of the lower Shahejie 3 Member are dominated by semi-deep-lake facies, with organic matter enriched in the northern and southern areas and TOC content concentrated at 0.9%–2.3%. The HST of the lower Shahejie 3 Member develops a braided-delta to semi-deep-lake depositional system, with siltstone delta-front facies along basin margins leading to poorer organic matter preservation, while the basin center retains semi-deep-lake calcareous–shale rhythmic assemblages dominated by organic-rich argillaceous shale. (3) Four types of vertical lithofacies association models were established: thick-bedded sandstone interbedded with thin mudstone, thick-bedded mudstone interbedded with thin sandstone, thick-bedded calcareous shale interbedded with thin mudstone, and thick-bedded mudstone interbedded with thin calcareous shale. In plan view, the mixed-sedimentary depositional system transitions sequentially from southwest to northeast-“silty (fine) sandstone facies-muddy shale facies-calcareous shale facies”-and exhibits a pronounced belt-like zonation pattern. In the western and southwestern steep-margin areas, braided river delta and nearshore subaqueous fan complexes are developed. Within the slope zone and basin beneath the fault belts of the lowstand and highstand systems tracts, slump-derived turbidite sand bodies occur. In the eastern and northeastern gentle-slope zones, calcareous-dominated sedimentary facies formed in semi-deep to deep lacustrine environments, with muddy shale and calcareous shale facies migrating toward the basin center. This study provides a quantitative theoretical basis for lacustrine mixed lithofacies prediction from well logs, offering guidance for organic-rich source rock and reservoir prediction, depositional pattern analysis, and oil and gas exploration.