Quantitative Characterization of Log Facies and Depositional Model of Mixed Siliciclastic-Carbonate Rocks Based on Fisher Discriminant: A Case Study on Daluhu Oilfield, Bohai Bay Basin

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 Daluhu oilfield, Bohai Bay basin. Integrating drilling, logging, and seismic data, 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, on the basis of a chronostratigraphic framework. The results are as follows. (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), Sonic transit time (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 developed a braided-river delta-semi-deep lake composite depositional system, with delta-front siltstone facies developed along the basin margins, resulting in poorer organic matter preservation, whereas the sag center was dominated by semi-deep lacustrine deposits, with organic-rich argillaceous shale being predominant. (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 siliciclastic-carbonate depositional system is characterized by braided-river delta and nearshore subaqueous fan complexes along the western and southwestern steep margins. Slump-induced turbidite bodies developed in the slope zones and lake basin below the slope-break belts of the lowstand and highst and systems tracts. In the eastern and northeastern gentle-slope areas, calcareous-dominated sedimentary facies developed in semi-deep to deep lacustrine environments, while muddy shale and calcareous shale facies shifted 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.