## 4 Results
### Mixing Degree Index of MSRs  
The mixing intensity refers to the degree of mixing between terrigenous siliciclastic and carbonate components within a given depositional environment. In our previous study, based on the relative ratio of terrigenous siliciclastic and carbonate minerals in the same layer, three types of MSRs were identified: mixed terrigenous siliciclastic rocks, mixed carbonate rocks, and highly mixed siliciclastic-carbonate rocks [35]. Currently, the comprehensive characterization system of \"component ratio + structural characteristics + sequence position\" is mostly used, but there is no unified standard for grading the mixing degree [36; 37]. Based on our study, there are two methods to represent the mixing intensity. For mixed successions composed of sandstone, carbonate rocks, and MSRs, the mixing degree can be quantified by calculating the ratio of MSR thickness to the total thickness of the corresponding formation. This ratio represents the mixing degree at different stratigraphic levels. The mixing degree index (MDI) can be calculated using the following formula:  
\[\text{MDI}=\frac{\text{Thickness of mixed siliciclastic -- carbonate rocks}}{\text{Total thickness of the sublayer}}\times 100\%; \tag{1}\]  
Taking the B2 sublayer in well BF as an example, the MDI can be calculated using the thickness of MSRs (2.8 m) and the total thickness of the B2 layer (7.9 m). The MSRs were distinguished by quantitative mineral interpretation from the well logs. Therefore, the MDI is 0.35%. In the second method, for rock containing terrigenous siliciclastic and carbonate components, MSRs are further categorized based on the proportion of the minor component (the relatively low content of carbonate or terrigenous clastic materials): high mixing degree (minor component proportion of 25-50%), low mixing degree (minor component proportion of 10-25%). The MDI for MSRs with high and low mixing degrees can be calculated accordingly. The contents of Rb and Al elements are closely related to the input of terrigenous clastic materials and serve as effective indicators of the mixing degree in mixed siliciclastic-carbonate rocks. These elements also reflect paleoenvironmental changes in the depositional environment [38]. Higher Rb and Al contents indicate a greater degree of mixing between terrigenous clastic and carbonate components (Figure 3b). The mixing degree index for the vertical and lateral distribution of MSRs varies by region and sublayer. In the C Oilfield, the mixing degree of MSRs was calculated for 46 wells in the southern region and 19 wells in the northern region (Table 2), distinguishing MSRs with high or low mixing degrees. The MDI for MSRs with high and low mixing degrees in each sublayer was calculated. Since the data were obtained from 46 wells in the southern zone and 19 wells in the northern zone, the maximum, minimum, and average MDI values were calculated for each subzone and each well. As shown in Figure 6, the B2, B3-1, B3-2, and B4 sublayers have the highest proportions of MSRs with a higher mixing degree compared to other sublayers. Meanwhile, the proportion of MSRs with a high mixing degree is relatively greater in the southern region than in the northern region. In the southern region, carbonate and terrigenous siliciclastics are mixed to the greatest extent. The B2, B3-1, B3-2, and B4 sublayers all exhibit higher proportions of MSRs with low mixing degrees as well, with average percentages between 10% and 20%.