## 1 Introduction  
The concept of mixed siliciclastic-carbonate sedimentation was first introduced by [PERSON] [1], referring to rocks formed through the mixed sedimentation of terrigenous clastic and carbonate components. Subsequently, mixed siliciclastic-carbonate rocks (MSRs) were defined as rocks composed of a mixing of terrigenous clastic materials and carbonate components (including allochemical grains) within a single depositional unit [2]. A depositional assemblage alternating between terrigenous clastic rocks, carbonate rocks, and MSRs is termed a mixed siliciclastic-carbonate succession [3, 4].  
Mixed sediments have been reported in various stratigraphic units across multiple basins worldwide, such as the Late Miocene Anjou Formation of the Tortonian stage in western France; the Upper Cretaceous Khasib and Tanuma formations in the Mesopotamian Basin [5]; the Mississippi Visean Terrace in southwestern Spain [6]; and the Sendji Formation of the Lower Cretaceous in the Lower Congo Basin [7]. Over the past three decades, significant progress has been made in understanding the mixed depositional environments, classification schemes, and controlling factors of MSRs, which have attracted considerable attention [3, 4, 8, 9]. Based on modern sedimentological studies, in shallow marine shelves without storm waves, modern mixed sediments are usually distributed at water depth ranging from 20 to 100 m [8]. MSRs are widely developed in transitional depositional environments, such as terrestrial lakes, shelves, slopes, semi-deep seas, and deep-sea settings, and are characterized by the lateral interweaving or vertical stacking of different depositional facies zones [10, 11, 12].  
Because of the complicated components in MSRs, lithological classification lacks a unified standard. It is primarily based on refinements of [PERSON]'s classification scheme, which considers four end-member components: sand, allochemical grains, micrite, and muddy clay [1, 13]. The formation and distribution of mixed rocks result from complex processes controlled by multiple factors. Previous studies have identified clastic material supply [14], climatic and aquatic conditions [15], paleogeographic settings [4], tectonic activity [16], sea (or lake) level fluctuations [9], hydrodynamic conditions [17], bioturbation [17], and diagenesis [16] as the main controlling factors of mixed rock deposition. However, the relationship between the mixing degree and reservoir quality remains underexplored.  
High-quality MSR reservoirs have been discovered in the Asmari Formation of Iraq. Previous research has primarily focused on reservoir characteristics, controlling factors of reservoir quality, and heterogeneity [18, 19, 20]. However, the mixing degree, mixed sedimentary processes, and distribution patterns of MSRs have not been sufficiently addressed. The understanding of the depositional environments and reservoir characteristics of MSRs remains incomplete, and the distribution of MSR reservoirs needs further investigation to better guide oil exploration and development in this area.  
This study aims to take the MSRs of Member B of the Asmari Formation in the C Oilfield, southern Iraq, as a case study to reveal the characteristics of mixed sedimentation in the transitional zone between a shallow-water delta and a tidal flat. Through core observations, thin-section identification, well-log data analysis, and geochemical characterization, this research classifies the lithofacies of MSRs in Member B of the Asmari Formation,identifies depositional environmental differences in MSRs, reveals the pore and throat shapes and sizes in rocks with varying degrees of mixing, and then clarifies the distribution patterns of MSR reservoirs. This study provide theoretical guidance for future oil and gas exploration and development in MSRs and offers new perspectives and methodologies for the study of mixed siliciclastic-carbonate rocks (MSRs).