## 2 Geological Setting  
The C Oilfield, part of the M oilfields group, is located in the southern Mesopotamian Zagros fold-thrust belt on the eastern Arabian Platform (Figure 1). The Zagros fold-thrust belt has undergone several geological stages. During NE-directed subduction, a NW-SE trending foreland basin formed, accompanied by the closure of the Neo-Tethys Ocean and the development of residual ocean basin deposits [21; 22; 23]. Member B of the Asmari Formation developed during the Chattian-Aquitanian stages, situated between the transgressive surfaces Ng10 and Ng30, with a geological age of approximately 23 to 15 million years [24; 25].  
Figure 1: Location map of the study area [1]. (**a**) Regional tectonic location of the M oilfields group; (**b**) A, B, and C are three oilfields of the M oilfields group, among which C is the study area of this study. The C Oilfield is located in the northeastern part of the oilfields group. The main lithologies of Member B of the Asmari Formation in this area include sandstones, dolostones, and mixed siliciclastic–carbonate rocks.  
During the early Oligocene, influenced by the Zagros orogen, large-scale regression led to frequent exposure of the Arabian Peninsula. The depositional center gradually migrated toward the Zagros foreland belt, while the western region experienced significant uplift and became land, and the eastern marine basin became isolated from the open sea. This caused the Avaz Delta to gradually retreat from the C Oilfield region [23; 26]. By the end of the Oligocene, the basin system had transformed into a restricted environment as the closure of the Neo-Tethys Ocean turned the Zagros foreland into a trough [23; 27], limiting seawater circulation. From the late Oligocene to the Miocene, periodic changes in sea-level and sediment supply resulted in the development of alternating or mixed sedimentary strata of carbonate and clastic rocks within the Asmari Formation in the C Oilfield [28; 29].  
The study area, located in the transition zone between a shallow-water delta and a tidal flat, experienced enhanced paleo-weathering and erosion during sea-level falls. The current burial depth of the Asmari Formation is 2800-3200 m, with a total thickness of approximately 380 m. Stratigraphically, the formation is subdivided from top to bottom into four members: A, B, C, and D (Figure 2). Among them, Member B, with a thickness of 110-140 m, serve as the primary oil pay zone. As shown in Figure 2, two main subfacies are identified within Member B, mixed tidal flat and delta facies, with lithologies consisting of sandstone, dolostone, mixed MSR, and mudstone. Based on core and well-log data, Member B of the Asmari Formation can be further subdivided into five sixth-order stratigraphic sequences from top to bottom: B4, B3-2, B3-1, B2, and B1(Figure 2). Sixth-order cycle represents the smallest-scale depositional units in high-frequency sequences stratigraphy, corresponding to short-term depositional rhythm driven by Milankovitch cycle, which usually corresponds to 4-20 kyr. Based on production variation, the C Oilfield can be divided into two regions: the northwest and the southern regions.  
Figure 2: Comprehensive stratigraphic column for well BF. The colors in the mineral composition profile represent the content changes in different minerals. Pink represents dolomitic, yellow represents sandy, purple represents calcareous, white represents gypsum, and gray represents argillaceous.