岩溶储层的孔缝差异充填特征对岩溶水系统具有重要的指示意义。岩溶水的流动性与滞留程度会造成碳酸盐岩储层溶蚀和充填程度的差异，从而影响储层物性。基于鄂尔多斯盆地大牛地气田奥陶系马家沟组构造与沉积背景，综合岩心观察、铸体薄片鉴定以及元素和同位素分析等手段，系统剖析了马家沟组纵向岩溶分带特征及孔缝充填规律，探讨了孔缝差异充填及其对岩溶水流动路径的指示。结果表明：①马家沟组纵向发育风化壳岩溶带（马五_1~5亚段）和顺层岩溶带（马五₆亚段）。风化壳岩溶带以垂向渗流溶缝和水平潜流溶孔为特征，受古地貌控制显著；顺层岩溶带以下渗岩溶水溶蚀作用为主，形成溶洞与角砾岩。②岩溶储层中溶孔与溶缝耦合发育，识别出孔缝均被充填、孔未充缝全充、孔全充缝未充及孔缝均未充4种差异充填类型，反映岩溶水饱和程度和流体沉淀作用的差异性。③岩相分析显示，研究区岩溶地质体的断裂通道是岩溶水流动路径之一；地球化学参数（如Fe/Mn值、碳氧同位素及锶同位素）显示，风化壳岩溶带Fe/Mn值较高（平均51.30），碳氧同位素显著偏负，指示氧化环境下强烈的溶蚀作用；顺层岩溶带Fe/Mn值分布不均（20.78~92.47），锶同位素比值（0.711 034）高于同期海水，顺层流动过程中岩溶水滞留程度较高，溶蚀作用相对减弱。④孔缝充填特征与古地貌具有耦合关系，岩溶高地与斜坡过渡带（如S401井）因垂向渗流受限，孔缝呈全充填特征，导致储层有效性降低；斜坡与沟槽过渡带（如D126井）保留半开放流体环境，溶蚀物质沉淀较弱，有效储集空间更发育。研究表明，孔缝差异充填特征可结合岩溶古地貌、断裂分布和元素变化等手段综合揭示岩溶水流动路径及滞留状态，为岩溶储层评价及油气勘探提供重要依据。

Differential filling characteristics of pores and fractures in karst reservoirs serve as critical indicators for the reconstruction of paleokarst hydrogeological systems. The mobility and retention degree of karst water control the dissolution and filling processes in carbonate reservoirs, ultimately governing reservoir quality. Based on the tectonic and sedimentary background of the Ordovician Majiagou Formation in the Daniudi gas field, Ordos Basin, this study integrated core observations, cast thin-section analyses, geochemical data (including elemental and isotopic compositions), and petrographic techniques to systematically investigate the vertical zonation of karst features and pore-fracture filling patterns. The implications of differential filling for tracing paleo-karst water flow pathways are discussed. Key findings are summarized as follows: First, the Majiagou Formation is divided into two vertical karst zones: a weathering crust karst zone (sub-members Ma 5_1~5) and a stratabound karst zone (sub-member Ma 5₆). Weathering crust karst zones are characterized by vertical seepage fractures and horizontal subsurface flow dissolution pores, which are strongly influenced by paleotopography. In contrast, the stratabound karst zone is dominated by downward-infiltrating karst water, forming dissolution caves and breccias. Second, the coupling between dissolution pores and fractures in the karst reservoir is evident. Four distinct filling patterns were identified: \mathrm{①} both pores and fractures were filled; \mathrm{②} pores were unfilled but fractures were filled; \mathrm{③} pores were filled but fractures were unfilled; and \mathrm{④} both pores and fractures were unfilled. These patterns reflect the differences in karst water saturation and mineral precipitation processes. Filling heterogeneity indicates variations in fluid chemistry and hydrodynamic conditions during diagenesis, and petrographic analysis revealed that fault systems act as primary conduits for karst water flow. Geochemical parameters further constrain fluid behavior; the weathering crust karst zone shows high Fe/Mn ratios (averaging 51.30) and significantly negative carbon-oxygen isotopic values, suggesting intense dissolution under oxidizing conditions. In the stratabound karst zone, Fe/Mn ratios vary widely (20.78~92.47), and strontium isotopic ratios (⁸⁷Sr/⁸⁶Sr=0.711034) exceed those of contemporaneous seawater, indicating prolonged water retention and reduced dissolution intensity during lateral flow. Fourth, the distribution of pore-fracture filling correlates with paleo-karst geomorphology. In the transition areas between karst highlands and slopes (e.g., Well S401), limited vertical seepage leads to the complete filling of pores and fractures, degrading reservoir effectiveness. In contrast, transition zones between slopes and grooves (e.g., Well D126) maintained semi-open fluid systems with minimal mineral precipitation, preserving effective storage space. This demonstrates that the differential filling features of pores and fractures, when combined with paleo-karst geomorphology, fault distribution, and geochemical proxies, can effectively reconstruct karst water flow pathways and retention states. These results provide a scientific basis for evaluating karst reservoir heterogeneity and guiding hydrocarbon exploration in similar geological settings.