## 1 Introduction  
Naturally fractured carbonate reservoirs are characterised by complex internal architecture due to diagenesis, karstification, and faulting processes. The Miocene carbonate reservoirs in Central Luconia Province, Sarawak, located in northwest Borneo Island, are mainly affected by syn-rift- or post-rift-induced faults and fractures [1]. The intensity and distribution of the fractures significantly impact the evaluation of these carbonate reservoirs as potential sites for carbon storage [2, 3]. The substantial secondary porosity in the carbonates is proposed as an option for the security of the long-term storage of carbon dioxide [4, 5, 6, 7]. In carbonates, the geometry, attitude, and distribution of fault-related fractures and subsidiary faults can significantly influence the genesis and evolution of karst features [8]. Fractures affect the porosity and permeability properties because they are prone to karstification, which induces permeability and porosity [9] while providing preferential paths for fluid circulation and potential storage capacity in fractured reservoirs. Estimating storage and flow requires fracture network modelling with matrix and karst porosity considerations. This could eliminate uncertainty in appraising potential carbon storage sites in carbonate reservoirs. The term \"carbon\" in this paper refers to \"carbon dioxide\". However, due to the existence of a vuggy system that produces intricate matrix-fracture interactions in carbonate reservoirs, simulating a reasonable fracture network and its connectivity in naturally fractured carbonate reservoirs remains a challenge for researchers [9; 10; 11; 12]. Other issues include the magnitude and resolution of the fractures applied in the modelling. Due to the limited dataset, only specific sizes or scales of fractures could be modelled, which is common, especially in the sub-seismic fracture networks [13; 14; 15; 16]. Ideally, to properly depict the fracture network within a carbonate platform reservoir, a comprehensive dataset with a resolution ranging from micro-scale to macro-scale is required; however, capturing all the data at once is ambitious and challenging. Three-dimensional (3D) seismic data are commonly employed in the industry as the primary source for subsurface structural interpretation at scales from several kilometres to a few meters [17; 18; 19]. However, faults and fractures below the seismic volume resolution are often excluded from the interpretation and are referred to as sub-seismic. They typically measure less than 25 m and are only detected through borehole [17; 18; 19; 20; 21] and core data. An analogue outcrop study is incorporated into the fracture network modelling approach to bridge the observational gap between well and seismic data. Due to changes in stratigraphy, facies, uplift history, and associated stress release during the post-uplift event, the statistical structural analysis performed on outcrop analogues cannot directly apply to deep subsurface prediction. However, analysing the analogue may lead to a better understanding of the subsurface condition and reduce ambiguity at the fracture scale. Analogue studies allow for a detailed structural characterisation of fracture networks at several scales, spanning from millimetres to metres and providing a better solution for upscaling in modelling the fracture network and reducing fracture underestimation [13; 15; 22; 23]. Furthermore, a better prediction of the subsurface can be formed by considering the impact and origin of secondary porosity and permeability. In summary, this study detailed the outcrop-based characterisation of a near-surface carbonate karst system in the Niah National Park, Sarawak, Malaysia. The study focused on the relationship between fractures and paleo-karst features, using an analogue from the Niah Cave-Subis Limestone complex acquired through field and digital structural analyses.