Fracture network mapping using image logs from Baghewala structure, Bikaner Nagaur sub-basin: Implications for well profile and hydrocarbon production

Fracture network develops in response to deformation in competent rocks and are often related to fault related folding (Allmendinger, 1982; Watkins et al., 2018). The Baghewala structure in the Bikaner-Nagaur sub-basin is a fault related anti-form that encompasses the Marwar Supergroup. The structure has a spatial extent of ~12 km² and is bounded by a major ENE-WSW trending reverse fault. The Upper carbonate (UC) Formation of Marwar supergroup here carries dominantly the dolomites and is traversed by extensive fractures that are visible in cores and image logs. This is a zone of severe mud loss and drilling complications.  

We study the image logs from 6 wells to decipher the fracture orientation, understand the deformation mechanism and the implications for drilling and hydrocarbon production. The interpreted fractures from image logs are observed to be dominantly high-angle (60°-90°) extensional fractures oriented along ENE-WNW direction with respect to the sub-horizontal bedding. Additionally, it is seen that the fractures are confined to dolomitic part of UC Formation. Spatially the wells nearer to the fold crest and fault show higher fracture intensity compared to the peripheral wells. The transpressional tectonics after the deposition of the Marwar Super group induced ~NW-SE compression that led to the formation of fault related Baghewala fold. Consequently, the outer arc extension resulted in formation of the syn-post folding fracture network (Price 1966). The fractures thus can be inferred as late-stage high angle fractures due to its orientation (Basa et al., 2019; Ahmed and Bhattacharyya, 2021) and relative prominence (Ismat and Mitra, 2001). Dolomites accommodate deformation by forming fracture networks and fracture intensity is higher, proximal to the fault zone and fold structure (Ahmed and Bhattacharyya, 2021). Thus, lithology and structural position played a role in the partitioning of fractures vertically and spatially.

We see the horizontal wells that are oriented sub-parallel to the fracture network have significant history of mud loss within UC compared to the wells whose profile trends at high-angle to fracture orientation. The deviated wells oriented along the major fracture network will encounter weak planes leading to drilling complications compared to profiles that are oriented across. Therefore, understanding orientation of fracture networks has implications in designing deviation profile of the wells. In low permeability rocks like dolomite, fractures can affect fluid flow due to increase or decrease in permeability (Hanks et al., 2004). The extensional fractures developed in the Baghewala structure can lead to increase in permeability in the reservoir zone also i.e. the competent Jodhpur sandstone. We see enhanced production in one well that is closer to the fault zone which may be due to the increased permeability effected by the fractures, but the data is limited to conclusively prove this. This study will help to design the well position and trajectory not only to avoid mud loss and drilling complications but also to increase production by optimally placing the wells in proximity to the higher fracture intensity after arriving at a Discrete Fracture Network (DFN) model.