Characterization of fracture networking and connectivity: Insights from Discrete Fracture Network Model and Multi-Scale analysis from the Buxa Dolomite of the Main Boundary thrust sheet, Siang widow, Arunachal Himalayan Fold thrust belt

Fracture systems develop at different stages of progressive deformation and are often genetically associated with folding. The frontal Main Boundary thrust (MBT) sheet is folded in a fault bend fold (Ahmed et al., 2018) and is exposed in Siang window in far eastern Arunachal Himalayan fold-thrust belt (FTB). The Buxa dolomite of the Lesser Himalayan sequence forms part of the MBT sheet and records four different sets of fractures (Basa et al., 2019). We present results from Discrete Fracture Network (DFN) model from the Buxa dolomite. Integrating fold test, cross-cutting, offset and abutting relationships, we have established that the low-angle fracture set (0°-20°) formed as a result of early layer parallel shortening.  These low-angle and the two sets of medium angle fractures (20°-60°) formed prior to the fault-bend folding. The late stage, high-angle fractures (60°-90°) developed synchronous to the fault-bend fold (Basa et al., 2019). We model the fractures formed before and during the folding event using 3D MOVE’s Fracture Modeling module to evaluate how the properties of secondary porosity and permeability, induced by fracture sets, fracture area/unit volume (P32) and overall connectivity are affected by the folding event. The input parameters of fracture orientation, intensity, length and aperture were measured from the field. For the aspect ratio, theoretical value of 1:2 (Olding, 1997; Olson, 2003) was considered.

Results from DFN analysis indicate that the average porosity increases from pre-folding (model-1) (~0.0028) to syn- to post-folding (model-2) (~0.0071). The permeability also increases from ~231 Darcy in model-1 to ~3988 Darcy in model-2. There is also a significant rise in P32 (~2.8m²/m³ to ~4.3m²/m³⁾ value from model-1 to model-2. The late high-angle fracture set led to increase in overall connectivity, including porosity, permeability and fracture intensity. This is also corroborated from the field results that reveal high-angle fractures are more conducive to vein formation (~41%) compared to the lower angle fracture-sets (~15 %).