Sub-Seismic Transverse Faults and Fractures Sets Identification Through Geomechanical Modeling Using Elastic Dislocation (ED) Theory: A Case Study from Kohat-Potwar Foldbelt Zone, Upper Indus Basin (UIB) of Pakistan
Authors: Khan Imran , Shahzad Rana Faisal , Khan Zahoor , Ali Iftikhar , Qureshi Muhammad Akram , Shams-Ul-Alam _ , Ahmed Shakeel , Abbasi Shabeer Ahmed , Awan Uzair Hamid 
Published: October 2023

The research work was carried out for sub-seismic transverse faults & fractures corridor identification, its characteristics and impact on wells placement & address failure reasons of 02 dry wells. On 3D-Seismic data, a structural model was developed and converted to a Geomechanical model after incorporating the carbonates rock geo-mechanical properties. Through forward modeling, the valid Geomechanical model was exercised by Elastic Dislocation (ED) theory (Solutions of Okada, 1985, 1992). From mapping fault geometry and slip distribution in 2D/3D seismic-reflection datasets, the application of ED theory can forecast the distribution of displacement, strain, and stress in the rock volume near major faults. The total strain, including background or remote strain as well as strain from displacement along the fault surface, was calculated. For deformation horizons, this total strain determines stress and failure.

The fracture sets were generated from failure planes in ED modeling. The majority are reverse fracture sets. The fault surfaces play a major role in the ED method for fracture analysis and consists of an array of panels, each contributing to the ED equation calculation. The main outcome is the sub-seismic faults & fractures corridor identification around larger faults. The ED model displays that the majority of predicted sub-seismic faults & fractures set orientation match to the major strike of regional fault planes, while some vary along the strike, being parallel to the main reverse fault directions (E-W or NW-SE), but oblique faults system is also existed along the central segments, agreeing with observed structure dip variations along the strike of faults. The thrusted anticlines' crestal region has more fracture densities than their flanks. Observing abrupt change of sub-seismic faults & fractures sets density & orientation in the Geomechanical model result, it was concluded that some transverse faults systems may exist sub-seismically, which is almost parallel to the compressional forces (N-S) direction and opposite to seismic scale faults trends (E-W or NW-SE) and the majority of predicated sub-seismic faults & fractures sets orientation. Secondly, in ED modeling we have noticed that well-01 (Producing) falls in high fractures density and well-02 & 03 (Dry) falls in low fractures density also having variation in fracture strike orientation, which is verified by the well's FMI results also.

The well planning to drill out the max fractures sets with proper face in the carbonate reservoir is a challenging job. The Eocene carbonate reservoirs in the studied fields have low primary porosity and permeability. The productivity of these reservoirs is dependent on permeable natural sub-seismic fractures. In the case study, an integrated approach was applied in the form of Geomechanical modeling based on ED theory to provide a reliable base for future well planning and have found out the failure reason of wells.