Effect of normal faulting on in-situ stress: A case study from Mandapeta Field, Krishna-Godavari basin, India

Abstract

The horizontal compressive stress orientations for Mandapeta field in Krishna-Godavari (K-G) basin had been calculated from borehole breakout data using four-arm dipmeter caliper logs. The minimum horizontal stress magnitude for Mandapeta field was calculated from pore pressure data using poroelastic equation. Three-dimensional (3-D) stress modeling using finite element analyses has been carried out for layered sediments and faulted layered sediments in the Mandapeta sub-basin. The length and width of the models are chosen as 5 km and 2 km respectively. Vertical depths of models are considered for sediments at depth 2 km to 5 km. This paper presents a numerical study of stress distributions (contours and trajectories) around fault models of layered geometries, submitted to overburden and unequal horizontal compressive loads. Reorientation and magnitude variation of in-situ stresses near faults and stress discontinuities across faults are observed. Changes in the stress patterns are observed at the interfaces between sediments due to faulting and sediment-basement layers due to contrast in elastic constants. Remarkable stress gradients are observed between the sediment and basement. Magnitudes of normal component of stress decrease near the faulting whereas shear stress magnitudes increase at the faulting site. There is increase of horizontal principal stresses in the downthrown faulted layers. The maximum horizontal principal stress rotates about 90° just adjacent to the fault. Fluid flow at depth of oil reservoir could be indirectly influenced by the stress contours and by geometries of the stress trajectories. Fluid migration in low permeability reservoir may occur in response to mean stress gradients induced by the tectonic loading. The areas of high mean stress gradient are located at the interface between sediment and basement whereas the areas of low mean stress gradient is exactly coinciding with the areas of upper sediments.