In Situ Stress Orientation From 3 km Borehole Image Logs in the Koyna Seismogenic Zone, Western India: Implications for Transitional Faulting Environment

Abstract

Knowledge of the in situ stress regime is critical to investigate the genesis of recurrent triggered seismicity over the past five decades in the Koyna seismogenic zone. Orientations of in situ horizontal stresses are determined for the first time from analyses of image logs in a 3 km deep scientific borehole KFD1 in the area. KFD1 passed through 1,247 m thick Deccan Traps and continued 1,767 m in the granitic basement. Stress-induced wellbore failures, breakouts and drilling-induced tensile fractures, are identified in the acoustic and microresistivity images of the granitic basement. Additionally, tightly constrained focal mechanisms of 50 earthquakes of M ≥ 3.6, reported in literature, are inverted to constrain the stress regime. Salient results are as follows: (i) wellbore breakouts and drilling-induced tensile fractures constrain N9°W ± 17°orientation for maximum horizontal principal stress (S Hmax ); (ii) consistency with the regional NNW-SSE orientation of S Hmax from inversion of earthquake focal mechanisms shows that the borehole data are representative for the Koyna region; (iii) breakout rotations at multiple depths below 2,100 m indicate that the borehole may have passed through localized fault damage zones; (iv) consistent strike azimuths of steeply dipping fractures with S Hmax orientation indicate favorable conditions for strike-slip to normal transitional faulting environment; and (v) stress inversion of 50 well-determined earthquake focal mechanisms supports transitional faulting environment in the Koyna seismogenic zone. Thus, stress orientation and fracture information from borehole data, together with stress regime constrained from inversion of earthquake focal mechanisms, shed new light on the faulting environment in the region.

Depth wide distribution and metabolic potential of chemolithoautotrophic microorganisms reactivated from deep continental granitic crust underneath the Deccan Traps at Koyna, India

“…The Koyna–Warna region has gained substantial research interest owing to the intermittent reservoir-induced seismic activity that started after the impoundment of the Shivajisagar (Koyna) artificial water reservoir in 1962 ( Gupta, 2017 ). A scientific borehole (3 km deep, named as Koyna pilot borehole, KFD1), was drilled in 2017 for setting up a deep borehole observatory in the region ( Goswami et al, 2020 ). Rock cores obtained from various depth intervals (1,679–2,908 mbs) through this borehole were used to reactivate and enrich microorganisms under chemolithoautotrophic conditions.…”

Microbial diversity and function in crystalline basement beneath the Deccan Traps explored in a 3 km borehole at Koyna, western India

“…KFD1 passed through the entire thickness of Deccan basalt (1247 mbs) and continued 1767 mbs into the crystalline basement (Roy, 2017). In situ geophysical well log data, wellbore images and chemical and noble gas isotopic composition of formation gases collected during drilling provided compelling evidence that the borehole has cut across localized fault and damage zones, particularly below 2100 m depth (Goswami et al, 2019(Goswami et al, , 2020Podugu et al, 2019). The fault damage zones were characterized by anomalous physical and mechanical properties, stress-induced shear-wave anisotropy, rotations in stress orientation and anomalous escape of 4 He gas of up to 7.8 ppmv when compared with the atmospheric abundance of 5.24 ppmv.…”

Heterogeneity Versus Anisotropy and the State of Stress in Stable Cratons: Observations From a Deep Borehole in Northeastern Alberta, Canada

“…Although they observed a mostly consistent orientation for borehole BOs, this indicator varied by as much as 90° over certain depth sections, which did not appear to correlate with any obvious variations in lithology or physical properties. Most recently, Goswami et al (2020) analyzed image logs through nearly 1.8 km of Neoarchean Dharwar craton granites and gneisses underlying a 1.2 km thick package of Cretaceous Deccan basalts. These rocks, however, appear faulted and fractured, possibly as a result of the rifting of the Indian subcontinent from Seychelles and the subsequent Deccan volcanism.…”