Can present-day stress orientations be determined in salt? Borehole geometry analysis from the Fore-Sudetic Homocline, Poland

Abstract

We analyse the regional-scale orientation of present-day horizontal stresses within the Zechstein salt deposits of the Fore-Sudetic Homocline, based on an analysis of the geometry of 23 boreholes. Using six-arm dipmeter logs (SADs), we identified systematic elongation of borehole cross-sections within salt intervals. We refer to these as breakout-like structures. These elongations demonstrate remarkable consistency within local groups of boreholes and are interpreted as resulting from natural horizontal stress anisotropy, with orientations that serve as indicators of the maximum horizontal stress (SHmax). Natural differential stresses in salt of <1 MPa can result in borehole wall stress differences of up to 4 MPa. We hypothesize that the maximum shear stress acting on the borehole wall immediately after drilling (up to 32.5 MPa) is sufficient to initiate cataclastic deformation at the grain scale and to generate microcracks. These microcracks may enhance borehole convergence and pressure-solution processes at a rate proportional to the shear stress and to the density of microcracks, which leads to observable ellipticity of boreholes in salt. Natural stress anisotropy is likely driven by differential lithostatic loading due to salt’s depth variations, generating horizontal pressure gradients of ~1 MPa/km instead. Another factor could be the gravitational gliding of competent layers down the slopes of salt-basement highs. These inferences are hypothetical but represent the first attempt to identify a proxy for estimating SHmax orientation in weak salt deposits at a regional scale.