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Evolution of Pressure and Stress in Salt-Suture Mudrocks

Abstract

Normal 0 false false false EN-US X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Calibri",sans-serif; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} We develop drained and transient evolutionary models to study stress and pressure in salt-suture mudrocks. Using drained models (pore pressure remains hydrostatic), we show that sediments along a suture are weak and highly sheared, because of significant extensional strains associated with the suture closing. We find that these extensional strains prevent volume compression and porosity decrease. In addition, they result in a low minimum principal stress. Using transient models, we find that high overpressures develop along the suture and in the basal suture area. We show that these high overpressures also result in low-strength sediments. We further illustrate that a dissipation path is established along the suture, allowing deeper suture sediments to drain towards the surface. This dissipation allows compression and porosity decrease in sediments. Once the suture closes, however, the basal area develops high pore pressures, near the lithostatic value. We built our transient evolutionary models using the Finite Element program Elfen. We model sediments as a poro-elastoplastic material and salt as solid visco-plastic. Sediment permeability properties are calibrated using experimental testing on Eugene Island material. Overall, we show that the low minimum principal stress together with the presence of overpressure result in narrow drilling windows along sutures. In addition, because of significant extensional strains, suture sediments have higher porosity values, lower calculated seismic velocities, and much lower acoustic impedance than mudrocks with equivalent overburden located outside the suture.