Evolution of fractures in a highly dynamic thermal, hydraulic, and mechanical system – (I) Field observations in Mesozoic Carbonates, Jabal Shams, Oman Mountains

We studied an exhumed high-pressure cell in outcrops of Cretaceous carbonates on the southern flank of Jabal Shams in the Oman Mountains. This more than 2 km thick sedimentary pile contains reservoir and source rocks in northern Oman and the United Arab Emirates. It develops a complex and rapidly changing anisotropy, due to mechanical stratigraphy and several generations of pervasive regional fault and fracture sets. Calcite cement healed faults and fractures before the next sets were formed. Burial extension within a high fluid-pressure environment led to the formation of four fracture generations by an anticlockwise rotating stress field. This was followed by bedding-parallel shear under lithostatic fluid-pressure conditions at a minimum temperature of 134–221°C deduced from primary and pseudosecondary fluid inclusions in quartz. The high pressure cell was drained along dilatant normal faults that were also repeatedly cemented and reactivated. The rapidly changing mechanical anisotropy, in combination with a chemically reactive system formed a complex feedback system in which the mechanical strength, strain and the permeability underwent major changes in this coupled thermal, hydraulic, and mechanical (THM) system.