Fracture characteristics and genetic mechanism of overpressure carbonate reservoirs: Taking the Kenkiyak Oilfield in Kazakhstan as an example

Taking the Kenjiyak Oilfield in Kazakhstan as an example, the fracture characteristics and genetic mechanism of carbonate reservoirs with overpressure were discussed. The study showed that carbonate reservoirs are formed under overpressure developed hydraulic fractures besides tectonic fractures and karstic fractures. The Carboniferous sub-salt hydraulic fractures in Kenkiyak are mainly formed in the Mesozoic and Cenozoic, on a macroscopic level they cut rocks into brecciated texture or are packed by calcite veins, on a microscopic level they are in a radiant distribution around pores. The formation and evolution of the fracture system in the region experienced three main stages, including structure transforming and uplifting, structure stretching and subsiding, and hydraulic cracking. In the stage of hydraulic cracking, salt movement and tectonic movement lead to the formation of overpressure zone, with the pressure coefficient of 1.84. Overpressure leads to the formation of hydraulic fractures in the Carboniferous low permeable carbonate reservoirs under the Permian non-permeable mudstones, and also leads to reopening, widening and prolonging of former tectonic fractures. Hydraulic fractures are mainly the extensional fracture, which has late opening time and low filling degree, thus is the main effective fractures in the study area.