Modes of occurrence of calcrete and dolocrete horizons within the Miocene Lower Fars Formation in North Kuwait

The Lower Fars Formation in the subsurface of north Kuwait consists of repeated fining upward sequences of sand, sandstone and shale. Two wells, one each from two north Kuwait fields, were selected to investigate the presence of calcrete and dolocrete in it. Total forty-four core samples of the host sediment, calcrete, and dolocrete were analyzed using thin section petrography, XRD, XRF, and SEM techniques. Additionally, granulometric analysis was also performed for the host sediments. Fabrics and textures of the studied calcrete indicate two genetic types, namely; phreatic groundwater and pedogenic that display several development stages with two end members: incipient and mature calcrete. It was concluded that the nature of the host sediments, the hydrogeochemical conditions, and the prevailing climate played significant role in the development of these calcrete horizons within Mutriba field. The occurrence of cyclic nodular dolocrete horizons within the South Ratqa sequence is mainly attributed to the difference in the chemistry of groundwater and nature of host sediments. Textures of the calcitized dolocretic horizons suggest that the migration of oil to the Lower Fars sequence took place after the completion of all diagenetic processes. Palustrine dolomicrite identified in the eastern part of the study area exclusively in two types as; nonfossiliferous and fossiliferous.