Diagenesis of Jauf sandstone in Hawiyah area (Saudi Arabia)

Jauf Formation of Lower Devonian age is an important hydrocarbon reservoir in Saudi Arabia. The sandstone reservoirs of Jauf Formation are mostly fine to medium-grained, moderately to well-sorted and texturally mature quartz arenites. Average composition of the framework grains of the sandstone reservoirs of Jauf Formation is 77% quartz, 2% feldspar and 0.1% rock fragments. Minor framework grains include carbonaceous materials, micas and heavy minerals. Major authigenic cements include calcite; chlorite, illite and quartz while minor amount of pyrite and illite/smectite clay are present.; The diagenetic processes include compaction, cementation and dissolution (secondary pore generation). The degree of compaction is evidenced by long, concavo-convex and sutured contacts. Compaction reduces pore space. Cementations include an early calcite cementation phase followed by clay coatings (chlorite and illite), quartz overgrowths cementation and late calcite cementation. In addition, some minor pyrite cementation has occurred in early stage of burial. Early-calcite cement is poikilotopic and has occupied most of the pore spaces in some samples reducing the primary porosity. Calcite is also present as isolated patches filling pore spaces as late calcite cement. Chlorite pore fillings and grain coatings have reduced both the porosity and permeability to some extent but also have helped to retain initial porosity at depths by retarding the development of quartz overgrowths. Illite cement is most commonly found as hair-like growth coating the grains and bridging pore throats. This reduces the permeability dramatically. Quartz overgrowths have reduced the porosity and permeability significantly. The late-calcite cement does not have a profound effect on the samples. The invasions of acidic fluids have resulted in partial dissolution of calcite cements and feldspar grains creating secondary porosity in many samples.