Lower Cretaceous sandstones of the Qishn Formation have been studied by integrating sedimentological, petrological and petrophysical
analyses from wells in the Masila oilfields of eastern Yemen. These analyses were used to define the origin, type of diagenesis and its relation
to reservoir quality. The sandstones of the Qishn Formation are predominately quartz arenite to subarkose arenite with sublitharenite and
quartz wackes displaying a range of porosities, averaging 22.33%. Permeability is likewise variable with an average of 2844.2 mD.
Cementation coupled with compaction had an important effect on porosity destruction after sedimentation and burial. The widespread
occurrence of early calcite cement suggests that the sandstones of the Qishn Formation lost significant primary porosity at an early stage
of its diagenetic history. In addition to poikilotopic calcite, several different cements including kaolinite, illite, chlorite and minor illite–
smectite occur as pore-filling and pore-lining cements, which were either accompanied by or followed the development of the early calcite
cement. Secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar grains. The
new data presented in this paper suggest the reservoir quality of Qishn sandstones is strongly linked to their diagenetic history; hence, the
reservoir quality is reduced by clay minerals, calcite and silica cements but is enhanced by the dissolution of the unstable grains, in addition
to partial or complete dissolution of calcite cements and unstable grains.