SEQUENCE STRATIGRAPHIC CONTROL ON THE FORMATION OF IRON-OXIDE FLUID BARRIERS: EXAMPLE FROM PARA- AND POSTGLACIAL MELAZ SHUQRAN AND MAMUNIYAT SANDSTONES, UPPER ORDOVICIAN, THE MURZUQ BASIN-SW LIBYA

An accurate prediction of porosity distribution in sandstone reservoirs is one of the most important and critical tasks facing petroleum geologists and engineers during hydrocarbon exploration, appraisal and production. Prediction of porosity distribution can be significantly improved and achieved by integrating diagenesis into depositional facies and sequence stratigraphy. A petrographic examination of 32 sandstone thin-sections from para- and postglacial, tide-dominated deltaic and shallow marine highstand system tract (HST) and transgressive system tract (TST) of the Upper Ordovician sandstones, Murzuq basin, SW Libya, demonstrates the absence of porosity along parasequence boundaries (PBs) and maximum flooding surfaces (MFSs) and relatively fair to good intergranular porosity values (ca. 3.3-20.7%) in the adjacent sandstones between PBs. However, the absence of porosity along PBs and MFS is closely related to pervasive cementation by eogenetic iron oxide. Such pervasive cementation by iron-oxide along these surfaces is attributed to low sedimentation rate and hence prolonged residence time under oxic geochemical zone that was concomitant with the formation of PBs and MFSs. This study suggests that sedimentation rate, which can be predicated by sequence stratigraphic analysis, provides a dominant condition controlling cementation by iron-oxide and thus porosity and permeability distribution. Prediction of the spatial and temporal distribution of iron-oxide cements and thus porosity and permeability distribution is very beneficial for hydrocarbon exploration, appraisal and production in tide-dominated deltaic and shallow marine sandstone reservoirs.