Depositional setting, mineralogical and diagenetic implication on petrophysical properties of unconventional gas reservoir of the silurian qusaiba formation, northwestern arabian peninsula

The Silurian Qusaiba Formation is known for its high potential as one of the regional unconventional hydro- carbon resources. Multiple samples cropping out in Tayma Quadrangle, Northwest Saudi Arabia, were analyzed extensively. Field work and comprehensive laboratory work was conducted including geochemical and petro- physical measurements all supported by petrographic imaging. Qusaiba Formation was classified from bottom to top into five lithostratigraphic units. Lithologically, Units 1 and 3 are composed mainly of thinly laminated shales, while units 2 and 4 are characterized with interbedding shale and siliciclastics in form of siltstone and sandstone. Uppermost Unit 5 exhibits mixed lithofacies of silty-shale, muddy-siltstone and siltstone. Clay min- erals constitute the main components in Units 1, 3, and 5 while quartz, feldspar, and carbonates are dominant in Units 2 and 4. Mica, pyrite and gypsum are distributed in all units in varying proportions. Inorganic geochemical indicators suggest that shale and muddy silt subunits are predominantly of mafic to intermediate parental ori- gins, while those of sandy and silty subunits are of intermediate to felsic origins. Qusaiba sediments appear to be derived mainly from passive margin with few from continental arc settings with limited continental facies. The variant distribution is mainly attributed to the siliciclastic input to the depositional basin. The applied redox and clay types identified a warm and humid low salinity shallow marine environment. Shale and muddy-siltstones subunits appear to be moderately to highly weathered while sandstone and siltstone units are slightly to moderately weathered. Petrophysically, tested samples lithologies are reflected by the different patterns of pore size distribution and wide porosity and permeability variations giving an insight into the depositional envi- ronment and diagenesis. Samples are characterized with different pore systems including intergranular inorganic effecting considerably the petrophysical properties, intragranular organic pores with the least effect and fracture pores inducing the highest influence. Petrographic analyses by different imaging techniques confirmed the mineralogical and petrophysical findings.