Linking diagenesis and porosity preservation versus destruction to sequence stratigraphy of gas condensate reservoir sandstones; the Jauf Formation (Lower to Middle Devonian), Eastern Saudi Arabia

This study attempts to unravel and predict the role of diagenetic alterations in porosity preservation anddestructioninthe deeplyburied (presentdaydepths 4154mto 5130m)gas condensate reservoir sandstones of the Devonian Jauf Formation, eastern SaudiArabia.This goal isachievedthroughlinking thedistributionof thesealterationstothedepositional facies andsystemstracts of the sandstones.Eogenetic alterations (depth<2km;T<70°C) include: (i) formation of grain-coating clays in tidal estuarine sandstones of the transgressive systems tract (TST)andhighstand systemstract (HST); (ii)cementationbyferroan dolomite/ankerite, particularly belowmaximumfloodingsurface in the tidal estuarineTSTsandstones;and(iii) dissolution and kaolinization ofmica,mudintraclasts and feldspars in theHST sandstones, which are attributed to meteoric water flux during fall in the relative sea-level. Mesogenetic alterations (depth≥2 km; T≥70°C) include the formation of graincoating illite and chlorite as well as quartz, Fe-dolomite/ankerite, anhydrite and siderite cements. Oxygen isotopes (δ¹⁸OVPDB=+9.1%) and age (101 Ma) of illite suggest precipitation from evolved formation waters with a d18OSMOW value of +1.2%. Quartz overgrowths are most abundant in the shoreface to coastal plain and HST sandstones (poor in grain-coating clays), whereas quartz outgrowths occur mainly in the TST and tidal estuarine, HST sandstones (rich in grain-coating clays). Sandstones with the best-preserved reservoir quality are encountered in the tidally influenced, channel and estuarine facies of the TST and HST sandstones, whereas sandstones with pervasive reservoir quality destruction occur in the HST, shoreface facies. Reservoir quality preservation in the TST and tidal estuarine HST sandstones is attributed to the presence of significant amounts of grain-coating clays, which prevented extensive cementation by quartz overgrowths. This study demonstrates that linking diagenesis to sequence stratigraphy allows better elucidation and prediction of the spatial and temporal distribution of the impact of diagenetic alterations on reservoir- quality modifications in tidal estuarine and shoreface sandstones.