Diagenesis and reservoir quality of mixed aeolian and fluvial sandstone deposits, Argana Valley, Morocco

Abstract

The mudstone-dominated middle and upper parts of the sedimentary fill of Argana basin contains meter-scale sedimentary cycles that usually comprise ephemeral lake shales at the base, playa mudflat mudstones in the intermediate part, and both fluvial and aeolian sandstones at the top. The fluvial and aeolian sandstone facies were petrologically studied. The sandstones are sublitharenites and, less commonly, quartz arenites and lithic arenites with average framework composition of Q85F2L14. The grains are generally moderately to well-sorted, sub rounded to rounded, very fine to coarse grained sands with typically grain-supported fabric. The aeolian facies show little compaction effects whereas the fluvial facies are more compacted. The main diagenetic phases that affected the sandstones were (1) ferruginous clay grain coatings (2) vadose infiltration of some clay into aeolian sands (3) precipitation of poikilotopic cement (4) cementation by anhydrite/gypsum/halite? (5) mechanical compaction through the presence of long contacts and bending or deformation of argillaceous fragments against rigid quartz grains (6) chemical compaction by smearing of argillaceous materials into pore-spaces, long, concavo-convex and suture contacts, pressure solution along quartz to quartz contacts and consequently (7) quartz overgrowth (8) blocky calcite cementation (9) dehydration of gypsum to anhydrite and dissolution of framework grains and cements. The distribution of porosity and reservoir quality is facies controlled with the aeolian facies having average porosities of 25% due to preservation of primary porosity following inhibition of quartz overgrowth by thin ferruginous clay coats or dissolution of eodiagenetic calcite cements. Fluvial facies on the other hand had average porosities of 17% and the poorer reservoir properties were due to pervasive quartz overgrowth, mechanical and chemical compaction and near surface precipitation of eodiagenetic calcite. The reservoir properties of the fluvial sandstones were diagenetically enhanced in some parts by framework grain and cement dissolution.