From Plate to Pore: Paleoclimate, Paleosols, and the Effects of Early Clay and Microquartz Grain Coatings from Deep Porosity in Permo-Carboniferous Unayzah Sandstone, Saudi Arabia
Stephen G. Franks
Advanced Research Center, Saudi Aramco, Dhahran, Saudi Arabia
Movement of the Arabian Plate from high to low latitudes during Late Carboniferous to Late Permian and the associated climatic changes had profound effects on reservoir quality of Unayzah sandstones. Periglacial braided stream sandstones of lower Unayzah C were deposited in a cold, relatively dry climate. Overlying Unayzah B was deposited as glaciers began to melt depositing glacio-lacustrine sandstones, siltstones, and diamictites. Further northward movement of the Arabian plate, yielded hot-arid conditions during deposition of Unayzah A, and at tropical latitudes warm and humid conditions prevailed during deposition of the fluvio-deltaic to marine Basal Khuff Clastics (BKC).
Clayey paleosols formed during BKC time produced abundant illuviated clay, sesquioxides, and early kaolinite which in some areas completely plugs pores in underlying sandstones. Less mature paleosols in Unayzah A, associated with brief humid intervals, produced thin clay coats around sandstone grains and preserved primary porosity by inhibiting quartz cementation during deep burial. In addition to pedogenic clay coats, microquartz grain coating associated with silcrete intervals in the Unayzah A also inhibited later quartz cementation. Within the silcretes, brecciated length-slow chalcedony infilled by length-fast chalcedony suggests initial silicification of evaporites under arid conditions followed by remobilization of silica and formation of microquartz during wetter periods.
Coated-grain sandstones in the BKC and upper Unayzah A are often friable and porous. They may have very good or extremely poor permeability depending on thickness of grain coatings. Sand production can be a problem in these friable intervals.
In contrast, Unayzah C paleosols are poorly developed or absent. Clay coats are uncommon, and microquartz is absent.
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