RESERVOIR POTENTIAL OF THE MAASTRICHTIAN PAB SANDSTONE IN THE EASTERN SULAIMAN FOLD‐BELT, PAKISTAN

Abstract

The hard, resistant, generally cliff‐forming Pab Sandstone Formation of Maastrichtian age is well exposed in parts of the eastern Sulaiman Fold‐Belt. Thin‐section, XRD and SEM studies of 66 samples from seven outcrops extending 200 km along strike were made in order to assess the reservoir quality of this formation. Petrographic studies indicated that the sandstones, classified as quartz arenites (locally subarkose), are texturally and mineralogically mature. The proportion of potassium feldspar varied from trace to 16%, and this mineral was confined to certain stratigraphic zones. Alteration of feldspars to clay minerals was common, particularly in the south of the study area. Plagioclase, heavy minerals, and lithic fragments were present in minor amounts.Diagenetic kaolinite, illite and chlorite were common, with kaolinite being the most abundant. Quartz overgrowths constitute the dominant cement. The reservoir quality of the formation largely depends on the composition of the sandstone, and its susceptibility to chemical alteration. Most of the porosity (up to 16%) results from the dissolution of feldspar grains; dissolution was restricted to the south of the study area, and the feldspars in the north show little alteration and no dissolution.Reservoir potential has also been influenced by the substantial input of silica‐rich solutions that completely destroyed the primary porosity of the sandstone. The observed porosity is mostly secondary, commonly formed by dissolution of feldspar grains, and occasionally cement/matrix.The sequence of diagenetic events is, from oldest to youngest: formation of pyrite; formation of early‐stage calcite; formation of dolomite; silica cementation; formation of clays; precipitation of late‐stage calcite; feldspar dissolution; and formation of iron oxides. The textural and mineralogical maturity, coupled with local well‐developed secondary porosity, may make the Pab Sandstone Formation a good reservoir rock.