Impact of diagenetic and depositional fabric on reservoir quality; a synthesis from Late Cretaceous and Early Paleocene sandstones in Eastern Sulaiman Range, Pakistan

Abstract

The Maastrichtian Pab Sandstone and the Danian Khadro Formation are among the major reservoirs of the region but are considered to be tight and are therefore poorly evaluated. A 551-m-thick section of the Pab Sandstone and 128-m-thick section of the Khadro Formation are measured and logged in detail. The measured units mainly composed of thin- to thick-bedded sandstone with subordinate mudstone and occasional marls that were deposited in the western passive margin of the Indian Plate. Eight and four depositional facies have been identified in the Pab and Khadro Formations, respectively, and are grouped into pro-delta, delta-front, and delta-plain facies associations which were deposited in fluvial-wave-dominated deltaic settings. Thirty-two samples of sandstones from Pab Sandstone and 20 from Khadro Formation are collected for reservoir and petrographic studies. The sandstone of studied formations are classified as quartz aernite, subarkosic, sublitharenite, and wackes. These are fine to coarse grain, subangular to rounded, and poorly to moderately sorted. The intense burial diagenesis resulted in compaction, extensive cementation, and authigenic mineralization which cause low values of porosity/permeability. Cements have filled all the available pore spaces and clay minerals block pore throats, thus resulting in major destruction of porosity and permeability. The late stage dissolution of unstable framework grains and cements has resulted in appreciable increase in secondary porosity. The petrographic studies and plug porosity/permeability results of outcrop samples suggest that laterally continuous and vertically stacked thick sandstone sequences of delta-plain and delta-front facies associations possess good values of porosity.