Depositional Environment, Diagenetic Alteration and Porosity Development in the Early Eocene Carbonates, Encountered in Adhi-7 Well, Potwar, Pakistan

Abstract

  The measured portion of the Early Eocene rock, encountered in Adhi-7 well, constitutes dominantly of limestone with subordinate partially dolomitized limestone and shale. On the bases of fossil contents, underlying and overlying rock units, these carbonates have been designated as Sakesar Limestone. Five microfacies have been delineated on the bases of petrographic analysis. Fossil assemblage, especially abundance of calcareous larger benthonic forams, locally common occurrence of green algae and echinoids etc., indicate warm shallow marine environment of deposition for these carbonates. Mud supported nature of the rock, i.e., occurrence of high content of matrix (in the form of carbonate mud) supports low energy environment, probably below wavebase.
Chemical compaction (pressure solution) has played dominant role during diagenesis, resulting in the development of nodular structure, stylolites, grain contact sutures, sutured and non-sutured seams. Dolomite crystals have formed locally along some of the stylolites and sutured seams. Primary porosity could not develop due to mud-supported nature of these carbonates, as these were deposited in low energy environment. Similarly secondary porosity could not formed due to the influence of chemical compaction during diagenesis. Presence of dark brown and black residual bitumen in association with the dolomite crystals along some of the stylolites, is probably the only evidence of some very localized effective secondary porosity within these carbonates.