Diagenetic Studies and Its Implications on the Reservoir Character of the Anisian-Norian (Triassic) Kingriali Formation, Salt Range (Pakistan)

Abstract

Abstract

Present studies deal with the field observations, petrographic studies and geochemical investigations dolomite in the middle Triassic Kingriali Formation (Salt Ranges, N. Pakistan). Field investigations revealed that medium to thick bedded dolomite with considerable good reservoir behaviour are observed in the study area as well as encountered in different exploratory wells drilled in the Potwar region.

Petrographic studies indicate three major phases of dolomitization that include; matrix selective dolomitization with high porosity values, followed by closely packed mosaic of non-planar to planar low porous RD-1 replacive phase, and finally modified by neomorphism process and result in the formation of RD-2 replacive phase during progressive dolomitization. Petrographic observations suggest that replacement of limestone by dolomite is the most common process of dolomitization as observed during this study. The process of matrix selective dolomitization, crystal overgrowth as overdolomitization and dolomite cementation, development of moulds and vuggs, presence of calcium sulphate cement, occurrences of two dolomite populations and formation of saddle dolomite strongly support the process of replacive dolomitization within Kingriali Formation.

Stable isotope analysis representing various dolomite phases (MZKD-2, MZKD-5, MZKD-6, MZKD-3) indicate slight depletion in C and O values (i.e., from -4.53 to -3.32) and interpreted that dolomitization resulted from slightly high temperature basinal brines and meteoric water. In addition, C & O isotope signatures of dolomite phases MZKD-1, MZKD-4 & MZKD-8 show purely marine signatures. Petrophysical interpretation shows considerably high values of porosity and permeability which is assumed to be as a result of different processes involved in the modification of shallow marine limestone to present day dolostone. Development of moulds and vuggs by dissolution of calcite at shallow depth, mole per mole replacement of high Mg unstable calcite and aragonite and fracturing due to active tectonism and burial compaction are the processes involved in porosity & permeability enhancement. 3D porosity values from plugs analysis ranges from 8.62 to 16.73% while values for 2D porosity calculations ranging from 2.16 to 30.37%, similarly values for air permeability ranges from 0.064 to 30md while Klinkenberg permeability values ranges from 0.037 to 27.2md which is in the range of a good hydrocarbon reservoir.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016