Reservoir heterogeneities due to diagenesis in Jurassic Samana Suk Formation Kahi section Nizampur Basin North West Himalayas Pakistan

sedimentary rocks, which is fundamentally influenced by both
depositional and diagenetic processes. The Jurassic carbonates are
targeted in many regions for oil and gas exploration. The current
study is carried out to elaborate on the diagenetic alterations and their
effect on reservoir properties. A thick outcrop of the Jurassic Samana
Suk Formation is studied in the Kahi section of Nizampur Basin
Northwest Himalayas, Pakistan, to study and relate the trend of
lithological variations and depositional settings. Field investigation
revealed that the Samana Suk Formation is extensively distributed in
the area and primarily made up of interbedded limestone and
dolomite units. The original unaltered limestone has a thick-bedded
and oolitic to bioclastic nature. Different types of dolomites have
been recognized based on colour contrast and sedimentary features.
Moreover, saddle dolomite cement, calcite cementation, and
mechanical and chemical compaction have also been observed. The
petrographic studies show different types of diagenetic alterations
that affected the Samana Suk Formation, including micritization,
bioturbation, mechanical and chemical compaction in the form of
fractures and stylolites, various calcite cementation, which includes
the various cement types that range from isopachous, blocky,
granular equant, fibrous, and dog tooth cementation, along with
dissolution that occurred in different diagenetic realms. Pyritization
was rarely observed. Moreover, different phases of dolomites were
identified, ranging in size and shape, i.e., finely crystalline to coarse
crystalline and planar euhedral to non-planar anhedral. The stable
oxygen isotope values of these dolomites show depletion from
original marine signatures and suggest burial-related fault-controlled
dolomitization events. Overall, diagenetic processes like dissolution,
fracturing and dolomitization increased the reservoir potential. On
the contrary, the overburden and cement precipitation result in a
decrease in the reservoir properties.