Geology/reservoir sandstone with secondary porosity
in Pakistan which sandstone has secondary porosity?
Good question but difficult to answer as there is not enough data about the secondary porosity reservoirs of Pakistan. However, we have to understand the processes responsible for creating secondary porosity in sandstone. Secondary porosity is developed in the rocks when primary porosity is often destroyed by compaction, pressure solution and cementation.
Shallow depth sandstone (less than 1 km) may often retain the initial depositional porosity whereas secondary porosity development is likely to develop from 1km to 3Km.
Post depositional processes which create secondary porosity are as follows;
1) Fracturing in the rock (see outcrops and wells logs and fracture image logs)
2) Shrinkage of the matrix (petrographic thin section analysis)
3) Dissolution of sedimentary grains and matrix (petrographic thin section analysis)
4) Dissolution of authigenic cements (petrographic thin section analysis)
5) Dissolution of authigenic replacive minerals.(petrographic thin section analysis)
In Pakistan majority of the oil and gas volumes flowed from reservoirs of Tertiary Permian and Cambrian carbonates. Production is mainly coming from Lower Goru Sandstone from Badin Block and Pab and Mughal Kot sandtstone (all Cretaceous) from fields inside or adjacent to Kirthar fold belt whereas Jurassic sandstone have also been found prolific reservoir in the Potwar and Kohat basin.
Considering the tectonic location of those sandstone reservoir fields in Indian plate, Lower Goru sands are mainly in the Cretaceous rift related basin followed by a relative quiescence of Indian plate before the Himalayan orogeny fractures are less likely phenomena to develop secondary porosity whereas other remaining four process would be active to create secondary porosity but we have not enough information or data to authenticate.
All other sandstone reservoir of Pakistan due to their position in the Indian Plate would have been affected by compressional tectonics (which was a more latest event in Late Tertiary; 30 to 5 Million years) and most likely to have fracture porosity specially those sandstone reservoir located in highly folded and faulted anticlines with relatively thicker sandstone bodies having more rigid and brittle lithological behavior. These are formations, namely; Jurassic Datta, Upper Cretaceous Pab and Mughl Kot formations.
Of the occurrence of other 4 process for secondary porosity development we have no option but to believe that process of creating secondary porosity in other sandstone reservoirs (mainly Lower Goru Formation)would have been actively going on through geological time.
In Summary, in the sandstone, fracture porosity likely to be only in those fields which are tightly folded and near the reverse faults whereas all other four processes would have been active in all other sandstone reservoirs unless we have data to show otherwise or define which process creating secondary porosity more actively.
Hope, this answer may help.
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