DEPOSITIONAL AND DIAGENETIC CONTROLS AFFECTING RESERVOIR PROPERTIES OF JURASSIC SAMANA SUK FORMATION, TAUDA CHINA SECTION, DISTRICT KURRAM, PAKISTAN

Abstract

The Middle Jurassic, Samana Suk Formation at Tauda China locality, Kurram District is selected for research study. It is located at north-west, Kurram District of Pakistan (33°32′69 ″N, 70°20’026” E). At study area, the Formation consists of thin to thick bedded, light to dark grey and massive limestone with reddish grey sandstone. It contains hard, oolitic, pisolitic limestone and has karstified topography (due to severe dissolution). In some parts, the limestone is partially recrystallized. Non-oolitic limestone is also present at the base of the Formation and some places limestone appear laminated (due to presence of sand). Its upper contact is disconformable with Chichali Formation and lower contact is not exposed in the study area. Brachiopod fossils are found at the base of the Formation. The microfacies identified from Samana Suk Formation include intraclastic ooidal grainstone, intraclastic bioclastic ooidal peloidal grainstone, peloidal bioclastic grainstone, bioclastic ooidal grainstone and bioclastic peloidal ooidal packstone, intraclastic peloidal ooidal wackestone and mudstone facies. On the basis of reported microfacies ramp margin depositional environment has been assigned to this Formation. The diagenetic fabrics observed in Samana Suk Formation are compaction, cementation, dissolution and recrystallization. Diagenetic history reveals prominent microbial activity during early marine diagenesis which caused micritization of ooids and deformation of their internal structural. Early diagenetic cement identified as fibrous and bladed type calcite cement. Shallow burial phase resulted in the physical compaction and brittle deformation of bioclast and quartz grain. Some pore filling calcite and recrystallized calcite are also the representative of this phase. Deep burial stage involves chemical compaction in the form of stylolites and stylolitic seams that are quite abundant in thin sections. While uplifting the rock to surface is evident from tectonically induced fractures and dissolution of calcite grain. Pyritization and cementations is the products of later stages of diagenesis. This Formation is strongly affected by diagenetic processes which has significantly reduced the pores and open fractures in the study area.