Mineralogical And Reservoir Characteristics Of Well Cutting Samples Of Sandstone Of The Permian Tobra Formation From Adhi Field, Potwar Basin, Pakistan
Abstract
Adhi Oil and Gas Field represents a NE – SW trending salt cored pop up structure which hosts three reservoirs i.e. Sakesar Limestone (Eocene), Tobra Formation (Permian) and Khewra Sandstone (Cambrian). This study is aimed at examining the depositional texture and diagenetic features of the well cuttings from the sandstone unit of the Tobra Formation and their influence on its reservoir characters using a combination of petrographic, XRD, SEM-EDX and grain size analysis techniques. On the basis of framework grains (Quartz80.81 Feldspar12.35 Lithic fragments6.6), the Tobra Sandstone samples are classified into sub-feldspathic arenites, sublithic arenites, and feldspathic wackes as the most abundant varieties while quartz arenite and subfeldspathic-sublithic arenite are encountered in few intervals. Grain size analysis of Tobra Sandstone samples shows sub-angular to sub-rounded and poor to moderate sorting as indicated by the polymodal nature of majority of the samples (~1.22 -≤5.2 Ф) with few being moderate to well sorted.
The matrix is dominantly consisting of ferruginous clays which shows variable concentration in the studied samples and ranges from 6 to 17% with a mean value of 6.6%. XRD and SEM-EDX analysis of the fine fraction shows illite as the principle clay constituent while kaolinite and montmorillonite occur as the minor mineral phases. Along with sandstone fragments, individual shale cuttings are also encountered in the studies Tobra samples which may occur as interlaminations with the sandstone layers. These shale fragments are also consisting dominantly of Illite, a clay mineral. Mineralogical and textural characteristics exhibit that the Tobra Sandstone bears poor to moderate reservoir potential with few being better ones. Several features such as the abundance of Illite, occurrence of planer to concavo-convex grain boundaries and the rarity of growth structures propose an intermediate burial compaction model for the studied sandstone unit of the Tobra Formation. Mineralogical results indicate that the detritus for the sandstone was derived from the craton interior of the Indian Plate as well as recycled orogen located to the south and southeast of the present-day Potwar Basin and Salt Ranges.
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