Application of integrated clustering and petrophysical-microscopic methods for facies determination of Campanian–Maastrichtian and Danian–Thanetian reservoirs: Constraints for tectonostratigraphic influences on sedimentation across the NW Indian Plate margin

Abstract

This paper focuses on reservoirs study in tectonostratigraphic perspective using soft computing multivariate statistical technique of clustering analysis (CA) to delineate electrofacies (EF) for rock typing. The EF knowledge was combined with petrophysics, rock fabric number (rfn) and application of microscopic studies comprising thin sections (TS), scanning electron microscopy (SEM), back scattered electron (BSE) and energy dispersive X-Ray spectroscopy (EDS) on fauna acquired from novel drill cuttings of selected wells. The methods were implemented on data of eight exploratory wells from eastern Sulaiman sedimentary basin (Pakistan) to investigate tectonostratigraphic influences and reservoir characteristics of the Campanian–Maastrichtian, and Danian–Thanetian reservoirs. The Danian-Thanetian (Upper and Lower Ranikot Formations) demonstrate good reservoir rocks in the Sulaiman Fold Belt (SFB) and the northern part of the Zindapir (ZA), however, in the Sulaiman Depression (SD), the Danian-Thanetian reservoirs are shale-rich bearing poor reservoir potential based on EF log correlation. The presence of glauconite (Gl) in the Paleocene Ranikot intervals suggests transgressive (TST) or early highstand system tract (HST) for this particular formation. This evidence is further supported by the occurrence of planktonic foraminifera (Pl), the abundance of which is indicative of late TSTs and early stage HSTs. The presence of Pl in the Campanian Parh Limestone indicates an open sea environment. The clay minerals (kaolinite, chlorite, montmorillonite) occurrence in Maastrichtian Pab Sandstone suggest the detrimental effect on reservoir quality, hindering permeability and fluid flow, proposing this particular formation as a tight reservoir. The kaolinite clay type in the Lower Ranikot Sands in the northern part of ZA characterize continental to near-shore sediments, indicative of fluviatile environment. The Maastrichtian Mughal Kot Formation in the SFB indicates arkosic sandstone (feldspar-rich) with some micaceous content and montmorillonite.The findings from the EF correlation of different wells demonstrate that the reservoir rocks in the SD to the southeast are more shale rich in comparison to those encountered in the SFB in the northeast and northwest. The rfn suggests class 1 type rock fabric (grain dominant) for the Cretaceous Parh Limestone. Due to incomplete logging data, it was not possible to derive rfn for Paleocene formations. The results validate the reservoir heterogeneities on microscopic and small-scale using drill cuttings and well logs.Geologically, active tectonics during the Cretaceous and Paleocene, local sea level fluctuations, diagenesis, clay mineralogy and paleo-topography have strongly affected the lateral variation of facies of Campanian–Maastrichtian, and Danian–Thanetian reservoirs. Moreover, regional active tectonics (Himalayan Orogeny) spanning the Cretaceous and Paleogene Periods also played a major role in crafting the reservoir architecture of various rock units. The results demonstrate the Campanian–Maastrichtian and Danian–Thanetian reservoirs as promising exploratory targets in the SFB. The comparison of the achieved outcomes showed a close agreement with the lateral facies variation in Cretaceous (Campanian–Maastrichtian) and Paleocene (Danian–Thanetian) across the study area.