Title : Integrated drill cuttings analysis for reservoir characterization in structurally complex fractured carbonates: A case study from the Northern Sulaiman fold and thrust belt, Pakistan
Abstract:
Reservoir characterization in structurally complex, coreless wildcat wells remains one of the most challenging aspects of frontier petroleum exploration. This study presents an integrated, cost-effective workflow for reservoir characterization applied to the 92KotPalak-1 well, a wildcat drilled in the tectonically active Northern Sulaiman Fold and Thrust Belt of Pakistan — a frontier region with significant hydrocarbon potential but limited subsurface data. The well encountered serious operational challenges and was completed without conventional core, necessitating an alternative approach to reservoir evaluation. Using drill cuttings as the sole data source, this study reconstructs paleo-depositional environments, evaluates diagenetic controls, and characterizes pore space distribution across five key formations: the Middle Ghazij, Upper, Middle, and Lower Dunghan carbonates, and the overlying Pab Sandstone.
A total of 35 drill cutting samples were subjected to a comprehensive multi-technique analytical suite. Petrographic analysis under plane-polarized light (PPL), cross-polarized light (XPL), ultraviolet (UV), and reflected fluorescence (RF) modes enabled facies typing, fossil identification, porosity classification, and diagenetic characterization. X-Ray Diffraction (XRD) quantified bulk mineralogy and clay assemblages. Mercury Injection Capillary Pressure (MICP) analysis assessed pore throat size distributions, capillary pressure behavior, and estimated relative permeability. Capillary Suction Time (CST) testing evaluated filter cake permeability and clay-fluid sensitivity using deionized water and 7% KCl solutions. All results were integrated to develop a rock-typing framework based on petrofacies, mineralogy, and hydraulic flow behavior.
XRD confirmed dominantly calcitic mineralogy in the Dunghan carbonates (averaging ~85 wt.%), with subordinate dolomite and quartz, while the Pab Sandstone is predominantly siliceous with minor micritic cement and heavy mineral opaques. MICP revealed low to moderate porosity of 2–9% and median pore throat radii ranging from 0.02 to 2.1 micrometers. Carbonate intervals exhibit stylolitic, intragranular, and fracture-associated porosity types, while siliceous zones display interparticle and dissolution pores. CST results confirmed poor permeability in carbonate zones and clay sensitivity in certain siliceous facies. Depositional environments were interpreted as inner to middle shelf for the Dunghan Formation — supported by fossiliferous grainstones and packstones with red algae, bryozoans, and benthic foraminifera — and as beach to upper shoreface for the Pab Sandstone, evidenced by well-sorted, quartz-rich sandstones with preserved porosity. Widespread diagenetic processes including micritization, pressure solution, calcite cementation, and stylolitization occlude primary porosity, though localized dissolution and fracturing enhance secondary porosity.
This work introduces a reproducible, low-cost cuttings-based workflow that bridges qualitative geological interpretation and quantitative reservoir rock typing in data-scarce frontier settings. The methodology delivers actionable petrophysical differentiation between low-quality carbonate intervals with fracture-dominated flow and higher-quality siliceous units that may serve as storage zones or flow conduits — directly informing well placement and fracture stimulation strategies in future development campaigns. The approach is scalable and applicable to other tectonically complex, coreless frontier basins globally. This study represents one of the first such integrated workflows documented for the Northern Sulaiman Fold and Thrust Belt, establishing a repeatable methodology for analogous structurally complex petroleum systems.
