Origin of Cement, Diagenesis and its Role in Fluid distribution in Shallow Heavy Oil Unconventional Reservoir in Kuwait

Abstract

A shallow unconventional heavy oil reservoir in Kuwait is primarily an unconsolidated sandstone reservoir with intervening cemented siltstone and sandstone, and thin shale layers. The process and relative timing of cementation in the reservoir played a key role in fluid distribution as the pore-filling cements originated prior to oil migration. Moreover, oil migration into the cemented zones was prevented by the presence of cement. This paper presents a study based on over 100 logs and 10 conventional cores in north of the reservoir. Detailed core analysis including petrography, XRD and SEM studies were considered understand the origin of cement, diagenesis and role in fluid distribution. In terms of origin and diagenesis, three types of cements were identified namely Calcite, Dolomite, and Argillaceous. Calcite and Dolomitic cements are admixture of calcium carbonate and calcium-magnesium carbonate with argillaceous components. Argillaceous cement is dominated by illite-montmorillonite and palygorskite with minor amount of kaolinite and chlorite. Argillaceous one primarily originated from feldspar, calcite from meteoric water rich in dissolved calcium (gypsum) and Dolomite as replacement of precursor calcite. Cement plays different roles in fluid distribution. Argillaceous cements cause pore-throat blockage due to presence of illite and palygorskite that form filamentous-fibrous aggregates. Cemented layers act as baffles in between oil layers capable of producing significant amount of trapped water in low pressure regime as they have significant amount of porosity and permeability. Finally cement layers hold water due to high capillary pressure and act as "water above oil" behaving as thief zone during thermal steam operation. Understanding origin of cement, diagenesis and its role in fluid distribution assist in evaluation of the layered nature of this complex fluid distribution pattern reservoir. Finally, integration of depositional environment, lithofacies, and cement distribution greatly enhance the assessment of lateral extension and characterization of these type of reservoirs.