Development Mechanism of Ultra-Low Permeability Limestone Reservoirs: A Case Study of the Cretaceous S-Reservoir in H Oilfield, Iraq

Abstract

Ultra-low permeability limestone reservoirs in the Middle East have huge untapped reserves. In Iraq, Field H has nearly one quarter of reserves in the S-Reservoir with permeabilities of (0.02-0.4 mD). It is difficult to identify optimal drilling locations, or sweet spots, that maximize recovery factors due to poor understanding of reservoir characteristics. Using the S-limestone reservoir, this study aims to clarify the environment of deposition, diagenetic evolution, reveal the development mechanism of ultra-low permeability, and predict the distribution of more favorable areas. Whole core, thin sections, scanning electron microscope (SEM), mercury injection (MICP), well logs, and seismic data were utilized for the analysis. Through core and thin section observation, lithology, biostratigraphic sequences, pore type, and sedimentary structures were determined. Through these observations the environment of deposition of the S-Formation is interpreted as mid to outer carbonate ramp. The lithology is mainly wackestone to packstone deposited in a low energy environment below fair weather wave base. Primary fossils are planktonic foraminifera tests as grains and coccolith microfossils in the matrix. Thin sections and MICP analysis helped delineate the porosity classes and diagenetic history. The pore throat size, determined by MICP data, was 0.03 - 0.1 μm. Primary porosity is around 10-25% and is driven by intragranular voids in foraminifera tests, followed by intergranular, and intercrystalline microporosity of the matrix. Foraminifera tests are generally intact and float in the mud matrix. As a result, large intragranular pores are well developed but not connected, causing overall medium-high porosity but ultra-low permeability. Early marine calcite cement is the primary diagenetic process which destroyed porosity and permeability. Little to no evidence of secondary dissolution was observed.