Residual-Oil Zone: Paleo-Oil Characterization and Fundamental Analysis

Abstract

Summary In most reservoirs around the world, paleo oil exists below the free-water level (FWL) and is considered residual oil to natural/geological waterflood. This nontrivial resource of residual oil will not flow by primary- or secondary-recovery means but requires carefully designed enhanced-oil-recovery (EOR) methods to mobilize it. To date, there is no detailed analysis of paleo oil in the literature simply because it is difficult to obtain a reservoir sample. This study provides a comprehensive paleo-oil analysis for samples obtained from reservoir sponge cores. The oil in the sponge core was extracted, analyzed, and compared with main-pay-zone (MPZ) oil. Critical data have been unveiled on paleo-oil characterization through fundamental studies on oil quality, fingerprint, filling history, available hydrocarbon components and compounds, and molecular-level characterization. It was found that the global composition and overall quality of paleo oil are very similar to those of the MPZ oil. However, the differences between the two oils were only apparent when the study was extended further to include molecular-level analysis and available hydrocarbon components and compounds. These differences may define the appropriate EOR methods to mobilize this oil and explain trapping mechanisms caused by fluid properties. Gas-chromatography (GC) studies revealed that paleo-oil extracts have the same pristane/phytane ratio as the MPZ oil, suggesting that they are of the same origins and share the same source rock. Further analysis showed a good match of the terpane biomarkers between paleo-oil extracts and MPZ oil but with slightly lower maturity levels. Paleo-oil quality was compared with MPZ oil by use of pyrolytic oil-productivity index (POPI) analysis which indicated the same °API range as the MPZ oil and the same light volatile, thermally distilled, and cracked components. Paleo and MPZ oils were also analyzed with low-field nuclear magnetic resonance (NMR) to qualitatively test the similarity of the oil components and to measure their apparent viscosities. Both oils have shown very comparable viscosity measurements and NMR signatures. The simulated distillation analysis showed that lighter components in paleo oil are less abundant than in MPZ oil, whereas medium-to-heavy components are relatively similar. A Fourier transform ion cyclotron resonance (FT-ICR) study, which zoomed into the heavier components, revealed that paleo oil has less aromaticity than MPZ oil and lacks aromatic sulfur and disulfur compounds, a negligible amount of nitrogen compounds, and no resin-type components. This study provides in-depth information about oil extracted from the residual-oil zone (ROZ), which does not flow by primary or secondary recovery means. To our knowledge, there is no available information in the literature that explains the components, compounds, quality, and behavior of this oil because it is difficult to obtain reservoir samples. These data shed light on a possible trapping mechanism caused by fluids in place. The study also used several methods and tools to confirm the conclusions and to ensure repeatability of results.