Characterizing the First Offshore Shale Oil Reservoir by the Integration of Advanced Logging Techniques and Core Measurements in Beibu Gulf Basin, China

Abstract

Abstract The exploration and development of shale oil reserves have been mostly conducted onshore and have seen significant success in recent years. For offshore shale oil exploration, the Liushagang formation in Beibu Gulf Basin, China, represents a first. Compared to an onshore shale oil reservoir, The Liushagang formation is more heterogeneous due to the development of multiple faults in this basin. Previous drilling result shows that the lithology, pore size, porosity, moveable oil volume, etc. vary greatly in different fault units. To better reveal the characteristics of this offshore shale oil reservoir and its potential, analyses of 2D NMR logs, high-definition oil-based mud image logs, gamma-ray spectroscopy, and core measurements like X-ray diffraction (XRD), scanning electronic microscopy (SEM), and NMR were conducted. The sedimentary environment of the Liushagang formation is lacustrine, which makes the reservoir more complex compared to the marine shale in terms of lithology, pore type, porosity, oil storage space, etc. The mineral assemblage is very important for shale oil reservoirs because it influences the pore types, pore structures, and the fracking fluids. The latest gamma-ray spectroscopy tool simultaneously measures the capture spectrum and inelastic spectrum, from which the formation elemental concentration is precisely measured. Using the formation elements and core XRD data, the mineral evaluation model has been customized to characterize the mineral components and their variations in the Liushagang formation. The new-generation NMR tool provides continuous 2D T1-T2 measurements, which enables the pore structure, and porosity measurements to be defined. This also allows the separation and quantification of different fluids in the pores by the 2D data analytics technique. Integrating with core NMR measurements and laser scanning confocal microscope (LSCM), it is possible to directly determine the bound oil, bound water, movable oil, and movable water which circumvents bias using resistivity methods due to the uncertainties of Archie equation parameters. Previous studies show that the lamination degree has a significant relationship with the reservoir quality within a lacustrine shale oil reservoir. With high-definition oil-based mud image logs and core plugs, the lamination degree was characterized by the innovative rock texture analysis method in addition to the fractures, structures, etc. Meanwhile, the lithofacies were determined by integrating the image log and minerals from gamma-ray spectroscopy. The integrated study has revealed the detailed characteristics of the offshore shale oil reservoir in lithology, lithofacies, total organic carbon (TOC), pore types, pore structures, fluid components, and rock texture aspects. Our testing results show that there is very promising potential in the first offshore shale oil reservoir in the Beibu Gulf Basin, China.