China ranks third in the world in terms of shale oil resources (Nie et al, 2016, Zhao et al, 2020, Zou et al, 2020); however, this is limited to onshore basins with widely discovered shale oil in the Permian, Triassic, Jurassic, Cretaceous and Paleogene settings (Wang et al, 2020). The shale oil resource potential in offshore basins is still an uncharted territory.
The first offshore shale oil appraisal well was drilled in the Beibu Gulf Basin in the western South China Sea in June 2022. Compared with the onshore shale oil reservoir, the offshore shale oil play is younger, showing less diagenetic compaction, and has higher pressure coefficient. It is featured by strong heterogeneity, complex mineralogical composition, and unclear pore structure. Accurate and continuous evaluation on total organic carbon, mineralogical composition, lithofacies, total porosity and brittleness index are crucial for the shale oil test zone selection and stimulation design.
To evaluate the oil-bearing potential, storage capacity, fluid movability and fracturing feasibility of shale oil formations, conventional formation evaluation alone, even with supporting core analysis and 3D borehole core pyrolysis are still insufficient. The advanced logging suites including spectroscopy, nuclear magnetic resonance (NMR), borehole imaging and crossed-dipole acoustic waveform analyses were integrated to enhance these evaluations.
Elemental weight fractions from spectroscopy logs are utilized to calculate total organic carbon, mineralogical composition, and matrix density. NMR logs are acquired to provide porosity (total and effective) as WY-1 oil saturation (total and movable). Borehole imaging logs provide the in-situ stress direction which is used for the stimulation design.
In this case study, 3D borehole core pyrolysis tests analyzed source rock maturity using 7 sidewall samples, and showed the source rock with high Ro, which means high maturity, indicating the high movability of the shale oil. The dry weight fraction of a total of 9 minerals were determined from the spectroscopy logs, namely quartz, potassium feldspar and plagioclase, pyrite, siderite and illite, kaolinite, dolomite and calcite. Among these minerals, quartz and illite are the two most abundant, with an average 38.2% and 30.3% respectively. The mineralogical composition was validated with X-ray diffraction (XRD), showing very good agreement.
Then, brittleness index and TOC of the target formation is calculated based on the mineralogical composition. The result shows TOC is inversely correlated with brittleness index. Total oil saturation and movable oil saturation were obtained from NMR logs. Finally, shale oil test zone was selected primarily based on the brittleness index and the percentage of movable oil saturation rather than TOC and total porosity. The test result confirmed the effective selection with production of oil 20m3/day and gas 1589m3/day in total.
Spectroscopy and NMR logging connect oil-bearing potential, reservoir petrophysical properties and fracturing feasibility throughout the entire shale oil evaluation. The shale oil test zone selection strategy for offshore basins is quite different from onshore basins, which can be a new reference and could be utilized for future appraisal wells.
