Multidimensional Evaluating First Offshore Shale Oil Play by Geomechanical, Petrophysical and Geological Integration, Beibu Gulf Basin, South China Sea

The shale oil resource is growing in offshore basins of China and considered to be a prosperous substitution for shale oil/gas on land. Unlike on land shale oil reservoir, the offshore shale oil play is featured by younger stratigraphy, lower diagenetic compaction, and higher-pressure coefficient. Strong heterogeneity, complex mineralogical composition, and unstable tectonic activities, and more naturally developed fractures are the main challenges for evaluating offshore shale oil. The common integration of geological and engineering evaluation on shale oil is not adequate, as natural fracture effectiveness can’t be verified without geomechanical analysis. To get more accurate petrophysical evaluation, the advanced logging spectroscopy log and nuclear magnetic resonance (NMR) log are indispensable. The oil-bearing potential, storage capacity, and fluid movability are analyzed by NMR log and total organic carbon (TOC), mineralogical composition, and matrix density are calculated by spectroscopy log. Core photos, thin sections together with spectroscopy log were used to characterize shale lithofacies. Borehole imaging log was utilized for geological evaluation especially for fracture identification. Based on the electrical resistivity image and standoff image, fractures are categorized into three types: fully open fractures, partially open fractures and closed fractures. Based on Coulomb failure function, these fractures are further integrated with effective normal stress and shear stress in Mohr-Coulomb for fracture stability analysis to geomechanically verify whether the fractures are critically stressed or stable. The fracture stability analysis shows the fractures dip azimuth from 280 to 330 deg are more stable (black) than other azimuthal directions, most of the effective fractures (fully open fractures and partially open fractures based on image logs) are tending to be critically stressed, the result of which is over 85% consistent to the fracture identification from borehole image log. The fractures that are both open and critically stressed are considered as effective fractures which will contribute more permeability to the shale oil reservoir, and on the contrary, the fractures that are partially open/closed and stable are considered as non-effective fractures.