Petrophysical characteristics of shale oil reservoirs in the Yanchang Formation, Ordos Basin

The mediate-low maturity continental shale oil reservoir in the 7th member of the Triassic Yanchang Formation is one of the main zones for shale oil exploration and development in Ordos Basin, China, but the seismic response mechanism of the reservoir remains unclear. Therefore, developing a new method for “sweet spot” seismic prediction combined with rock physics is necessary. To determine the petrophysical characteristics of continental shale in the 7th member of the Yanchang Formation in the study area, a series of tests, such as a systematic acoustic test on shale oil samples in the target segment, X-ray diffraction analyses, analysis of thin optical sections, and scanning electron microscopy, were conducted to summarize the patterns of seismic elastic property changes. Results show that the shale oil samples of the 7th member of the Yanchang Formation are primarily composed of lithic feldspar sandstone with feldspar dissolution and intergranular pores, widespread micron pores, and throats. Such composition indicates a positive correlation between porosity and permeability. The velocities of the samples are affected by their porosity and mineral composition. Velocity increases with the increase of the calcium content and decreases with the increase of the clay content, indicating a negative correlation with the porosity and total organic carbon (TOC) content on the condition of the same rock structure (quartz skeleton or clay skeleton). The elastic properties of the rock are horizontally isotropic (T1 medium), and the velocity is vertically anisotropic. In addition, the directional arrangement of clay controls the anisotropy of rock velocity. TOC is mainly distributed in the primary intergranular pores, and it has no contribution to the anisotropy of rock velocity. The results of this paper can provide a reference for the seismic prediction of continental shale oil reservoirs in the 7th member of the Yanchang Formation.