Logging Response and Mechanism of Pressure Relief From Sandstone Bands Intercalated in Overpressured Shale Oil Reservoir

Abstract

Overpressured shale sequences are commonly interbedded with sandstone layers that exhibit markedly contrasting physical properties. Although these interbeds are known to play a significant role in overpressure dissipation, the underlying pressure‐release mechanisms remain poorly understood. In this study, we systematically investigate the pressure‐dissipation behavior, well‐log response characteristics, and microscopic controlling mechanisms of sandstone bands in overpressured shale formations. To this end, we integrate core observation, thin‐section petrography, petrophysical measurements, formation pressure analysis, and variable‐confining‐pressure nuclear magnetic resonance (NMR) experiments. Our results demonstrate that the magnitude of pressure release is jointly governed by the porosity and thickness of the sandstone bands, which exhibit a complementary functional relationship. The vertical extent of pressure disturbance shows a non‐monotonic trend with increasing porosity, first rising and then declining. High‐porosity interbeds facilitate rapid pressure equilibration, which, in turn, curtails the lateral propagation of pressure perturbations—a dynamic regime that can be characterized as “rapid release with localized dissipation.” Within the pressure‐dissipation zones, well‐log responses display a distinctive assemblage, characterized by a marked increase in acoustic velocity, essentially unchanged bulk density, decoupling of velocity from density, and a null response in the DC index. On the basis of these observations, we propose a novel identification chart for sandstone‐interbed‐related overpressure that effectively discriminates this type from undercompaction‐ and fluid‐expansion‐induced overpressure. Variable‐confining‐pressure NMR experiments further reveal that shale porosity decreases by 12.21% as confining pressure rises from 0 to 40 MPa, dominated by the collapse of nanoscale throats (<20 nm), whereas sandstone porosity decreases by only 10.25% owing to its rigid grain framework. The fundamental contrast in throat stress sensitivity and cyclic deformation behavior between sandstone bands and the surrounding shale matrix constitutes the core dynamic mechanism governing the formation, persistence, and temporal stability of the pressure‐depletion funnel. Collectively, these findings provide a new theoretical foundation for the efficient development and overpressure evaluation of overpressured shale oil reservoirs containing sandstone bands.