Insight into Fluid Occurrence and Pore Structure of Lacustrine Shale from the Cretaceous Tengger Formation, A’nan Sag, Erlian Basin, China

Fluid occurrence and pore structure are prerequisites for exploring the enrichment pattern of shale oil. The Cretaceous Tengger Formation in the A'nan Sag, Erlian Basin, China, contains abundant shale oil resources. In this study, a series of NMR T2 and T1–T2, combined with Rock-Eval, scanning electron microscopy (SEM), and low-temperature nitrogen adsorption–desorption (LTNA/D), were conducted to clarify the fluid occurrence and pore structure characteristics of the shale oil reservoirs. Meanwhile, the influences of the pore structure on fluid occurrence were discussed. Results suggest that the in situ occurrences and distributions of pore water and oil can be well analyzed by the NMR T1–T2 technique combined with water and oil restorations. The selected shales are characterized by high contents of capillary-bound water and bound oil, and bound oil mainly contributes to shale oil, followed by adsorbed and movable oil. The shales at the as-received state have lost half of the capillary-bound water, as well as most of the bound and movable oil, while adsorbed oil is less affected. The pore spaces primarily consist of interparticle pores related to brittle granules, followed by intraparticle pores in clay mineral aggregates. The T2 spectra are characterized by the dominant peak p2 ranging from 0.7 to 20 ms, corresponding to the developed mesopores (100–1000 nm). Fluid occurrence is closely related to pore structures. Capillary-bound water primarily occurs in micropores (<25 nm) and minipores (25–100 nm) and coexists with adsorbed oil. Mesopores and macropores (>1000 nm) are mainly saturated with bound and movable oil, respectively. The developed large pores generally correspond to less capillary-bound water but more shale oil, especially bound and movable oil. These findings may improve the understanding of enrichment mechanisms of shale oil and provide the references for sweet spot exploitation in the A'nan Sag, Erlian Basin.