Insights into fluid types in unconventional resources reservoirs

Lacustrine shale oil resources are crucial for maintaining energy supply. The types and contents of fluids are key factors in estimating the resource potential and oil recovery of organic-rich shales. Accurately identifying the fluid types in shale oil reservoir successions, which are characterized by significant lithological heterogeneity, is a significant challenge. Although numerous methods for determining porosity and fluid saturation have been proposed in previous studies, many are only applicable in limited situations and have limited accuracy. In this research, an advanced logging technique, Combinable Magnetic Resonance logging (CMR-NG), is employed to evaluate fluid types. Two-dimensional Nuclear Magnetic Resonance (2D-NMR) experiments were conducted on reservoir rocks under different conditions (as received, after drying at 105℃, and after kerosene imbibition) to define the fluid types and classification criteria. In addition, the contents (proportions) of various types of fluids were estimated. Subsequently, the contributions of organic matter and mineral compositions were investigated using corresponding Rock-Eval pyrolysis parameters and various mineral contents obtained from X-ray Diffraction. Then, the contents of different fluid types were calculated using CMR-NG (Combinable Magnetic Resonance logging, also known as 2D NMR logging). Based on the fluid classification criteria under experimental conditions and production data, the most favorable model and optimal solution for logging evaluation were selected. Finally, the fluid saturations were calculated for a single well. The results indicate that six fluid types (kerogen-bitumen-Group OH, irreducible oil, movable oil, clay-bound water, irreducible water, and movable water) can be recognized through the applied 2D-NMR test. The kerogen-bitumen-Group OH is mainly affected by pyrolysis hydrocarbon (S2), and irreducible oil is influenced by soluble hydrocarbon (S1). However, due to the effects of underground environmental conditions on the instruments, kerogen-bitumen-Group OH and clay-bound water cannot be detected by CMR-NG. The Q8 and Q9 layers of the Qing 2 Member of the Cretaceous Qingshankou Formation, Gulong Sag, Songliao Basin, China are identified as the most favorable layers for shale oil. This study provides insights into the factors controlling fluid types and contents, offering guidance for the exploration and development of unconventional resources, such as geothermal and CCUS (carbon capture, utilization, and storage) reservoirs.