Evaluation of natural fracture effectiveness in deep lacustrine shale oil reservoirs based on formation microresistivity imaging logs

The lacustrine shale oil reservoirs of the Fengcheng Formation in the Mahu Sag, Junggar Basin have burial depths exceeding 4 500 m.Natural fractures in these reservoirs, serving as important storage spaces and seepage channels, are critical to the enrichment and high yield of shale oil. There is a lack of systematic study on their effectiveness evaluation despite existing characterization of these fractures in previous works, severely restricting the further exploration and exploitation of shale oil in the Mahu Sag. Given this, we conduct a systematic study on the distribution patterns and effectiveness evaluation of natural fractures in the study area using formation microresistivity imaging (FMI) logs. The results indicate that there exist two types of natural fractures in the lacustrine shale oil reservoirs in the study area: cross-layer fractures and intralayer fractures. The cross-layer fractures are characterized by a large scale, with heights generally reaching up to several meters or above, and their distribution is governed by faulting. The intralayer fractures are found within brittle beds, and their heights are limited by the thickness of rock layers, largely less than 50 cm. Vertically, the fracture density in a single well is positively correlated with the brittle mineral content. Laterally, the fracture density gradually decreases with increasing distance from faults. Fractures with different orientations exhibit greatly varying degrees of filling. The NW-SE-trending fractures, among others, are mostly not filled with minerals, thus boasting high effectiveness. As the burial depth increases, fracture apertures generally trend downward. The evaluation results reveal that the NEE-SWW-trending fractures exhibit the largest aperture and, accordingly, the highest effectiveness.