Fractal characteristics of Upper Cretaceous lacustrine shale from the Songliao Basin, NE China

Abstract

To improve our understanding of the characteristics of nanopores and their influence on liquid hydrocarbon occurrences in lacustrine shale, we carried out fractal analysis on 21 lacustrine shale cores from the Upper Cretaceous Qingshankou Formation (K2qn) in the Songliao Basin, NE (northeast) China, using TOC (total organic carbon) and Rock-Eval analysis, XRD (X-ray diffraction), low-pressure nitrogen adsorption experiments, and the FHH (Frenkel-Halsey-Hill) method. The relationships between composition of lacustrine shale (TOC, minerals, oil content), the pore structure parameters (BET surface area, pore volume, pore average diameter), and the fractal dimensions were investigated.Two fractal dimensions D1 and D2 (at relative pressures of 0.5–1 and 0–0.5, respectively) were obtained, with D1 ranging from 2.5055 to 2.8888, and D2 between 2.1026 and 2.733. D1 is greater than D2, indicating that small pores are more homogeneous than larger pores. Both D1 and D2 increase with increasing BET surface areas and total pore volumes, and increase with decreasing average pore diameters. The fractal dimensions of lacustrine shale are negatively correlated with the clay content, but show no obvious relationship with the quartz content. The relationship between fractal dimensions (both D1 and D2) and TOC content is a U-shaped curve, with minimum D values at 2.5–3 wt.% TOC. Fractal dimension D1 can be used to characterize the shale oil content, and lacustrine shale samples with a lower fractal dimension have greater free oil. Therefore, considering the influence of TOC content on fractal dimension, and the relationship between oil content and fractal dimension, we conclude that lacustrine shale in the oil generation stage with 2.5–3.0 wt.% TOC is probably the preferred shale oil exploration target in the Songliao Basin. The results indicate that fractal analysis leads to a better understanding of the pore structure and oil storage capacity of lacustrine shale.