Fractal characteristics of the pore structures of fine-grained, mixed sedimentary rocks from the Jimsar Sag, Junggar Basin: Implications for lacustrine tight oil accumulations

Abstract

The tight reservoir property and oil accumulation are greatly affected by the complexity and heterogeneity of pore structure. Fine-grained, mixed sedimentary formation samples from the Permian Lucaogou Formation in the Jimsar Sag were systematically studied focusing on their fractal characteristics and effects on the storage space, minerals and diagenesis, and its implication for tight oil accumulation. Fractal dimensions were calculated by the MICP model equation. The lacustrine fine-grained, mixed sedimentary samples are characterized by single and multi-fractal structures. Compared with the multi-fractal reservoir, the pore-throat structure of single fractal reservoir is more uniform and the pore throat size is relatively smaller. Minerals and diagenesis have apparent influences on fractal characteristics of the lacustrine MSR tight oil reservoir. The enrichment of terrestrial minerals can increase the heterogeneity of storage space and might be the reason for the multi-fractal pore structures. The enrichment of tuff in mixed sedimentary rock (MSR) can greatly improve the property and reduce the fractal dimension of tight reservoirs through dissolution. Large-pore filling calcite has dual effects on destroying reservoir space and reducing heterogeneity, which makes the weak correlation between calcite and fractal dimension. Fractal characteristics implicate that pore structures play a very significant role in controlling the accumulation of tight oil. MSRs with D or De more than 2.586 and porosity (average pore-throat radius) less than 6.3% (0.05 μm) are mostly incapable of effectively accumulating oil.