Heterogeneity of Pore Structure in Braided River Delta Tight Sandstone Reservoirs: Implications for Tight Oil Enrichment in the Jurassic Badaowan Formation, Central Junggar Basin

Abstract

The influence of diagenesis on the heterogeneity of pore structures in tight sandstone reservoirs is essential for accurately assessing hydrocarbon potential. This study employed polarized light microscopy and X-ray diffraction to characterize lithofacies in tight sandstones from the Jurassic Badaowan Formation in the Mo-Yong region of the central Junggar Basin. Additionally, the study integrated low-temperature nitrogen gas adsorption and mercury injection capillary pressure to comprehensively analyze pore structure attributes across various lithofacies within a braided river delta setting. Finally, the incorporation of a multifractal model allowed for a quantitative assessment of pore structure heterogeneity, examining the implications of diagenetic processes on the enrichment and distribution of tight oil. The investigations revealed significant variability in pore structures among lithofacies of the Badaowan Formation in the Mo-Yong region. Medium-coarse lithic sandstone (M-CLS) and medium feldspar–lithic sandstone (MFLS) predominantly feature meso-pores and macro-pores, playing a crucial role in hydrocarbon accumulation. In contrast, fine-medium feldspar–lithic sandstone (F-MFLS) and fine-medium calcareous sandstone (F-MCS) are characterized by the predominance of micro-pores, exhibiting weak connectivity. Feldspar dissolution markedly altered the pore architecture, notably enhancing the connectivity while reducing the heterogeneity of meso-pores and macro-pores. Secondary enlargement of quartz augmented the heterogeneity and reduced the connectivity of meso-pores and macro-pores, whereas the presence of micro-fractures in quartz could decrease this heterogeneity and enhance connectivity. Conversely, an increase in clay minerals and calcite reduced the volume and connectivity of meso-pores and macro-pores, thereby augmenting the heterogeneity of the pore structure. Multifractal analysis demonstrated the profound impact of diagenetic processes on the scale-dependent heterogeneity of pore structure, providing essential insights into the adsorption and flow mechanisms of tight oil within complex pore matrices. The analyses clearly identified variations in pore volume and heterogeneity across lithofacies as pivotal in governing the distribution of tight oil. Particularly, the well-developed meso-pores and their lower heterogeneities in MFLS designate it as the most prospective reservoir lithofacies. These findings offer new perspectives and solid theoretical support for the exploration and development strategies of deep tight sandstone reservoirs in braided river delta environment.