Deep Sandstone Reservoir Characteristics and Pore Structure Classification Based on Fractal Theory: A Case Study of the Bayingobi Formation in Guaizihu Sag, Yin'e Basin.

Abstract

The paper focuses on deep oil and gas resources in the Bayingobi Formation of Guaizihu Sag in the Yin'e Basin. Previous studies overlooked the differences between the pores and throats, mainly focusing on pore analysis. This work aims to analyze the pore structure and petrophysical properties of the reservoir using various methods. The study utilized the constant velocity mercury intrusion method to quantify pores and throats separately. Scanning electron microscopy and casting thin section techniques characterize the pore and throat morphology. The analysis compares the pore structures in reservoirs with different petrophysical properties. Additionally, pore and throat types are classified based on fractal dimensions, and factors influencing their development are discussed. Results reveal feldspar lithic sandstone as the predominant rock type with a low compositional maturity. The sandstone reservoirs exhibit low porosity (10.23%) and ultralow permeability (0.99 mD). Primary reservoir pore spaces include intergranular pores, dissolution pores, and microfractures. Pore radius averages at 195.32 μm, while throat radius is 3.76 μm. Pore structures are categorized as micropore small-throat, small-pore small-throat, and large-pore coarse-throat types. The study area generally exhibits a high pore-to-throat ratio, impacting reservoir petrophysical properties significantly. Pore development is primarily influenced by early diagenesis, organic acid dissolution, and hydrocarbon filling. Weak compaction and cementation transformations provide a material and spatial basis for the subsequent dissolution. The presence of thick organic-rich mudstone above and below the reservoir contributes to organic acid dissolution and hydrocarbon filling.