The Pore Structure Development and Evolution of the Q4 Member in Lower Cretaceous, Northern Songliao Basin

To better characterize the pore development and evolution characteristics in the fourth member of the Quantou Formation (Q4), an accurate evaluation of the reservoir properties and oiliness for different types of reservoirs was conducted in this study. Polarization microscope and scanning electron microscope experiments were used for lithology, mineral composition, pore types, and morphology observation. High-pressure mercury intrusion experiments were used to characterize the porosity, permeability, and pore size distribution, and the Beard and Weyl model was used to calculate the porosity evolution characteristics. The results reveal the following: (i) siltstone and pelitic siltstone dominate the Q4 tight reservoir, while feldspar and quartz are the main rock-forming minerals. (ii) Intergranular pores, intergranular dissolution pores, intragranular dissolution pores, intercrystalline pores, and microfractures are the main pore types, with a porosity of 4% to 12% and a permeability of 0.001 to 1 mD. (iii) After the Q4 reservoir was divided into Grade I, II, and II reservoirs from good to poor, their current average porosity values were found to be 14.82%, 8.98%, and 3.57%, respectively. The results from this work are of great significance for accumulation space and oiliness evaluation, and they provide theoretical support for the exploitation of Q4 tight reservoirs in the study area.