Study on pore structure characteristics and formation mechanism of high quality shale oil reservoirs in Chang 7 Member of Ansai Oilfield

The Yanchang Formation's seventh member in the AnSai Oilfield is a crucial development layer, with the genesis and pore structure of its reservoir playing a significant role in controlling reservoir quality, which in turn has a profound impact on shale oil production. The AnSai Oilfield faces challenges such as a lack of conventional resource potential and considerable difficulty in reserves replacement. Currently, the primary objective for reserve augmentation is the shale oil reservoir; therefore, understanding the pore structure and genesis of different reservoirs is of great importance for the exploration and development of the oilfield. This study focuses on the interbedded shale oil reservoir of the seventh member of the Yanchang Formation in the AnSai Oilfield, located in the Ordos Basin. Through the use of scanning electron microscopy, cast thin sections, low-temperature nitrogen adsorption, high-pressure mercury injection, and nuclear magnetic resonance experiments, we elucidate the impact of varying pore sizes on reservoir quality and reveal the genesis of different reservoirs from the perspectives of depositional environment and diagenesis.The results indicate that: (1) The main types of reservoir pores include feldspar dissolution pores, residual intergranular pores, intergranular dissolution pores, clay intercrystalline pores, and a small number of microfractures. Notably, feldspar dissolution pores primarily develop micron-sized pores, whereas clay intercrystalline pores are predominantly nanometer-sized. (2) The overall drainage pressure of the reservoir is relatively high, with lower mercury saturation and a predominant throat radius in the nanometer range. Pores with diameters under 500 μm tend to exhibit open, plate-like, and fissure-like geometries, with a small quantity of ink-bottle-shaped pores also developed. (3) Reservoir pore diameters are mainly below 300 μm, and as the physical properties of the samples improve, the proportion of larger pores within the reservoir gradually increases. (4) The genesis of high-quality reservoirs can be primarily categorized into two types: The first type, located to the northeast near the sediment source, experiences strong hydrodynamics and good particle sorting, facilitating the development of chlorite films, which protect the original intergranular pores between particles and preserve a substantial number of residual intergranular pores following compaction, thereby forming high-quality reservoirs. The second type, situated to the southwest away from the sediment source, is characterized by increased water depth and weakened hydrodynamics. This area, being close to hydrocarbon source rock development zones, is prone to organic acid dissolution from the source rock, leading to the development of numerous dissolved pores and the formation of a significant amount of high-quality reservoir.