Impact of Magmatic Intrusion on Thermal Effects in Sandstone Reservoirs

The impact of magmatic intrusions on the porosity and reservoir quality of sandstone formations remains insufficiently understood, particularly in the context of thermal effects across different temperature zones. Existing studies have primarily focused on general porosity trends, but the precise mechanisms through which heat from magmatic intrusions alters reservoir properties have not been fully elucidated. This study addresses this gap by investigating the thermal effects of magmatic intrusions on sandstone reservoirs in the BZ34-9 oil field, located in the Huanghekou Sag of the Bohai Bay Basin, and the outcrop of the Lishu Fault Depression in the Songliao Basin. Through a combination of field research, NMR (nuclear magnetic resonance), X-ray diffraction, and micro-CT, alongside thermal simulation models and high-temperature experiments, this research demonstrates that proximity to magmatic intrusions significantly reduces sandstone porosity. Specifically, porosity in a high-temperature zone (HTZ, 400-600 °C) decreases by 30-50%, while in a low-temperature zone (LTZ, normal temperatures to 400 °C), it decreases by 16-20%. Thermal simulations using three geological models (thick sandstone with lateral thermal effects, thin sandstone and mudstone interbedding, and vertical thermal effects) reveal that heat propagates more widely in sandstone-dominated strata and that temperature decay occurs rapidly with increasing distance from the intrusion. High-temperature experiments further show that mineral expansion and clay dehydration below 700 °C reduce porosity, while at temperatures around 900 °C, calcareous cement decomposes, leading to a sharp increase in porosity. However, temperatures exceeding 1200 °C cause rock melting and the formation of isolated bubbles that destroy reservoir connectivity. This study provides new insights into the thermal modification mechanisms in sandstone reservoirs, offering a theoretical framework for enhancing oil and gas exploration strategies. The findings have important implications for the development of oil fields affected by magmatic intrusions and can inform the exploration and management of the BZ34-9 oil field.