Pore structure characteristics of low oil saturation reservoirs in Badaowan Formation of Mahu Sag, Junggar Basin, China

Abstract

ABSTRACT The Badaowan Formation in the Mahu Sag of the Junggar Basin has revealed a low oil saturation reservoir with promising exploration prospects. To understand the pore structure characteristics and their impact on oil content, high-pressure mercury injection and nuclear magnetic resonance (NMR) experiments were used to identify and classify pore structures. In previous studies, the disparity in NMR transverse relaxation time(T 2) between centrifuged and saturated samples has frequently been utilized to establish a correlation with the capillary pressure curve. This study contends that the contrast in T 2 values between dried and saturated samples, as well as the capillary pressure curve, both reflect the attributes of interconnected pores and exhibit a strong correlation. Consequently, it is more justifiable to devise a conversion formula between T 2 and pore radius based on this premise. The study demonstrates that sandstone and conglomerate reservoirs in the Mahu Sag exhibit complex pore structures, with sandstone having higher porosity but poorer permeability compared to conglomerate. Each lithological reservoir exhibits two typical pore structure types, with Type 1 being superior to Type 2. Furthermore, our findings indicate that better pore structure correlates with higher oil content within a single lithological reservoir; additionally, conglomerate reservoirs show higher oil grade compared to sandstone due to permeability’s greater impact on oil content in this area. This research provides valuable insights for identifying high-quality reservoirs within the Badaowan Formation in Mahu Sag.