Characteristics and Mechanisms of Hydrocarbon Accumulation in Tight Oil Reservoirs of the Carboniferous Kalagang Formation, Malang Sag, Santanghu Basin, China

ABSTRACT As hydrocarbon exploration progresses in the Santanghu Basin, tight oil reservoirs in Carboniferous volcanic rocks have become a focal point for exploration and development efforts. This study aims to account for significant industrial oil flow from the tight tuff layers of the Kalagang Formation in well blocks Ma 71 to 33 in the Malang Sag within the basin. Through experiments including organic carbon analysis, rock pyrolysis, and nuclear magnetic resonance (NMR), this study systematically investigated the principal characteristics and determinants of the tight oil reservoirs, culminating in the development of hydrocarbon modes, thus providing a reference for tight oil exploration and production in the study area. The results indicate that the Kalagang Formation is composed predominantly of tuff, especially vitric tuff. The storage spaces of this formation include dissolution pores, tectonic fractures, and dissolution fractures, with devitrification pores less common. Compaction plays a crucial role in rendering the tuff reservoirs tight. Nevertheless, differentiated diagenetic alterations have improved the microscopic storage spaces and their interconnectivity. Well blocks Ma 3301 and Ma 33, having undergone significant weathering (leaching), exhibit superior reservoir physical properties and connectivity compared to well blocks Ma 71 and Ma 73. In contrast, well blocks Ma 71 and Ma 73 experienced more significant alterations due to organic acid dissolution and devitrification under the influence of source rock types and hydrocarbon generation. The comprehensive analysis indicates that tight oil reservoirs in the Malang Sag were formed due to the close superimposition of source rocks and reservoirs, along with the effective connection of source rock‐rooted faults. This results in the formation of two sets of proximal tight tuff reservoirs with the ‘lower‐source rock and upper‐reservoir’ and ‘self‐sourced reservoir’ modes.