Hydrocarbon generation thermal simulation experiment of massive organic-rich tuff and laminar tuffaceous shale: A case study of the Chang 7 member of the Ordos Basin

The Chang 7 Member of the Yanchang Formation in the Ordos Basin consists predominantly of airfall and water-borne tuff along with tuffaceous shale, which exhibit considerable hydrocarbon potential, yet their thermal evolution and hydrocarbon generation behaviors remain inadequately studied. As an emerging unconventional exploration target, this interval was investigated through systematic thermal simulation experiments utilizing a novel programmable dual-pressure constant-flow system, which enables precise control of fluid pressure and automated monitoring of hydrocarbon generation and expulsion processes under semi-open/semi-closed conditions. The study focuses on low-maturity, organic-rich massive tuff and laminated tuffaceous shale from Well Z40, revealing that although all samples are sedimentary in origin, the airfall tuffs are rich in vitric pyroclasts—including accretionary pellets (AP1)—while water-borne tuffs are dominated by feldspar crystal pyroclasts. Key findings demonstrate that massive tuff displays markedly higher hydrocarbon expulsion efficiency compared to laminated tuffaceous shale, with an earlier onset of expulsion and contributions derived from both kerogen cracking and heavy oil components. Thermal evolution analysis further indicates that massive tuff attains higher organic maturity under identical conditions. In laminated shale, expulsion efficiency correlates increases with TOC content, reflecting the role of organic matter in pore development and fluid connectivity. The superior expulsion performance of massive tuff is attributed to its higher volcanic glass content, low clay abundance, and dispersed organic matter distribution, which create highly efficient migration pathways. In contrast, laminated tuffaceous shale exhibits stratified organic matter closely associated with clay minerals, requiring higher thermal maturity (R_o > 0.6%) and elevated TOC to form an interconnected organic network that facilitates efficient expulsion. This study highlights that variations in pore structure, organic matter distribution, and thermal maturity represent the primary controls on differential expulsion efficiency between organic-rich tuff and tuffaceous shale in the Chang 7 Member, providing crucial insights for the exploration and development of shale oil in volcanic–lacustrine basins.