Diagenetic processes controlled by volcanic detritus of the Lower‐Middle Permian Shanxi and Shihezi sandstone formations, North Ordos Basin, China

Volcanogenic detritus in sandstones not only conveys important provenance information but also significantly impacts diagenetic processes. This study specifically focuses on such understudied effects by carrying out grain size, petrographic, fluid inclusion, scanning electron microscopy, cathodoluminescence imaging, physical property and geochemistry of the second member of the Shanxi Formation (P 1 s 2 ) and the first member of the Lower Shihezi Formation (P 2 h 1 ) from the Xinzhao area of the north Ordos Basin. These two members represent potential tight‐sandstone gas reservoirs containing different volcanic materials derived from the Central Asian Orogenic Belt. Petrographic and geochemical analyses testify the deposition in an intracontinental basin close to the continental arc of the orogenic belt and reveal the closure of the Palaeo‐Asian Ocean after the early Middle Permian. Magmatic crystallisation ages indicate that the P 1 s 2 and P 2 h 1 sandstones correspond to a magmatic lull phase testified by low pyroclastic production and to a period of intense magmatic activity evidenced from enhanced sediment supply and river overload, respectively. Both members experienced post‐depositional compaction, cementation, dissolution and replacement and remain in the mesodiagenetic stage. The P 1 s 2 sandstone suffered from only weak compaction and dissolution because of the abundance of more rigid and insoluble felsic fragments, whereas the P 2 h 1 sandstone experienced strong compaction and dissolution owing to the higher content of ductile and metastable mafic debris, and it shows cementation by clean kaolinite, grain‐coating chlorite and pore‐filling analcite. Because these cements are soluble, the P 2 h 1 sandstones display a high total porosity and permeability despite extensive compaction. The presence of mafic volcanogenic materials may thus favour the formation of superior‐quality, litho‐quartzose sandstone reservoirs. This contributes to understanding deeply the diagenetic processes in volcanoclast‐rich sandstones of the North Ordos Basin. The knowledge gained in this study may serve as a paradigm for analogous tight sandstone reservoirs forming at intracontinental basins near active margins globally.