Permian sedimentary tuff tight reservoirs in the Santanghu Basin, NW China

Sedimentary tuffs comprise important tight oil reservoirs in the Middle Permian Tiaohu Formation in the Santanghu Basin. This paper describes three research advances made on these reservoirs. (1) There are many sets of sedimentary tuff in the second member of the Tiaohu Formation and they are the “sweet section” for tight oil exploration. Vitric, crystal-vitric, and argillaceous tuffs are the main reservoir types, of which the first two are more favorable. (2) A model was established to show how the volcanic edifice controls the type and distribution of the volcanic lacustrine tuff deposition. The crystal-vitric tuff is distributed in the lacustrine deposits nearest to the volcanic crater, and the vitric tuff is distributed on both sides of the active volcanic belt a certain distance from the crater, and this is the most favorable distribution area for sedimentary tuff. Later devitrification resulted in formation of a large number of micro-pores, and this is the main reason for the presence of favorable reservoirs. (3) The sedimentary tuff reservoir has undergone successive geological processes from formation of secondary dissolution pores, then hydrocarbon charge and accumulation, to compaction. The tight oil accumulation is the result of the coupling of the diagenetic evolution of the tuff in the Tiaohu Formation and the hydrocarbon generation and charging from the shale in the Lucaogou Formation.