Effects of deposition and diagenesis on sandstone reservoir quality: A case study of Permian sandstones formed in a braided river sedimentary system, northern Ordos Basin, Northern China

Abstract

• The reservoir quality of sandstones with coarser grain and good sorting is better. • Compaction reduced pore spaces of sandstones formed in the braided river by 84.49%. • An appropriate amount of kaolinite was conducive to the development of reservoir. • Quartz cements do not have a major control on the reservoir quality. • The poorer the sorting, the greater the compaction index and the cementation . The impacts of deposition and diagenesis on sandstone reservoir quality have been extensively discussed over the years. But the research on how depositional factors control the reservoir quality by affecting diagenesis is rarely involved. The Permian System in the northern Ordos Basin is composed of clastic rocks, and sandstones formed in a braided river sedimentary system (braided channel and braided bar) are one of the most important reservoirs. Sandstones formed in the braided channel have inferior sorting, thinner depositional thicknesses, and higher contents of rock fragments and calcite cement. The total thin-section porosity, porosity and permeability of the sandstones are 5.9%, 9.76% and 0.75 × 10 −3 μm 2 , respectively. The grain size and sedimentary thickness are positively correlated with the reservoir quality, and the secondary pores of the well-sorted sandstone are more developed. Sedimentary compaction resulted in average reductions of 85.69% and 82.76% in sandstones formed in the braided channel and braided bar, respectively. An appropriate amount of kaolinite (<6%) was conducive to the development of reservoir quality. Calcite cements have reduced the pore spaces, whereas quartz cements do not have a major control on the reservoir quality. Diagenesis directly impacts reservoir quality, whereas depositional factors determine the preference and strength of diagenesis. The strength of dissolution is greater in the well-sorted sandstone, and the poorer the sorting, the greater the compaction index and cementation. The grain size, sorting, and deposition thickness control the sandstone reservoir quality by affecting the compaction strength, cement content, and dissolution.