Property and origin of tight sandstones in Middle-Upper Ordovician, northeastern Tarim Basin

In the northeastern Tarim Basin,a thick set of turbidite composed of sandstone,siltstone and mudstone (shale) was found in the Middle and Upper Ordovician,attracting more and more attentions for exploration.The reservoir property and origin of sandstones from the Querquek Formation were studied in detail through systematic sampling of Kuruktag outcrops.The sandstones from the Querquek Formation mainly included fine-grained arkose,lithic sandstone,debris feldspar and feldspathic lithic sandstone.The dissolved pores of feldspar particle and calcite cement worked as the main pore space,and the pore structure was mainly micro pore and throat.The average porosity was 1.72%,and the average permeability was 4.584×10^-3μm²,indicating extra-low porosity and extra-low permeability reservoir.The low compositional and mid-poor textural maturities of sandstone resulted from near source and rapid deposition were the inherent reasons for the formation of Querquek tight sandstone.The severe compaction resulted from ultra-compensation,the high content of plastic particle and the late weak dissolution were the important factors for poor physical property reservoir.The weak dissolution and fracturing did not improve the physical property of reservoir obviously,and were the important reasons for poor reservoir with low porosity and permeability.