Study On Diagenesis And Porosity Structure Of Upper Triassic Yanchang Formation Of Northern Pingqiao Oil-field In Ordos Basin

Chang 4+52Chang 62 pay zones in Yanchang Formation of the Upper Triassic in northern Pingqiao of Ordos Basin are of low porosity and very low permeability and low oil saturation. So it is an urgent need to find out the main factors affecting the reservoir physical properties and more favorable reservoir belt.On the basis of the study of high resolution sequence stratigraphy, this paper discusses the reservoir petrology's diagenesis and secondary pore of the Chang 4+52~Chang 62 pay zones in detail by using the methods of analysis of mercury injection, cast section, popular section, scanning electron microscope, x-ray diffraction and etc. The reservoir rock is mainly composed of feldspar fine sandstone, which is characterized by low compositional maturity and relatively high textural mature. The main diagenetic minerals of the reservoir rock are chlorite, laumontite ,calcite and authigenic quartz and so on. The pattern of diagenetic and porosity evolution are established. The primary porosity of the reservoir rocks was 35%, which was reduced by 21.7% by compaction and pressure solution and 8.2% by cementation. The secondary pores were produced by organic acids' dissolution during organic matter maturing stage , resulting an increase of porosity by 5%. The preserved average porosity is 10.1% at present.The main reservoir space of Chang 4+52~Chang 62 sandstone in the area is secondary pores, especially the secondary intergranular pores. Chang 4+52 sandstones have small pore and fine throat, however, Chang 61-Chang 62 sandstones have fine pore and tiny throat.Reservoir properties are mainly controlled by sedimentary microfacies and diagenesis;the effect of the tectonic movements on the reservoir is relatively limitted. The main sedimentary microfacies are submerged distributary channel, debouch bar, submerged natural levee and flood-plain splay. The research shows that the physical property of subermerged distributary channel sandbodies are the best and debouch bar comes second. The cementation of laumonite, calcite and chlorite, compact and pressure solution, as well as the dissolution of laumontite, feldspar are also of main factors which affect reservoir properties. The more favorable reservoir sandbodies are submerged distributary channel sandbodies and debouch bar sandbodies on delta front.