DIAGENESIS AND RESERVOIR EVALUATION ON THE UPPER PALAEOZONIC RESERVOIR SANDSTONES IN DANIUDI GAS FIELD, THE ORDOS BASIN

The diagenesis stages of the reservoir sandstones in the Upper Palaeozonic strata in Daniudi Gasfield,northern Ordos Basin,were studied.The results show that the vitrinite reflectance(R_o) of the Taiyuan Formation and the First Member of Shanxi Formation is between 1.4% and 1.5%,the inclusion homogenation temperature is 140℃ and the percentage of montmorillonite is usualy less than 30% in illite-montmorillonite mixed-layer mineral,indicating the stage B of metarmosis.The Lower Shihezi Formation(including P₁x¹,P₁x² and P₁x³ Members) is in the stage A of metarmosis with the R_o being 1.3%-1.4%,the inclusion homogenation temperature being 126℃ and the percentage of montmorillonite being 32%-36% in illite-montmorillonite mixed-layer mineral.The inclusion homogenation temperature of the Second Member of Shanxi Formation is complicated,but still can be considered as in the stage A of metarmosis.The main diagenesis processes that control the reservoir properties include compaction,cementation(silicide,carbonates,clay minerals),dissolution and fracturing.The types of pores are primary intergrain pore(residual),intercrystalline micro-pore and intergrain or intragrain solution pore(including moldic pore).The reservoir generally belongs to the type of low-porosity and low-permeability.The microfractures in formation greatly improve the reservoir properties and contribute to higher gas yield in P₁x³ and P₁x² than in P₁x¹ and other reservoirs.