DIAGENESIS OF ORDOVICIAN PALEO-KARST RESERVOIR IN THE SULIGE AREA, THE ORDOS BASIN

The Sulige area is located to the northeast of the ancient central uplift in the Ordos Basin.The main sedimentary environments during Ordovician in the area include under-water uplift and inter-uplift depression as well as inner shelf basin.The Ordovician mainly consists of host carbonate rocks and karst breccias.Dissolution pores are the most important pores,and fractures are important reservoir spaces.Porosity ranges from 0.02% to 16.70%,averaging 2.19%.Permeability ranges from 0.0014×10^-3 to 706.35×10^-3μm²,averaging 1.90×10^-3μm².Diagenesis,which the Ordovician carbonate sediments(rocks) have experienced,includes compaction,dolomitization,cementation,replacement,dissolution and dedolomitization.The slightly lower δ¹⁸O values,higher concentration of Mn and Fe,and obviously lower concentration of Sr for dolomite of whole rocks indicate that the dolomite is influenced by meteoric leaching and burial diagenesis.The δ¹³C values are not low since carbon comes from CaCO₃ of original sediments.The δ¹⁸O and δ¹³C values of dolomite cements are generally lower than those of dolomite of whole rocks,suggesting that the dolomite cement is formed during karstification at epidiagenetic expose phase.The rocks have experienced early diagenesis,epidiagenesis and late diagenesis.Reservoir rocks are characterized with low porosity and permeability,due to weak karstification and deep burial.