| Based on the core observing and describing,thin section analysis,scanning electron microscopy,reservoir measured physical analysis and oilfield water analysis,the characteristics of anhydrite cementation and its forming mechanism,distribution and effect on reservoir properties of the third member of Shahejie Formation of Wendong area,Dongpu Depression was studied.The results show that the main material form of anhydrite cementation in the Es3 sandstone reservoirs are patchy,which mainly developed in 2700 to 4400 m,middle diagenetic stage of A and B.The anhydrite precipitated in the fluid environment,for the CaSO4 has been Super-saturated,which the saturation coefficient of calcium sulfate in sandstone reservoirs is larger than 140.The main sources of its materials include the ancient sedimentary water,gypsum-salt rocks dissolution,cation alternative adhesive action,dissolution of feldspar and carbonate cements and the albitization of plagioclase.The distribution of reservoirs which anhydrite cement developed are mainly relate to the distribution of gypsum-salt rock.Reservoirs in the vertical,which anhydrite cement developed mainly distributed the upper,lower part of gypsum-salt rocks,and/or between the gypsum-salt rocks;On the plane,they mainly distributed at edge of gypsum-salt rocks,and/or the area where gypsum-salt rocks and sandstones inter-fingered together.It has been controlled by solution of gypsum-salt rock and transportation of hydrocarbon filling and over-pressured fractures.Thermochemical sulfate reduction reaction(TSR)and local dissolution of anhydrite cements help to improve reservoir properties.However,the improvement were relatively weak,for the temperature of third member of Shahejie Formation were just reached to the lowest reaction temperature of TSR and areas of low SCCS value are limited.In contrast,the content of anhydrite cement showed a significant negative correlation with reservoir porosity and permeability,when it exceed more than 5%. |
