Study of Controlling Mechanism of Carbonate Cementation on Porosity Evol ution in Lower Tertiary Sandstones of the Liaohe Basin

Key words: 
• Sandstone /
• Carbonate cementation /
• Oryganic acid /
• Oxygen and carbon iso-topes /
• Porosity evolution

Abstract: In sandstone carbonate cementation is an important porosity reduction prcxess. Timing of carbonate precipitation and dissolution influences on development and evolution of porosity di-rectly. In Liaohe Basin Low一Tertiary sandstones carbonate cement average content is 10%,the maximum can be to 30% . The extent of carbonate cement distribution varies widely with present depth from 1300 m to 3700 m, and species of carbonate cement are calcite,ferriferous calcite,dolomite and ferriferous dolomite.The SEM,cathodoluminescene emission,oxyen and carbon stable isotopes,etc. analyti-cal methods are adopted in the research of the development mechanism of the carbonate cement in Liaohe Basin reservoir sandstones. Through research,the author indicates that(1)there were two episodes carbonate cementation took place ire sandstones,and the formation of car-bonate cement is related to the sandstone composition,evolution of organic matter and the characteristics of water;(2)there are different influence of two episodes carbonate cementa-tion nn porosity,and effect on the emplacement of hydrocarbon. Early episode precipitated carbonate cement caused pore loss and provided a condition for dissolution secondary porsity,and emplacement of hydrocarbon,late episode carbonate cement precipitation is related to CO₂ generated by deoxidation of organic matter and repecipitated in the dissolution porosity,so that a large amount of porosity lost in the B episode of Mid一diagenesis.