Annnalysis on Mechanism of Feldspar Dissolution and Its Influencing Factors in Feldspar-rich Sandstone Reservoir

Porosity formed by the dissolution of unstable framework grains is one of the most important genetic types of secondary porosity,and feldspar is found to be the most extensive ones of the unstable framework grains. Songliao and Liaohe basins are two large petroliferous inland basins of Mesozoic and Cenozoic,in which has been constructed a clastic rock formation with great thickness.In sandstone there is above 20% unstable feldspar fragments,which is the requisite condition for the development of the secondary porosity. But it has been studied that the FGD of the grains, may not cause the increase of the reservoir permeability of the sandstone(Siebert,1984). The dissolution of a unit volume of feldspar may produce 0. 66 volume of Kaolinite and 0. 33 volume of quartz (Sullivan,1991).Therefore.only when the dissolved materials are transferred after dissolution the porosity of the sandstone can be effectively increased. This paper is based on analyzing the state of thermodynamics of feldspar dissolution process.The SEM,X一diffraction,oxygen and carbon stable isotopes, etc. analytical methods are adopted in research of the controlling factors of feldspar dissolution in Cretaceous sandstone in the region of Songliao basin and Palaeogene sandstone in the Liaohe basin. Through research,the author indicates that favors to development degree of feldspar dissolution are dependent on:(1)the organic and inorganic reaction time;(2)the characteristics and fluid condition of pore water;(3)the primary porosity of sandstone,and(4)the temperature and pressure,etc. These factors play different roles in different types of feldspar. The most important parameters are fluid condition,type and abundance of organic acid in fluid .which were closely related with the evolution of clay mineral and organic material in the diagenesis of mudstone.