Double-porosity System of Mixed Sediments in Saline Lacustrine Basin and Its Significance to Reservoir.

The Lower Ganchaigou Formation of Eocene in Yingxi area of Qaidam Basin is mixed sediments in saline lacustrine basin. It consists of lacustrine carbonate, detrital material and mud, and all the rocks contain many salt minerals. Based on core observation, physical property analysis, capillary pressure test, imaging logging and field emission scanning electron microscopy data, the mixed sediments contain double-porosity system. One is fracture and salt mineral dissolution pore with big pore radius, another one is crack between the salt and dolomite intergranular pore with small pore radius. The two types of pores build the "narrow lower-wide higher double-platform" structure on the capillary pressure curves. On the whole, the physical property of the rock is very poor with the characteristics of "low porosity-ultra low permeability". The pores and throats of the rock are generally fine. Electron microprobe analysis shows that the element composition of dolomite has the geochemical characteristics of "high contents of silicon and aluminum content, and low contents of iron and manganese". The average mass fractions of FeO and MnO in the dolomicrite are 1.322% and 0.060%, which are obviously lower than that of Fe and Mn in the hydrothermal dolomite. It is considered that the dolomicrite has a penecontemporaneous metasomatic formation mechanism. A large number of shrinkage intergranular pores are produced during the dolomitization process. This kind of matrix pore as the storage space is an important support factor of sustained and stable production of oil and gas in Yingxi area of Qaidam Basin. [ABSTRACT FROM AUTHOR]