Porosity characteristics of the Eocene mixed-sediment reservoirs in the saline lacustrine basin, Qaidam, China

In recent years, significant progress has been achieved in the field of deep exploration in the Yingxi area of the Qaidam Basin. The Eocene was characterized by mixed sediments in the lacustrine basin. The area consists of lacustrine carbonate, detrital material, and mud. All rocks contain numerous salt minerals. Via core observation, physical property analysis, capillary pressure tests, image logging, and field emission scanning electron microscopy, results reveal that the rocks contain a double porosity system. The first are fractures and salt mineral dissolution pores possessing a large pore radius, and the second are cracks between the salts and dolomite intercrystalline pores with a small pore radius. These two types of pores caused the double-platform feature with a lower narrow portion and upper wide portion on capillary pressure curves. Overall, the physical properties of the rocks are deficient, with low porosity-ultra-low permeability. Moreover, pores and throats are generally acceptable. Mineralogical composition analysis based on electronic probe shows that the rocks possess a high content of both silicon and aluminum, but low content of iron and manganese. The average FeO content in the dolomicrite is 1.322%, and that of MnO is 0.060%, both of which are lower than average Fe or Mn content in hydrothermal dolomite. It is believed that the dolomicrite possesses a penecontemporaneous metasomatic formation mechanism. The dolomitization process produces a large number of shrunken intercrystalline pores. These kinds of matrix pores, functioning as storage space, constitute an important support factor for the sustained and stable production of oil and gas in the study area.