Determination of petrophysical property cutoffs of tight lacustrine carbonate reservoir, Qaidam Basin, China

The study area is located at the northwest end of Qaidam Basin. The exploration target horizon of the study area is the Paleogene Lower Ganchaigou Formation, which is a set of shore-shallow to semi-deep lacustrine sediments. The rock composition is characterized by the mixed deposition of argillaceous, carbonate and clastic particles.
The main reservoir types are micro- and nano-scale dolomite intercrystalline pores. a large number of intercrystalline pores were formed during the process of penecontemporaneous dolomization. Samples were taken from the drilling cores at a sampling depth of 3100m-4050m. The purpose of this method is to distinguish the movable fluid from the bound fluid by multiple centrifugation and NMR(nuclear magnetic resonance) tests, and to determine the lower limit of the pore throat radius of the movable fluid distribution according to the relation between the fluid and the pore throat, and to determine the lower limit of the physical property through the relation between the physical property and the pore throat radius.
The pore throat distribution range of the samples with carbonate intercrystalline pore in the lower Ganchaigou Formation of Yingxi area in the process of mercury injection was 11 nm to 220 nm, and the pore throat distribution range in the process of mercury ejection was 40 nm to 1500 nm.

Through saturated fluid state NMR test, it is believed that the pore throat radius of intercrystalline pore developed samples is 0-160nm, and the fluid is planar distribution, and the movable fluid accounts for 13.7%. The lower limit of pore throat containing movable fluids radius were determined to be 47nm through the separation point of accumulated curve of saturation state and centrifugal state, and it was analyzed that the lower limit of pore throat containing movable fluids were mainly affected by the median radius.The lower limit of porosity was calculated by substituting the lower limit of pore throat of movable fluid distribution into the fitting formula of the median radius of pore throat and gas porosity. There is a good correlation between permeability and test pressure. The curve of permeability and test pressure shows that when the gas permeability is less than 0.02mD, the permeability decreases rapidly with the increase of pressure, indicating that the gas flow characteristics of samples with permeability below 0.02mD and samples above 0.02mD are quite different.