MICROSCOPIC PORE STRUCTURES AND MOVABLE FLUID DEPOSIT CHARACTERISTICS OF TIGHT SANDSTONE RESERVOIR IN CHANG-8 OIL RESERVOIR, HUJIANSHAN OILFIELD, ORDOS BASIN

The author investigated and outlined the complex geological characteristics of Chang-8 oil reservoir in Hujianshan Oilfield, Ordos Basin, including its significant anisotropy, complex microscopic pore structures, low movable fluid saturation, and intensive fluid distribution characters, etc. To answer these questions, the author carried out experiments by means of normal physical characteristics, casting thin-section, SEM analysis, conventional mercury injection and NMR, etc. Observations were made particularly on the microscopic pore structures of Chang-8 oil reservoir in Hujianshan Oilfield, and its influence to the saturation level of movable fluids. The study suggested that the reservoir spaces of Chang-8 were predominately primary intergranular pores and feldspar vugular pores. These pore structures were categorized into four groups of reservoir type that represent varied reserving and filtration capacities, i.e. Types I (the highest), II, III and IV (the lowest), according to their varied capillary pressure curves and corresponding parameters. Each had a NMR T-2 spectrum similar to its corresponding distribution pattern of mercury saturation at pore throat radius, suggesting a positive correlation between the T-2 value and the pore throat diameter. Pore throat diameter was thus highlighted as a major factor affecting that determined the distribution of movable fluids, along with an intensive positive correlation between permeability and movable fluid saturation that provided an intuitive parameter representing the filtration capacity of pore structures.