Abstract:

The oil and gas was accumulated under the surface condition of high-temperature and high-pressure.This process is not only complex but also difficult to observe. Petrophysical properties are important parameters for evaluating reservior quality and for controlling petroleum accumulation in the lithologic sandbody.Under the condition of geology, this influence is mainly represented in reservoir space and nature of sandbody. As we have seen,the sandstones with better petrophysical properties have high storage capacity, larger storage space and better permeability,and oil and gas easily enter into the reservoir spaces of sandstones.Moreover,sandstone cann't be filled with oil and gas from external source rock until the porosity and permeability in sandstones reach their critical values. With respect to the control of interior conditions of lithologic sandstone on hydrocarbon-bearing capacity, Chen Zhangming suggested by experimental analysis that the coarse-grained sandstone is better in hydrocarbon-bearing capacity and the fine-grained sandstone possibly has no hydrocarbon-accumulation. Zeng Jianhui found, by the petroleum accumulation test of lithologic sandbody, that the sandstone cann't be filled with hydrocarbon until the sandstone grain diameter is up to certain critical value. Stainforth indicates that the capillary transmission under the action of hydrocarbon concentration gradient is key mechanism that cause the expulsion of hydrocarbon from source rock, and this mechanism is also suitable for the petroleum accumulation of lenticular sandstone.In order to make more intensive study of petroleum accumulation mechanism and model in lenticular sandstone, the nuclear magnetic resonance (NMR) technique was introduced in this model test. This technique is used to describe quantitatively the magnitude and space distribution of rock porosity and permeability as well as the transmission and percolation mechanism of rock pore fluid. By using this technique,the distribution of oil (in lenticular sandbody )and water(in enclosing rock)and their replacing process can be more directly observed and the process and mechanism of petroleum accumulation can be deeply analyzed. Lentiular pools refer to petroleum accumulation in lenticular reservior or other irregular reservoirs confined by impermeable and low-permeable strata. Test model of nuclear magnetic resonance was built according to characteristics of the geological model. The experiment was carried out in group. Under the conditions of T=30℃,P=20 MPa,and 40% of oil saturation in argillaceous rock, the artifical cement cores (5 centimeters in diameter and 4 centimeter in height) with permeability of 988×10^-3,585×10^-3,203×10^-3,56×10^-3,13×10^-3 μm² were separately measured. From the results of experiment, under the pressure difference of penetration and diffusion caused by hydrocarbon concentration difference, there is a tendency for oil in enclosing rock to migrate towards core (sandstone). When oil migrated to the sand-shale boundary, the capillary force difference exist in contact belt between large and small pores as the migration channel of sand lens is pore. Under the action of the capillary force difference and hydrocarbon concentration difference, oil and gas first entered into the sand lens with larger pores,where the intergranular water is replaced to enter into the shale with smaller pores. With the progress of the above process, oil and gas constantly entered into sand lens and pore water is continuously replaced until sand lens are saturated by oil and gas. There is no hydrocarbon accumulation in the sandstone bodies with smaller pore throat radius and poor petrophysical properties.

Key words: lenticular sandstone, petroleum accumulation modeling, petroleum accumulation mechanism, nuclear magnetic resonance (NMR), Jiyang depression