Pore Characteristic Analysis of Deep Tight Sandstone Reservoir in the Dongpu Depression, Bohai Bay Basin, China

Abstract

With the increasing demand for oil and gas, deep reservoirs are becoming one of the important exploration areas. To further evaluate the quality of deep tight sandstone reservoirs, thin section (TS), high pressure mercury injection (HPMI), and scanning electron microscopy (SEM) were performed on deep tight sandstone reservoir samples (>3500 m) from the Dongpu Depression. We analyzed the pore-size distribution and pore-throat structure characteristics. Fractal dimensions of different pore size calculated from capillary pressure curves were also used in this study to evaluate the microscopic heterogeneities of tight sandstone reservoir. The results show that the studied deep tight sandstones possess poor reservoir property with average porosity and permeability of 5.98% and 0.177 ×10^-3 μm² respectively. Intergranular pores, intragranular pores, intercrystalline pores, dissolution pores, and microfractures can be observed via TS and SEM. There are many halite aggregates in the pores observed by SEM images, which filled between particles in conglomerate shape and some have been dissolved. Compared with cement like calcite and clay minerals, the effect of halite on the reservoir physical properties is greater when blocking pores. The distribution range of effective throat radius is narrow according to HPMI test. The entire pore throat radius of experimental samples is less than 1.0 µm and mainly distributes between 0.01 µm and 0.1 µm. The pore throat, which contributes greatly to permeability, occupies a smaller volume. Additionally, too small or too large throat sorting coefficients are not conducive to reservoir seepage. There is a correlation between pore structure parameters and reservoir physical properties. Experiments show that the entry pressure, saturation median pressure and mercury withdrawal efficiency are favorable indicators to reflect the reservoir property of deep tight sandstones. The calculation results of fractal analyses display that mesopores (100-1000 nm) and transitional pores (10-100 nm) possess average fractal dimension of 2.6 and 2.57 respectively, which are obviously heterogeneous and complexity than micropores (with average fractal dimension of 2.26), and the fractal dimension of mesopores is apparently influenced by pore throat structure. Compared with reflecting the reservoir physical property, fractal dimension is more proper to characterize the pore throat structure of deep tight sandstones.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018