Micro- and Nanoscale Pore Structure Characterization of Carbonates from the Xiaoerbulake Formation in the Tarim Basin, Northwest China

Abstract

The evaluation of pore structure is an essential part in the assessment of carbonate reservoirs. The structures (geometry and connectivity) of nm to μm-scale pore networks in outcrop samples of carbonates from Xiaoerbulake Formation in Tarim Basin of China were studied by using optical microscopy, field emission-scanning electron microscopy (FE-SEM), as well as mercury intrusion porosimetry (MIP) with fractal analyses of the data, and spontaneous imbibition tests (distilled water). The results demonstrate that the lithologies are micritic dolomites, fine-to-medium-to-coarse crystalline dolomites, microbial dolomites, and dolarenite. At micro- to nanoscales in size, pore types are dominated by intergranular, intercrystalline, and intragranular (e.g., dissolution) pores. These pore networks have pore-throat diameters from 0.01 to >10 μm. Compared with a nanoscale pore network, the μm-scale pore networks are relatively well connected and serve as the most important permeability pathways. Although the pore volume accounts for most of the total porosity, the permeability of nanoscale pore networks is low. The existence of micro-nano-fractures could improve connectivity, especially for the nanoscale pore networks, by linking the intragranular (dissolution) pores which are mostly in the range of nm-scale.

Changes in Nanoscale Pore Structures and Mesopore Connectivity as a Result of Artificial Maturation of a Source Rock from the Shanxi Formation, Ordos Basin

“…Shale micropores are usually characterized by the LCA method because CO 2 has greater accessibility to micropores due to its higher experimental temperature than N 2 . , As shown in Figure a, the adsorption curves of the original shale and the samples after pyrolysis are shown as a type I pattern having microporous characteristics . In this work, the Dubinin–Astakhov (D-A) model was used to interpret the adsorption curve and obtain the pore volume and SSA of shale. , Table contains the micropore volume and SSA of shale samples at different pyrolysis temperatures. Table contains the micropore volume and SSA of shale samples at different pyrolysis temperatures.…”

“…Before the test, the samples were crushed to 35–80 mesh. Samples were first placed into the instrument to be degassed at ∼110 °C for about 24 h and then subjected to increasing (adsorption) and decreasing (desorption) relative pressures of either N 2 or CO 2 . − The Dubinin–Radushkevich (DR) model was used to calculate the specific surface area (SSA) and volume of micropores. , The Brunauer–Emmett–Teller (BET) model and Barrett–Joyner–Halenda (BJH) model was used to determine SSA and meso–macropore volume, , respectively. Both the micropore and meso–macropore size distributions were determined by the density functional theory (DFT) model.…”

“…70 In this work, the Dubinin−Astakhov (D-A) model was used to interpret the adsorption curve and obtain the pore volume and SSA of shale. 11,58 The pore volume ranges from 0.289 to 0.521 cm 3 /100 g and SSA range from 5.12 to 9.06 m 2 /g. The results of pore volume and SSA show similar changing trends as temperatures change.…”

Distribution of Retained Oil in Shales from the Lucaogou Formation, Jimusar Sag, Northwest China: Insights from Mineralogy and Pore Structure

“…To obtain an extremely flat surface for better observations, the sample surfaces were initially ground using a lapping machine (Leica EM TXP) with 9, 2, and 0.5 μm abrasive sandpapers and finally ion-milled by an ion beam milling system (Leica EM TIC 3X) for 2 h. During the milling process, two accelerating voltages of 5.5 and 2.0 kV were selected alternately for four rounds of milling. , The edge of each sample was coated with a conductive carbon adhesive to improve the electrical conductivity for better imaging.…”