Insights into Pore Structures and Multifractal Characteristics of Shale Oil Reservoirs: A Case Study from Dongying Sag, Bohai Bay Basin, China

Abstract

The pore structure and its complexity and heterogeneity control the occurrence states and fluidity of shale oil. The multifractal theory effectively characterizes the complexity and heterogeneity of the shale pore structure. In this study, serial technologies were applied to detect the pore systems of shale obtained from Dongying Sag, Bohai Bay Basin in China. The multifractal characteristics of the pore structure of the shale were analyzed based on its nuclear magnetic resonance (NMR) T 2 spectrum. The analysis results show that shale oil reservoirs can be classified into four types based on their T 2 spectra. Type I shales T 2 spectra show large p2 (1−20 ms), moderate p3 (>20 ms), but little p1 (<1 ms), characterized by large NMR and connected porosity, the lowest BET specific surface area (SSA), and the largest average pore throat diameter and S 1 contents. Large p2, moderate p1, and tiny p3 are the main distinctions of type II shales with the largest NMR porosity, large connected porosity, and BET SSA. The T 2 spectra of type III shales have large p1, moderate p2, and little p3, corresponding to large NMR porosity and BET SSA and the largest total organic content (TOC) and S 1 contents but lower connected porosity. Type IV shales have the most significant contents of micropores with the relatively largest p1 in the T 2 spectra characterized by the lowest NMR and connected porosity, the largest BET SSA, the lowest TOC, and S 1 contents but the largest clay mineral contents. Both types III and IV shales are unfavorable shale oil reservoirs. D q decreases monotonically as q increases, indicating the multifractal nature of shale pore structures. D 0 varies from 0.88 to 1.00 (mean: 0.95), and Δα ranges from 1.24 to 2.82 (mean: 1.79), suggesting complex and heterogeneous pore structures. Types I and II shales have lower D 0 values than types III and IV shales. Thus, type I organic-bearing massive felsic and type II organic-rich layered calcareous shales are favorable for shale oil reservoirs with large pores and large porosity. They have the least complex pore structures among the four shale types considered.

Classification and Control Factors of Reservoir Types in Hybrid Sedimentary Rocks: A Case Study on Lucaogou Formation in the Jimusaer Sag, Junggar Basin

“…Figure illustrates the mercury intrusion curve, revealing a significant amount of mercury entering the sample at 0.01–0.1 MPa, with a pronounced peak in the pore size distribution curve at approximately 10–100 μm. According to previous studies, the pore size distribution curve within this range (10–100 μm) may represent a false intrusion volume (skin effect) formed by rough pits on the sample surface due to the high surface tension of mercury (Figure B). Therefore, a correction is necessary before using the mercury intrusion and pore size distribution curves.…”

A Comparative Study on the Microstructure and Petrophysical Properties of Undeformed and Naturally Deformed Shales

“…Mineralogy, TOC, and pore structure results for the samples are summarized in Tables and . Shale composition is another important potential factor and can significantly affect pore structure evolution. − Our shale samples vary in their TOC contents and mineralogical compositions (Table ), but no important linear correlations were found between the pore structures and the TOC, brittle minerals, or clays in both undeformed and deformed samples (Figure ). The relationship of the pore structure with the shale component is quite complex.…”

Pore Structure Characterizations of Shale Oil Reservoirs with Heat Treatment: A Case Study from Dongying Sag, Bohai Bay Basin, China

“…For core plugs, NMR measurements were performed, while powder samples with grain sizes of 40–60 and >100 mesh underwent LTNA/D and Rock–Eval tests, respectively. To obtain dry samples for testing, both core and powder samples were first washed oil with a mixed solution of dichloromethane and acetone (3:1 in volume) and then dried at 110 °C under vacuum conditions for 24 h. This temperature was chosen as it helps preserve the original pore structures of shales, which are typically tough to destroy. − After these samples cooled to room temperature, NMR, LTNA/D, and Rock–Eval tests were carried out. Subsequently, the dry core samples were saturated with n -dodecane ( n -C 12 ) at 10 MPa for 24 h after vacuuming.…”

“…T 2,gm can be calculated as follows lg 0.25em T 2 , gm = ∑ f i lg T 2 , i ∑ f i where T 2, i is the T 2 relaxation time in the T 2 spectrum, ms, and f i is the signal amplitude at T 2, i . Moreover, referring to r 35 and r 50 in the mercury capillary curve, T 2,35 and T 2,50 were calculated …”