Abstract
Organic-rich lacustrine shales in the Upper Triassic Yanchang Formation with thermal maturity mainly in the oil window are the main shale oil and shale gas system in the lacustrine strata of the Ordos Basin, China. Pore systems are important for the storage and transfer of shale oil and gas. The main objectives of this study are to identify the pore types and pore structures and investigate the controlling factors for pore types, pore structures, and total porosities of the lacustrine Yanchang Shale. In this study, organic-rich mudstones, mudstones with siltstone interlayers, siltstone, and sandstones were selected from 15 wells in the southern Ordos Basin. X-ray diffraction, pyrolysis, scanning electron microscopy (SEM), low-pressure nitrogen adsorption analysis, and helium porosimetry were conducted to investigate the mineral compositions, pore types, pore structures, porosities, and controlling factors. Siltstone and sandstone interlayers heterogeneously developed in the Yanchang Shale. The petrology, mineral composition, geochemistry, pore type, pore structure, and porosity of siltstone interlayers are different from those of mudstones. The siltstone and sandstone interlayers usually have more quartz and feldspars, greater detrital grain sizes, and relatively better grain sorting but are lower in clay minerals, total organic carbon (TOC), amount of free liquid hydrocarbons values (S1), and total residual hydrocarbons values (S2), compared to mudstones. Interparticle (interP), intraparticle (intraP) pores, and organic pores (OPs) were developed in both siltstones and mudstones. OPs were observed in samples with lower thermal maturity (e.g., 0.5–0.85%). The inorganic pore size is greater than that of OPs. Additionally, the inorganic pore diameters in siltstone interlayers are also greater than those in mudstones. Organic-rich mudstones generally have higher pore volumes (PVs) of pores with sizes less than 10 nm, pore volumes of pores with sizes between 10 and 50 nm (PV, 10–50 nm), and specific surface area (SSA), but they have lower PVs of pores with sizes greater than 50 nm, total PV, and porosity when compared to siltstone and sandstone interlayers. The dominant pore type in mudstones is OPs and TOC (first order), sources and OM types (second order), and thermal maturity (third order), while the abundances of rigid grains with greater sizes and grain sorting are the main controlling factors of pore structures, SSA and PV. Both inorganic pores and organic pores are abundant in the siltstone interlayers. The pore size distribution (PSD), PV, and porosity of siltstone interlayers are related to the abundance of rigid grains (first order), grain sorting (second order), grain size (third order), and carbonate cement content. The total PV and porosity of Yanchang Shale reservoirs may have increased with the increased abundance of siltstone and sandstone interlayers.
Highlights
Pore systems in organic matter-rich shales that comprise nanometer- to micrometerscale interP pores, intraP pores, and organic pores, and fractures play an important role in both the storage and migration of shale oil and gas [1,2,3,4]
Loucks et al [13] reported that the dominant pores are intraparticle pores and that there is a lesser abundance of organic matter pores, while interparticle pores are rare in the Yanchang Shale
Lei et al [12] and Yu et al [20] identified that the pore types, pore size distributions, and porosities are different between silty laminae and mudstones and concluded that the silty laminae are dominated by macropores (>50 nm), whereas mudstones without silty laminae are dominated by mesopores
Summary
Pore systems in organic matter-rich shales that comprise nanometer- to micrometerscale interP pores, intraP pores, and organic pores, and fractures play an important role in both the storage and migration of shale oil and gas [1,2,3,4]. Loucks et al [13] reported that the dominant pores are intraparticle pores and that there is a lesser abundance of organic matter pores, while interparticle pores are rare in the Yanchang Shale They proposed that clay mineral content, thermal maturity, and lithology are the major controls of pore type and pore structure [18,20]. Other studies have reported that abundant OPs developed in the Yanchang Shale [7,8,15,16,21], and Ko et al [7] even concluded that OP was the most important pore type in the Yanchang Shale with lesser abundances of interP and intraP pores. Mudstone samples without siltstone interlayers, siltstones, and sandstones separated from the samples with siltstone or sandstone interlayers from 15 wells with different burial depths and thermal maturities were selected, and their mineral compositions, geochemistry, pore types, pore structures, and porosities are characterized, and the main controlling factors of pore types and pore structures are discussed
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