Impact of Minerals and Sealing Systems on the Pore Characteristics of the Qiongzhusi Formation Shale in the Southern Sichuan Basin

Abstract

The characteristics, distribution, and preservation of pores are vital in controlling the storage and distribution of shale gas. The Qiongzhusi Formation shales taken from different members with similar tectonic and thermal evolutions were used to evaluate the response of pore characteristics to minerals and sealing systems using field-emission scanning electron microscopy and gas adsorption. Because of differences in mineral structure and arrangement, feldspar, organic matter (OM)–clay, OM–rutile, and OM–apatite aggregates facilitate multiple types of pores in the shale and influence the relative proportions of surface porosity for different types of pores owing to differences in mineral structure and arrangement. Rigid frameworks and pressure shadows formed by rigid minerals and OM–mineral aggregates preserved OM and pores to some extent. The sealing capacity of the floor controls the sealing system and hydrocarbon expulsion efficiency of the Qiongzhusi Formation in different members. During thermal evolution, the amount of hydrocarbons generated and expelled affected the stress equilibrium state between the pore pressure and external stress, influencing the compaction intensity of shales. The OM pore development characteristics were evolved with variation in the stress equilibrium state in different sealing systems. Once the stress equilibrium state was disrupted, the OM pores deformed, narrowed, or even closed under the influence of compaction owing to the loss of overpressure support. The pore characteristics of the Qiongzhusi Formation shales responded significantly to different sealing systems. A few OM pores are flat and slitlike in the open system, whereas numerous OM pores are round and elliptical in the semiopen system. Meanwhile, the average diameter of the OM pores in the open system was reduced by approximately 40.2% compared with that of the semiopen system. Furthermore, the pore volume and specific surface area of the mesopores for open system shales were reduced by 38.4% and 37.7%, respectively, compared to the semiopen system. These findings will improve the understanding of the distribution and preservation of pore in shale and help assess the sweet-spot members for the Qiongzhusi Formation shale gas.

Pore Characteristics of Oil Shales in Jilin Province, Northeast China: Investigations Using Gas Adsorption, Mercury Intrusion, and NMR Cryoporometry

“…The four oil shale-bearing formations in Jilin Province are in the immature to low-maturity stages of thermal evolution, with the most mature formation being the Qingshankou Formation ( R o ∼ 0.76%). ,, Moreover, Lu et al observed that only rare OM pores occur in the Qingshankou Formation among these four formations by using Ar ion polishing and FE-SEM imaging techniques. Thus, it is difficult to establish the relationship between pore structure and OM using total organic carbon (TOC) data, as other scholars have completed previously. ,,,,,,− Therefore in this section, we mainly discuss the influences of mineral composition on pore structure.…”

“…A possible explanation for this phenomenon might be that OM-clay mineral complexes that form by the van der Waals energy widely exist in the continental environment and can effectively preserve pores. , Another possible explanation for this phenomenon is that rigid authigenic minerals, such as pyrite filled between the cleavages of clay minerals, can form pressure shadows to resist compaction to protect the clay mineral pores (Lu et al, their Figure c). , …”

“…63,67 Another possible explanation for this phenomenon is that rigid authigenic minerals, such as pyrite filled between the cleavages of clay minerals, can form pressure shadows to resist compaction to protect the clay mineral pores (Lu et al, 21 their Figure 6c). 21,66…”

Rare Earth Element Characteristics of Shales from Wufeng–Longmaxi Formations in Deep-Buried Areas of the Northern Sichuan Basin, Southern China: Implications for Provenance, Depositional Conditions, and Paleoclimate

“…The end of the Ordovician and the beginning of the Silurian was an important period in Earth history, − and it is marked by marine mass extinction, large-scale glaciation, sea-level change, extensive volcanism, ocean anoxic event, and extensive deposition of marine black shale. − Black marine shale, as a source rock for oil and gas, is widely accumulated in paleogeographic environments. − To this day, such a black shale has been widely developed as unconventional reservoirs in many countries, such as the United States, Canada, Australia, China, Germany, Russia, etc. − Thus, the study of the sedimentary environment and the mechanism of organic matter accumulation for the marine shales of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the northern Sichuan Basin not only can provide important information about the climatic and paleoenvironmental change of the Ordovician–Silurian but also provides evidence for the prediction of favorable areas for oil and gas exploration.…”

Insights into the Multiscale Conductivity Mechanism of Marine Shales from Wufeng–Longmaxi Formation in the Southern Sichuan Basin of China

“…The marine shale in the southern Sichuan Basin has the characteristics of continuous distribution with integrated source and reservoir, while the depositional environment is mainly a deepwater shelf with static water (Han et al 2016;Wang et al 2020). The structure and composition of shale are complex, where low-resistivity thin layers have developed in the vertical direction (Zhao et al 2014;Li et al 2016;Liu et al 2022). In addition to quartz, feldspar, calcite and dolomite, the rock composition also includes conductive phases (pore water, clay minerals, pyrite, and overmature organic matter); the mineral composition is complex, and the framework minerals are conductive (Liu et al 2019;.…”