Pore Water and Its Influences on the Nanopore Structures of Deep Longmaxi Shales in the Luzhou Block of the Southern Sichuan Basin, China

Abstract

In the Luzhou Block of the southern Sichuan Basin, the deep Longmaxi shales have become important exploration targets in recent years. However, the water-bearing properties of these shales are still unclear, which significantly limits evaluations of reservoir pore structures and gas-in-place (GIP) contents. In this study, twelve fresh shale core samples were collected at the well site, and the pore water (CPW) and equilibrium water (CEW) contents, as well as the pore structures of the shales, were analyzed under both as-received and dried conditions. The results indicate that the deep shales have low water-bearing extents with a pore water content (CPW) of 3.82–16.67 mg/g, and that both the organic matter (OM) and inorganic matter (IM) pores can be used for pore water storage. The extent of influence of pore water on nonmicropores and IM pore structures is more significant than that on micropores and OM pore structures. Meanwhile, the pore water obviously reduces the retention effects of nanopores and may block nanopores with pore widths < 0.5 nm. An average of 40% of pore spaces were taken up by pore water in the studied deep shales in the Luzhou Block, and the residual pore surface area and pore volume of the shales were mainly contributed from micropores and nonmicropores, respectively.

Effects of Pore Water on the Heterogeneity in Nanopore Structures of Deep Marine Shales: A Multifractal Study on As-Received Shale Samples from the Southern Sichuan Basin, China

“…Schematic maps showing the location (a, b) and stratigraphic column (c) of the well L216 in the southern Sichuan Basin (adapted with permission from ref . Copyright 2022, MDPI).…”

“…37 The deep WL shales were developed in the whole Luzhou block, with a thickness of 500−650 m. The burial depth of the WL shales is between 3500 and 4500 m and gradually increases from north to south. 38 (1)…”

“…The main strata in the Luzhou block include Cambrian, Ordovician, Silurian, Permian, Jurassic, and Cretaceous strata . The deep WL shales were developed in the whole Luzhou block, with a thickness of 500–650 m. The burial depth of the WL shales is between 3500 and 4500 m and gradually increases from north to south …”

Gas-In-Place Content of Deep Shales and Its Main Controlling Factors: A Case Study on the Wufeng-Longmaxi Shales in the Middle Luzhou Block of the Southern Sichuan Basin, China

“…Schematic maps showing the location (a) and stratigraphic column (b) of Well L202 in the middle Luzhou block, southern Sichuan Basin (modified from ref ).…”

“…At present, it is difficult to accurately evaluate the GIP contents of deep shale reservoirs. Because of the heterogeneity in the development environments of deep shale strata, the organic and inorganic compositions are significantly different for deep shales in various strata and tectonic regions. − The type, content, and maturity of the organic matter of shales not only affect shale gas generation but also control the development of organic matter-hosted (OM) nanopores, which influences the enrichment and storage of shale gas. − In addition, the clay minerals of shales generally develop inorganic matter-hosted (IM) nanopores and provide certain space for shale gas storage. Although the brittle minerals of shales barely develop pores, they can prevent shale pores from collapsing as a result of compaction at deep burial depths, which benefits shale gas storage; meanwhile, a high content of brittle minerals is beneficial to hydraulic fracturing of shale reservoirs and enhances the development of shale gas .…”