Shale Gas Potential of the Major Marine Shale Formations in the Upper Yangtze Platform, South China, Part III: Mineralogical, Lithofacial, Petrophysical, and Rock Mechanical Properties

Abstract

The marine black shale formations on the Upper Yangtze Platform, South China, are currently exploration targets for shale gas. Here, we report on the mineralogy, lithofacies, petrophysics, and rock mechanics of samples collected from the Ediacaran (Upper Sinian), Lower Cambrian, and Lower Silurian black shale intervals. All three formations are composed of high proportion of quartz, low content of clay, and rare or nonexistent content of carbonates. The Ediacaran and Lower Cambrian shales deposited in restricted deep water marine platform to marine basin environments are characterized by a higher quartz content and lower clay content than the Lower Silurian shales that were deposited in a more restricted marine basin environment. The carbonate content varies from 0 to over 50%, with the higher values measured in the Lower Silurian samples. These stratigraphic units were formed during bottom water anoxic conditions; therefore, they were rarely influenced by bioturbation. Lithologically, laminated and nonlaminated siliceous mudstones predominate, with minor contributions of other lithotypes. Pores generally have diameters in the nanometer (nm) to micrometer (μm) range, and numerous pores occur in organic matter. Most of the measured samples have porosities less than 4%, although a few samples show porosity in excess of 10%. Pores with radii less than 50 nm contribute significantly to total pore volume and total porosity. Permeability is extremely low, and helium permeability coefficients (Klinkenberg corrected permeability coefficient) are less than 20.2 nD (nano-Darcy, ∼2 × 10 −20 m 2 ). The rock mechanical properties of the samples are characterized by high brittle behavior, which coincides with their high compressive and tensile strengths and elastic properties. The Lower Cambrian shale is generally more brittle than the Lower Silurian shales, which possess a relatively higher content of clay minerals. The rock mechanical properties of the measured samples, however, depend on the overall mineral compositions and physical properties.

Pore Structure and Fractal Characteristics of Niutitang Shale from China

“…It is typical marine sediment carbonaceous and black shale in the lower section and dark-grey silty shale in the upper section [22,23]. In recent years, many studies have been carried out on the lithofacies, geochemistry and mineralogy of the Niutitang Formation [21,24,25], although there is a lack of information on pore structure and fractal characteristics. Therefore, the aim of this study is to examine the pore structure and fractal characteristics of the Niutitang Formation from northwestern Hunan Province, China.…”

Composition and pore characteristics of black shales from the Ediacaran Lantian Formation in the Yangtze Block, South China

“…Marine shales are widespread in the Yangtze Block, mainly in the Ediacaran Doushantuo Formation, the Lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation. These are some of the most promising strata for shale gas exploration in China (Dong et al, 2014;Tan et al, 2014aTan et al, , 2014bZou et al, 2011). Stratigraphic studies have indicated that the black shales in the Ediacaran Doushantuo Formation were mainly deposited in deep-water slope and basin settings (Jiang et al, 2011;Zhu et al, 2007); however, unlike the Cambrian and Silurian shales Han et al, 2013;Jiao et al, 2014;Liang et al, 2014;Ma et al, 2015;Tang et al, 2015;Tian et al, 2013Tian et al, , 2015Wang et al, 2015), the Ediacaran shales have not been described in great detail (Tan et al, 2014a).…”

“…Numerous studies have been performed to shed light on the porosity and pore structures of shales and the impact of shale composition on the pore development (Chalmers and Bustin, 2007;Chalmers et al, 2012a;Clarkson et al, 2013;Mastalerz et al, 2013;Milliken et al, 2013;Bustin, 2007, 2009a;Tan et al, 2014a). Since shales comprise fine-grained particles and abundant organic matter (OM), they commonly possess very low porosity and permeability due to continuous compaction and heating in the sedimentation process.…”

Mechanical Properties of Longmaxi Black Organic-Rich Shale Samples from South China under Uniaxial and Triaxial Compression States

“…This difference accounts for the shift of the minimum strength orientation as pressure increases, and the locus of minimum strength value was predicted by the variable τ 0 and tanφ theory showed in the former curves. In the research of Tan et al, the uniaxial strength of Longmaxi black shale was affected by both the sample composition and the porosity, and brittle character was also revealed by triaxial compression test with confining pressure increasing up to 36 MPa [20].…”

“…Longmaxi formation from the Lower Silurian is composed of high proportion of quartz, low content of clay, and rare or nonexistent content of carbonates [20]. According to Tan's research, the Lower Silurian shales were deposited in a restricted marine basin environment and were formed during bottom water anoxic conditions; therefore, they were rarely influenced by bioturbation.…”