Development Characteristics of Shale Lithofacies in the Longmaxi Formation and their Main Controlling Factors in the Changning Area, South Sichuan Basin, SW China

Based on core observations, thin sections, X-ray diffraction (XRD), and seismic data, the lithofacies types in the organic-rich Longmaxi shale (Lower Silurian) in the Changning area of the southern Sichuan Basin were identified. The factors controlling the spatial variations in the shale lithofacies and the influences of the shale lithofacies on shale gas development were also analyzed. Results indicate that there are seven main types of shale lithofacies in the Long11 sub-member of the Longmaxi Formation, including siliceous shale (S-1), mixed siliceous shale (S-2), carbonate-rich siliceous shale (S-3), clay-rich siliceous shale (S-4), carbonate/siliceous shale (M-1), mixed shale (M-2), and argillaceous/siliceous shale (M-4). A vertical transition from the carbonate shale association + mixed shale association at the bottom of the sub-member to a siliceous shale association and mixed shale association + siliceous shale at the top generally appears in the Long11 sub-member. The shale lithofacies of the Long11 sub-member also laterally change from the central depression (low-lying area) to the geomorphic highland in the east and west parts of the Changning area. The spatial variations in shale lithofacies in the Long11 sub-member of the Changning area were mainly controlled by palaeogeomorphology and relative sea level. The geomorphic highland area is dominated by carbonate-rich siliceous shale and mixed siliceous shale, but the depression (low-lying area) is mainly dominated by mixed siliceous shale and argillaceous/carbonate shale.

Characteristics of Pore Structure and Gas Content of the Lower Paleozoic Shale from the Upper Yangtze Plate, South China

This study is predominantly about the differences in shale pore structure and the controlling factors of shale gas content between Lower Silurian and Lower Cambrian from the upper Yangtze plate, which are of great significance to the occurrence mechanism of shale gas. The field emission scanning electron microscopy combined with Particles (Pores) and Cracks Analysis System software, CO2/N2 adsorption and the high-pressure mercury injection porosimetry, and methane adsorption were used to investigate characteristics of overall shale pore structure and organic matter pore, heterogeneity and gas content of the Lower Paleozoic in southern Sichuan Basin and northern Guizhou province from the upper Yangtze plate. Results show that porosity and the development of organic matter pores of the Lower Silurian are better than that of the Lower Cambrian, and there are four main types of pore, including interparticle pore, intraparticle pore, organic matter pore and micro-fracture. The micropores of the Lower Cambrian shale provide major pore volume and specific surface areas. In the Lower Silurian shale, there are mesopores besides micropores. Fractal dimensions representing pore structure complexity and heterogeneity gradually increase with the increase in pore volume and specific surface areas. There is a significant positive linear relationship between total organic carbon content and micropores volume and specific surface areas of the Lower Paleozoic shale, and the correlation of the Lower Silurian is more obvious than that of the Lower Cambrian. The plane porosity of organic matter increases with the increase in total organic carbon when it is less than 5%. The plane porosity of organic matter pores is positively correlated with clay minerals content and negatively correlated with brittle minerals content. The adsorption gas content of Lower Silurian and Lower Cambrian shale are 1.51–3.86 m3/t (average, 2.31 m3/t) and 0.35–2.38 m3/t (average, 1.36 m3/t). Total organic carbon, clay minerals and porosity are the main controlling factors for the differences in shale gas content between Lower Cambrian and Lower Silurian from the upper Yangtze plate. Probability entropy and organic matter plane porosity of the Lower Silurian are higher than those of Lower Cambrian shale, but form factor and roundness is smaller.

Characteristics of Organic Matter Pores and the Relationship with Current Pressure System of Lower Silurian Longmaxi Shales in Dingshan Field, Southern Sichuan, China

Organic matter pores (OMP) provide significant storage space for hydrocarbons in lower Silurian Longmaxi shales in the Dingshan field of southern Sichuan, China. The distributions of organic matter and the different OMP structure parameters were characterized through Ar-ion polishing, scanning electron microscopy (SEM), and image analysis software for shale samples of different wells. The research results indicated that organic matter has been divided into two categories based on its occurrence, location, and its relationship with authigenic minerals: organic matter in situ and migrated organic matter. OMP for organic matter in situ are mainly micropores mostly arranged isolatedly, while in migrated organic matter pores show larger sizes and higher roundness. The development of OMP in samples is predominantly controlled by the formation pressure. The existence of overpressure alleviated the stress on the rock skeleton, causing the compaction of some migrated organic matters to lag or decrease. This played a positive role in protecting the development of pores in the interior and edge of the rock skeleton, and it can also induce the development of microfractures in shale. The protective effect of formation pressure on organic pores was provided for understanding the exploration and exploitation of Longmaxi shales in the study area.

The smooth, spire-bearing brachiopods after the terminal Ordovician extinction through lower Llandovery in the central Oslo region, Norway

Strata of the Solvik Formation in the central Oslo region (upper Hirnantian to upper Aeronian) are rich in atrypides that elucidate the evolution of this group. A few athyridide brachiopods are also present. Eleven taxa of smooth spire-bearers are described taxonomically using fossils and peels. Among these occur one new genus, Eisaella, and five new species: Eisaella uniplicata, Thulatrypa huangi, T. vikenensis, ?Becscia pentagona, and Shelvothyris bivittata. Atrypides make up 30% (24 genera) of the total brachiopod genera in the Solvik Formation, more than one third of the known global atrypide fauna during that interval. Most are members of the family Lissatrypidae, illustrating the evolution of this group from upper Hirnantian and lower Silurian strata. These show close affinity to each other attesting to ongoing radiation. The new genus Eisaella is a likely ancestor of Lissatrypa, while Meifodia displays a gradual evolution through the formation. Atrypides were most diverse at the base and top of the Solvik Formation due to heterogony in environments and water depth, but were less diverse in the comparatively constant environmental setting of the late Rhuddanian time. The basal assemblages with few atrypides from shallow facies may be compared to the shallow Cathay Fauna of China. Most of the abundant Norwegian atrypide fauna is found in deeper water with few comparable faunas known globally. The Oslo region may have served as a center for the evolution and spread of atrypides immediately following the end-Ordovician glacial drawdown in sea level. UUID: http://zoobank.org/508dba95-5501-403d-9a4d-deeb324847c9