No abstract

No abstract

No abstract

Carbonate deposits, which unconformably overlie the Palaeozoic bedrocks, extensively occur in the base of the Tertiary lake succession in the half-graben Shulu Sag, central Hebei Province, North China. This study focuses on the basal carbonate successions on the hinged western slope. Based on seismic, borehole and core data, nine facies are identified in the carbonate successions, and are further grouped into five facies associations: mid-proximal alluvial fan, distal alluvial fan, fan fringe, moderately deep lake and deep lake. The first two facies associations constitute alluvial fans formed by debrisflows at the edge of lake and are dominated by mounded-to lobate-shaped, matrix-to clastsupported carbonate rudstones with minor calcretes in the lowermost rudstone units and basinward increase in interfingering with lacustrine carbonate facies. The fan fringe, moderately deep lake and deep lake associations are dominated by pebbly carbonate arenites (or rare carbonate arenites), calcisiltitecalcilutites, and varve-like calcilutites, calcareous shales and oil shales, respectively. Widespread occurrences of fine-grained limestone packages containing varve-like organic-rich laminations, minor authigenic glauconite and pyrite, and planktonic and plant fossils suggest a meromictic, anoxic deep lake under a semi-humid to humid climate, probably with a connection to marine basins. Similarities in lithology and fossil assemblages (e.g. trilobites) of lithoclasts with those of the Mid-Upper Cambro-Ordovician bedrock carbonates suggest that the clastic and dissolved carbonate loads were sourced from this Lower Palaeozoic catchment, and shed off the surrounding highlands into the basin. These carbonate facies associations represent the lake lowstand and transgressive deposits of the basal third-order sequence (Ia) in which the highstand deposits are composed of lacustrine siliciclastics. During the lake lowstand stage (or initiation of basin-filling) under an intermediate climate, carbonate alluvial fans occurred mostly subaerially at the bottom of the hinged slope with a narrow, shallow lake zone basinwards, and locally were perched within the palaeovalley on the mid-upper slope. During the transgressive (deepening) stage under a semi-humid to humid climate, carbonate alluvial fans became smaller in size and episodically stepped backwards upon the slope, with greatly expanded and deepened lake. Nevertheless, the carbonate system was switched to an exclusively siliciclastic system during the highstand stage. The exhumation and erosion of the Mid-Lower Cambrian bedrock dominated by siliciclastics was probably the cause due to further uplift of the drainage basin. All these facts indicate

No abstract

Pyrite is a common mineral in sedimentary rocks and is widely distributed in a variety of different morphologies and sizes. Pyrite is also widely distributed in the Es3x shale of the Eocene Shahejie Formation in the Zhanhua Sag, Bohai Bay Basin. A combination of geochemical and petrographic studies has been applied to address the formation and distribution of pyrite in the Es3x shale. The methods include thin section analysis to identify the representative samples of the shale-containing pyrite, total organic carbon (TOC) content analysis, X-ray fluorescence, X-ray diffraction, electron probe micro-analysis, and field emission scanning electron microscopy (FE-SEM) coupled with the energy dispersive spectrometer, to observe the characteristics, morphology, and distribution of pyrite in the lacustrine shale. The content of pyrite in the Es3x shale ranges from 1.4 to 11.2% with an average content of 3.42%. The average contents of TOC and total organic sulfur (TS) are 3.48 and 2.53 wt %, respectively. Various types of pyrites are observed during the detailed FE-SEM investigations including pyrite framboids, euhedral pyrite, welded pyrite, pyrite microcrystals, and framework pyrite. Pyrite framboids are densely packed sphere-shaped masses of submicron-scale pyrite crystals with subordinate large-sized euhedral crystals of pyrite. Welded pyrite forms due to the overgrowth and alteration of pyrite crystals within the larger pyrite framboids. Pyrite microcrystals are the euhedral-shaped microcrystals of pyrite. The framework pyrite is also observed and is formed due to the pyritization of plant/algal tissues. Based on the growth mechanism, the pyrites can be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. The presence of pyrite, especially the abundance of pyrite framboids, suggests that the environment during the Es3x shale deposition in the lacustrine basin was anoxic. Their dominant smaller size suggests the presence of an euxinic water column, which is consistent with the lack of in-place biota and high TOC contents. This research work not only helps to understand the pyrite mineralization, role of organic matter, and reactive iron in pyrite formation in the shale but also helps to interpret the paleoredox conditions during the deposition of shale. This research work can also be helpful to other researchers who can apply these methods and conclusions to studying shale in other similar basins worldwide.

No abstract

The impacts of thermal maturity on the pore size, volume, and distribution in transitional shale are recognized. Combined with previous research results, Shanxi shale of the upper Paleozoic formation in the Ordos Basin, China, was selected to identify the evolution of shale pore size and types, and clarify the factors affecting the pore development. Samples of Shanxi shale from the northeastern Ordos Basin (NOB) were selected for experimental analysis, including total organic carbon (TOC) content, vitamin reflection (R o ), X-ray diffraction (XRD) analysis, argon ion polishing-scanning electron microscopy (AIP-SEM), energydispersive spectroscopy (EDS), and low-pressure carbon dioxide (CO 2 ) and nitrogen (N 2 ) adsorption analyses to illustrate the geochemistry of organic matter, mineral composition, pore morphology, size, and distribution. The data show that the Shanxi shale in the NOB is in the oil generation stage and has good conditions for gas generation. The interparticle (interP) and intraparticle (intraP) pores are related to mineral matrix rather than organic matter with a relatively high value of pore volume. In addition, combined with the characteristics of organic geochemistry and reservoir development of Shanxi shale in the southeastern Ordos Basin (SOB), the factors affecting shale reservoir development and the effect of thermal maturity on the evolution of the pore type and size can be explained. The presence of quartz inhibits the increase in pore volume in the low thermal maturity stage, but it is beneficial to pore preservation in the high thermal maturity stage, and different clay mineral components have different effects on pore development. Furthermore, the pore types of shale in the low maturity stage are mainly interP and intraP pores related to inorganic mineral, and organic pores are rarely developed. With the increase in thermal maturity, the number and volume of organic matter pores increase, but inorganic pores are still the main pore type. The effect of thermal maturity on the evolution of the pore type and size will enrich a basis of understanding of gas storage, transport mechanics, and fracturability that can be beneficial to the evaluation of shale reservoirs.