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The Sinian Doushantuo and the Cambrian Niutitang formations in southern China have a good shale gas resource potential. Although >20 wells have already been drilled in these formations, the shale gas production has been much lower than expected, due to the high thermal maturity of the rocks and the severely disturbed geological structure of the corresponding area. An industrial gas flow was discovered only in the Weiyuan region. To achieve the shale gas breakthrough of paleo-shale strata, it is necessary to find a shale reservoir with low thermal maturity and weak geological deformation. On the basis of the study on the Weiyuan, Huangling and other paleo-uplifts, a shale gas accumulation mechanism named "shale gas accumulating around the edges of paleo-uplifts" was proposed. It has been suggested that the shale gas is usually contained within the slope belt of a paleo-uplift margin characterized by a shallow burial depth, stable structure activity, and a low level of thermal maturity. Taking into account this accumulation mechanism, a large number of wells have been drilled on the southern edge of the Huangling paleo-uplift (western Hubei province). Organic-enriched shales with high shale gas content and low thermal maturity, have been found within the Sinian Doushantuo and the Cambrian Niutitang formations. To tackle the problems deriving from the low exploration level, complex geological structure, low temperature, and low pressure in the region, we proposed and applied innovative fracturing process, new complex fracturing techniques and new recovering technology systems. High and stable shale outputs were obtained from the Niutitang Formation (well Yangye-1HF, HF: Horizontal Well Fracturing) and from the Doushantuo Formation (well Yangye-2HF): 7.83 × 104 and 5.53 × 104 m3/day, respectively. These outputs were achieved following horizontal well section fracturing, which resulted in a significant breakthrough for the production capacity of the low-pressure shale gas reservoir in the complex tectonic region outside of the Sichuan Basin. • The shale gas accumulated around the edges of the paleo-uplifts. • The reservoir with complex fractures and low pressure can be reconstructed by new technology. • The Cambrian and Sinian organic-enriched shale has a good shale gas potential. [ABSTRACT FROM AUTHOR]

It is important to consider the influences of depositional environment on lithofacies characteristics, lithofacies distribution patterns, and organic matter enrichment when defining reservoir properties and predicting "favorable reservoirs" and "sweet spots". In this study, we examined the Niutitang shale of the early Cambrian Niutitang Formation in western Hubei Province, middle Yangtze Block, to understand how depositional environment controlled organic matter accumulation in the Niutitang shale. Through a combination of detailed sedimentologic investigations with geochemical analyses, we identified four primary lithofacies, siliceous shale, siliceous-clay mixed shale, calcareous shale, and argillaceous shale, from the Y1 drill core. The sedimentological analyses, including systematic observations of the color of fresh rocks, grain size, sedimentary textures and structures, biogenic and diagenetic (e.g., veins and pyrite) characteristics in cores and thin sections with mineralogical analyses by X-ray diffraction (XRD), and changes in geochemical proxies, including TOC (wt.%), Mo EF , U EF , Ni/Al and Zn/Al, suggest that siliceous shale and siliceous-clay mixed shale were deposited in euxinic and anoxic deep water with high surficial paleoproductivity which primarily controlled by long term global sea level changes, and that the calcareous shale and argillaceous shale were deposited in shallow and oxygen-rich water conditions with low energy and low paleoproductivity in the surface water. The Niutitang shale changed from the siliceous shale-dominated lithofacies association (LA1), to the argillaceous shale-dominated lithofacies association (LA2), and then back to the siliceous shale-dominated lithofacies association (LA1), suggesting a variation of sedimentary environment from euxinic–anoxic deep-water environment (highstand), to relatively oxic and shallow water environment (lowstand), and then back to euxinic–anoxic deep-water environment (highstand). This may reflect changes in sea level and mechanisms of organic matter enrichment during sea level highstand and lowstand. The low Mo/TOC values of the siliceous shale and siliceous-clay mixed shale lithofacies are similar to the Mo/TOC values of marine sediments in Framvaren Fjord, suggesting that the study area was located in a moderately restricted basin during deposition of the Niutitang shale. The Fe/Ti and Al/(Al + Fe) ratios and the Al–Fe–Mn diagram combined with our detailed petrographic studies and the good correlation between TOC and quartz contents are indicative of a mainly biogenic origin with a minor detrital source for the silica in the Niutitang shale. TOC is well correlated to Mo EF , U EF , Ni/Al and Zn/Al values in LA1, but it is only moderately correlated to Mo EF and U EF values and weakly correlated to Ni/Al and Zn/Al values in LA2. The correlation relationships suggest that good preservation in euxinic–anoxic water conditions and high primary paleoproductivity were the major controlling factors of organic matter enrichment, whereas organic matter accumulation during sea level lowstand was mainly controlled by water redox conditions because primary paleoproductivity remained persistently low in the more restricted basin with shallow water and limited nutrient upwelling. The correlations between TOC content and clay minerals within the different lithofacies associations suggest that adsorption of carbon components by clay minerals promoted the accumulation of organic matter to various extents in LA2 but probably did not impact organic matter enrichment in LA1. • Biogenic origin and minor detrital source for the silica in the Niutitang shale. • Mechanisms of organic matter enrichment are different in highstand and lowstand. • TOC in shallow restricted basin is primary controlled by water redox conditions. • A model was developed to explain the differential enrichment of organic matter. [ABSTRACT FROM AUTHOR]

Exploratory research was conducted in this paper on the correlation of chemical elements with sedimentary microfacies and shale gas sweet spots through cluster analysis, factor analysis, and so on, using a large amount of geochemical data of elements obtained from shale gas exploration of the Cambrian Niutitang Formation in Western Hubei. This paper screened the chemical element associations and parameter systems that can reflect sedimentary microfacies and sweet spot evaluations, proposed a classification method for the chemostratigraphic facies of shales, and finely depicted the sedimentary chemical microfacies of the shales in the Cambrian Niutitang Formation. A chemostratigraphic lithofacies paleogeography map of SQ1 (the first sequence strata) in the Cambrian Niutitang Formation in Western Hubei was compiled. Sweet spots were predicted and evaluated, and favourable areas were predicted, thereby providing important support for making a breakthrough in the exploration of the Cambrian shale gas in the Yichang area. Chemostratigraphy has a higher precision and a low characterization cost of reservoir sedimentary microfacies; especially in areas lacking drilling data and seismic data and having a low exploration degree, chemostratigraphy is an important means for identifying of shale sedimentary sequences and predicting shale sweet spot sections. • The paper has proposed a division scheme for chemostratigraphic facies of shales. • The optimized elements and their associations as geochemical characteristic index in Western Hubei were proposed • Two shale gas sweet spot facies belts were proposed after comprehensive analysis. [ABSTRACT FROM AUTHOR]