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[Objective] The impacts of a metal mine on the surrounding grassland were studied, and the ecological environment was evaluated using remote sensing technology as a significant aid to ecological restoration and environmental protection of mining areas. [Methods] The study was conducted at a desert-steppe gold mine located in an ecologically fragile area. Spatial and temporal dynamics of normalized difference vegetation index (NDVI) in the mine area were analyzed based on long time series remote sensing data to conduct a comprehensive evaluation of the ecological quality of the study area using the remote sensing ecological index (RSEI). [Results] The impacted area of desert-steppe gold mining was determined as the area with a radius of 5 km outward from the mine boundary. There was a fluctuating increase in NDVI values in the mine and impact areas that varied with the duration of mining in the steppe from 2009 to 2021. The annual average value of NVDI within the mine area during the underground mining operation period was 51.9% higher than during the open pit mining operation period. In addition, the impact of open pit mining on vegetation in the study area was higher compared with underground mining. The RSEI values in the mining area and the impacted area initially increased, then decreased, and then stabilized, resulting in an overall non-significant increase. The spatial distribution of RSEI indicated that the areas with poor ecological environmental quality in the mine and impact areas were mainly the waste dump and surrounding area. [Conclusion] Gold mining influenced the surrounding desert grassland within a radius of 5 km from the mine. The ecological environment of the mine and the impacted area gradually improved through management efforts during mining. Ecological restoration and environmental management of the waste dump and the surrounding area should be strengthened during the process of mine ecological construction.

[Objective] The relationship between land use change and ecological process in the ten tributaries area in Inner Mongolia was studied to reveal the relationship between human activities, land use, and ecological environment in order to provide a basis for the optimization of land use structure and regional ecological environment protection. [Methods] Landsat images from 2000 to 2020 were selected. Land use data and transfer matrices in different periods were obtained by using the method of supervised classification combined with a land statistical module. The dynamic change of land use and the comprehensive index of land use degree were analyzed. On this basis, the NDVI value, ecological environment quality index, and other indicators were selected by using mathematical statistics and other methods to comprehensively evaluate the ecological environment quality of the ten tributaries. [Results] ① During the 20 years of the study period, the proportion of forest land and grassland area changed the most, and the proportion of water area was the smallest, with frequent transitions between land use types, An obvious "upward trend" for forest land and an obvious "downward trend" for grassland and unused land were observed; ② The ecological environment quality indexes of the transformation of land use types in 2000, 2010, and 2020 in the ten tributaries were 0.526 1, 0.537 3 and 0.543 3, showing an overall upward trend. In terms of spatial distribution, the ecological environment quality was high in the east and low in the west, and high in the south and low in the north; ③ The annual average precipitation and annual average wind speed had significant positive impacts on the ecological environment quality. The annual average temperature had a significant negative impact on the ecological environment quality. [Conclusion] With the rapid development of urbanization occurring from 2000 to 2020, the ecological environment quality of the ten tributaries has also developed in a good direction. The ecological environment quality in the study area has improved overall and deteriorated locally. The transformation between construction land, forest land, grassland, and unused land is the main reason for changes in ecological environment quality. Environmental protection policies such as returning farmland to forest have continuously improved the environmental quality of the study area.

Based on analyses of characteristics, hydrocarbon charging history and geological conditions for the formation of Sinian–Cambrian reservoirs in the north slope area of central Sichuan paleo-uplift, the natural gas origin, accumulation evolution, accumulation pattern and formation conditions of large lithologic gas reservoirs have been investigated. Through comprehensive analyses of natural gas composition, carbon and hydrogen isotopic compositions, fluid inclusions, reservoir bitumen, and geological conditions such as lithofacies paleogeography and beach body characterization, it is concluded that: (1) The natural gas in the Sinian–Cambrian of the north slope area is mainly oil cracking gas, and different contribution ratios of multiple sets of source rocks lead to different geochemical characteristics of natural gas in different reservoirs. (2) Although the both Sinian and Cambrian gas reservoirs in this area are lithologic gas reservoirs under monocline background, the former has normal-pressure and the latter has high-pressure. There are three types of source-reservoir-caprock combinations: single source with lower generation and upper reservoir, double sources with lower generation and upper reservoir or with side source and lateral reservoir, double sources with lower generation and upper reservoir or with upper generation and lower reservoir. The Permian–Triassic is the main generation period of oil, Early–Middle Jurassic is the main generation period of oil cracking gas and wet gas, and Late Jurassic–Cretaceous is the main generation period of dry gas. (3) The Sinian–Cambrian system of the north slope area has two favorable conditions for formation of large lithologic gas reservoirs, one is that the large scale beach facies reservoirs are located in the range of ancient oil reservoirs or near the source rocks, which is conducive to the “in-situ” large-scale accumulation of cracked gas in the paleo-oil reservoirs, the other is that the large scale mound-beach complex reservoirs and sealing layers of inter beach tight zones match effectively to form large lithologic traps under the slope background. The research results confirm that the north slope area has large multi-layer lithologic gas reservoirs with more than one trillion cubic meters of natural gas resources and great exploration potential.

In addition to traditional gypsum salt rocks and shale rocks, many studies have confirmed that carbonate rocks also work as oil and gas seals. Breakthrough pressure is a characteristic parameter for the evaluation of the sealing ability of cap rocks. Different lithology types and lengths of carbonate rocks were subjected to breakthrough pressure measurement experiments at different temperatures, results showed that lithology, permeability and fracture development have obvious effects to rock characteristics. At the same temperature, sample length does not have a good correlation with breakthrough pressure, however, at different temperatures, the breakthrough pressure of most samples gradually decreases with the increase of temperature. Therefore, formation temperature has a non-negligible effect on the sealing ability of cap rocks. It is a key factor that needs to be considered for the evaluation of the sealing capacity of cap rocks. The carbonate rock with a high argillaceous content has better sealing capacity under high geothermal conditions.

The Upper Paleozoic in the north part of Tianhuan depression in the Ordos Basin, NW China has lower hydrocarbon generation intensity and complex gas-water relationship, the main factors controlling the formation of tight sandstone gas and the distribution of tight sandstone gas in the low hydrocarbon generation intensity area are studied. Through two-dimensional physical simulation experiment of hydrocarbon accumulation, analysis of reservoir micro-pore-throat hydrocarbon system and dissection of typical gas reservoirs, the evaluation models of gas injection pressure, reservoir physical property, and gas generation threshold were established to determine the features of tight gas reservoirs in low hydrocarbon intensity area: (1) at the burial depth of less than 3 000 m, the hydrocarbon generation intensity of (7−10)×108 m3/km2 is high enough to maintain effective charging; (2) tight sandstone in large scale occurrence is conducive to accumulation of tight gas; (3) differences in reservoir physical property control the distribution of gas pool, for the channel sandstone reservoirs, ones with better physical properties generally concentrate in the middle of sandstone zone and local structural highs; ones with poor physical properties have low gas content generally. Based on the dissection of the gas reservoir in the north Tianhuan depression, the formation of tight gas reservoirs in low hydrocarbon generating intensity area are characterized by “long term continuous charging under hydrocarbon generation pressure, gas accumulation in large scale tight sandstone, pool control by difference in reservoir physical property, and local sweet spot”, and the tight gas pools are distributed in discontinuous “sheets” on the plane. This understanding has been proved by expanding exploration of tight sandstone gas in the north Tianhuan depression. Key words: Ordos Basin, Tianhuan depression, hydrocarbon generation intensity, tight sandstone gas, reservoir-forming mechanism, distribution pattern