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Sweet spot evaluation is of great significance for efficient exploration and development of shale gas reservoirs. Taking the shale reservoir in Ordovician Wulalike Formation on the western margin of the Ordos Basin as the research object, eight experiments including rock section analysis, X-ray diffraction, SEM scanning electron microscopy, low-temperature nitrogen adsorption, isothermal adsorption, total organic carbon (TOC) content test, organic matter vitrinite reflectance (Ro) test and triaxial rock mechanics test were carried out in this study. It is found that the rock type in the target area is gray brown mud shale, with the pore size in the ranges of 2-4 nm and 35-61 nm, and intergranular pores, clay mineral interlayer pores and intragranular pores are developed. The TOC content is 1.01%, the average Ro value is 1.75%, and the brittleness index is 47.8%. By analyzing the influence of different factors on the selection and evaluation of shale reservoir sweet spots, it is concluded that the content of siliceous minerals, clay minerals, pore specific surface area, TOC content and Ro value play a decisive role on the adsorption perfor-mance of the reservoir, the pore size and the number of pore types control the reservoir property, the content of brittle minerals and rock mechanics parameters affect the compressibility of the reservoir. According to the two indexes of adsorption property and reservoir property evaluated by geological sweet spots and the compressibility index of engineering sweet spots, the parameter indexes corresponding to different characteristics are finely classified, and a classification and evaluation scheme of geological sweet spots and compressibility sweet spots of the marine shale gas reservoir in Ordovician Wulalike Formation in the Ordos Basin is preliminarily established. The results show that all characteristic para-meters in the target area meet the grade Ⅱ standard. It could be a sweet spot for shale gas development.

Permian Wujiaping Formation in Hongxing area of eastern Sichuan is rich in high-quality shale, and multiple wells have achieved high productivity of industrial gas. It is an important replacement block for the natural gas production capacity of the Jianghan Oilfield. However, due to the complex structure and lithology, deep burial depth of the target layer, and large fluctuations in the attitude of thin reservoirs, complex situations such as well leakage and collapse occuring frequently, low mechanical drilling speed, long drilling cycles, and difficulty in ensuring the drilling rate of high-quality reservoirs seriously restrict the benefit development. Based on the summary of previous data, the drilling difficulties were analyzed, the optimal design of drilling engineering was carried out, and some key rock breaking tools were selected to optimize the density and sealing performance of drilling fluid. The "three opening" wellbore structure with three different diameters, "irregular tooth PDC and mixed drill bits + high torque equal wall thickness screw + reinforced parameter drilling", simple pressure control by a rotary control head, and low density and strong sealing drilling fluid technology for "both leakage and collapse treatment" were formed. We have established a geological engineering integrated guidance technology based on "vertical depth prediction, segmented slope control, and five charts and one table", and proposed the direction for further improvement. The on-site application shows that: (1) the "three opening" wellbore structure with three different diameters meets the safety drilling requirements in Hongxing area, laying the foundation for drilling speed increase. The parameter drilling was enhanced by high-performance PDC, and the mechanical drilling speed of each opening is significantly improved. The drilling speed of the third opening section in the Permian formation increased by 15%, and the horizontal section increased by 22%; (2) The density reduction and nano sealing drilling fluid technology effectively reduced the number and time of complex treatments, and the time for complex treatments was reduced by 20.5% compared to the early implementation of wells; (3) The penetration rate of high-quality reservoirs significantly increased to 93.27%, an increase of 19.85% compared to the previous period.

Marine shale gas, represented by the shale gas from Wufeng-Longmaxi formations in southern China, is the main field for shale gas exploration and development in China. In recent years, marine shale gas exploration and development have been constantly facing new problems and challenges. Expanding new fields of shale gas resources and increasing shale gas production are still top priorities. The distribution characteristics, resource potential, and favorable area prediction of shale gas in typical marine formations in China are sorted out, with the results as follows. The geological and recoverable resources of shale gas in Wufeng-Longmaxi formations in Sichuan Basin and its peripheral regions are (17.50-33.19)×1012 m3 and (3.50-6.14)×1012 m3, respectively, with deep shale gas resources accounting for over 50%, mainly distributed in the high and steep structural belt in eastern Sichuan and in the low steep structural belt in southern Sichuan. The geological and recoverable resources of shale gas in the Permian Wujiaping Formation are (8.7-24.6)×1012 m3 and (1.3-3.7)×1012 m3, respectively, with exploration potential in areas such as Kaijiang-Liangping, Longjuba, Jiannan, and Sanxing in eastern Sichuan. The geological and recoverable resources of shale gas in the Qiongzhusi Formation are (5.69-12.71)×1012 m3 and (0.89-1.06) ×1012 m3, respectively, with favorable areas mainly distributed in areas such as Jingyan-Jianwei-Weiyuan-Ziyang in southwestern Sichuan, Nanjiang in northern Sichuan, and Yichang in the middle Yangtze region. The ancient marine formations such as the Doushantuo Formation, Hongshuizhuang Formation, and Xiamaling Formation have certain potential of shale gas resources, which are potential continuation of shale gas resources. The favorable areas for shale gas in the Doushantuo Formation are mainly distributed in western Hunan and Hubei and southeast Chongqing, while the favorable areas for shale gas in the Hongshuizhuang Formation are mainly distributed in the Chengde-Kuancheng area of Hebei. The favorable areas for shale gas in the Xiamaling Formation are mainly distributed in the areas of Zhuozhou in Hebei, Fangshan, Mentougou, Changping in Beijing, and Lanqi-yingzi. Based on this, three suggestions are further proposed for the future exploration direction of marine shale gas: the first is to focus on deep to ultra-deep and normal pressure shale gas, move towards new layers, and expand the field of shale gas exploration; the second is to deepen and enrich the exploration theory of deep to ultra-deep shale gas, normal pressure shale gas, and shale gas of new layer in the new area; the third is to focus on the improvement and innovation of deep to ultra-deep shale gas drilling and production technology and supporting equipment, and reduce the drilling and production costs of single wells of normal pressure shale gas.