Identification and division of favorable intervals of the Chang 7 gas shale in the Yan’an area, Ordos Basin, China

As early as 2011, people have recognized the great potential of continental shale gas in the Ordos Basin and obtained commercial gas flow from some exploration wells. However, over the years, shale gas has not yet reached commercial production in this basin. The complexity of the geological conditions and the lack of understanding of the continental shale gas enrichment characteristics are the most important factors restricting shale gas from reaching commercial production capacity. Therefore, the identification and division of favorable intervals of continental shale is the key to achieving exploration breakthroughs. The Chang 7 shale of the Yanchang Formation in the Ordos Basin is the first continental shale to be targeted for shale gas exploitation in China. A large amount of drilling data and experimental analysis test data have been accumulated during exploration in recent years. Therefore, in this paper, the favorable development intervals of the Chang 7 gas shale have been divided and predicted through the analysis of multiple geological factors. The results show that two types of favorable intervals can be identified in the Chang 7 shale. One type is the thick organic-rich shale interval, in which thin sandy laminae are developed. Shale gas mainly exists in the shale as adsorbed gas and remains trapped in the interval due to the poor physical conditions. This interval can be recognized by the logging curve, characterized by a high Gr (Gr > 180 API), high resistance (Rd > 100 Ω·m), high time difference (AC > 280 μm/s), and low density (Den < 2.4 g/cm 3 ). The other type is where sandy lamina and organic-rich shale are interbedded in a 'sandwich' arrangement, where the shale gas exists mainly in sandy lamina as free gas. This interval can be recognized by the logging response characteristics of a medium to high Gr (180 API > Gr > 120 API), high time difference (300 μm/s > AC > 250 μm/s), high resistance (Rd > 100 Ω·m), and medium–low density (2.6 g/cm 3 > Den > 2.4 g/cm 3 ). These research results can effectively explain the enrichment mechanism of the Chang 7 shale gas and guide the exploration and development of the hot gas shale.