Shiyou shiyan dizhi (Nov 2024)
Characteristics of Chang 7 shale gas reservoirs in Triassic Yanchang Formation of Ordos Basin and its exploration and development prospects
Abstract
As the second-largest sedimentary basin in China, the Ordos Basin has enormous potential for oil and gas exploration. The Chang 7 member of the Triassic Yanchang Formation in the basin is extensively distributed with organic-rich source rocks, covering an area of 40 000 to 50 000 km2. These source rocks are characterized by high organic content, with organic matter types mainly being type Ⅰ to Ⅱ1. The Ro values mostly range from 0.9% to 1.2%, indicating that they are in thermal maturity stage. The high content of retained hydrocarbons provides a strong material basis for the development of large-scale shale oil and gas reservoirs. Although the organic-rich mud shale layers in the Chang 73 sub-member have good gas-bearing properties, a systematic analysis of their gas-bearing characteristics is lacking. Using the southern part of the basin's western margin as a case study, multiple analytical methods such as rock geochemistry, organic geochemistry, and isotope analysis were used to identify the geological and geochemical characteristics of the source rock reservoirs in the Chang 7 member. The gas-bearing characteristics and shale gas occurrence states of the Chang 7 member were analyzed, and the shale gas resource potential was preliminarily calculated. The results indicated that the black shale of the Chang 73 sub-member in the study area exhibited good gas-bearing properties, with dissolved shale oil gas being the main component, along with minor amounts of adsorbed gas of kerogen clay minerals and free gas. The average volume of desorption gas was calculated to be 1.91 m3/t. The Chang 73 sub-member contained both shale oil and gas resources, with approximately equivalent quantities. It is recommended to consider both oil and gas development in future exploration and development research. In the central part of the lake basin, where thick black shale of the Chang 7 member occurred, gas content reached 2 m3/t. These reservoirs contained rigid minerals, micropores, and fractures, with high gas abundance and substantial resource potential, making this a favorable area for shale gas exploration. The total shale gas resource of the Chang 7 member was preliminarily estimated to be about 4.25×1012 m3, indicating promising exploration prospects. The favorable exploration areas were identified in the Jiyuan, Gucheng and Zhengning area.
Keywords
