Abstract:

The Chang 7 Member in the ZhidanGanquan region of the Ordos Basin is a set of dark mudstones and shales deposited in a deep lacustrine setting. With high TOC, type Ⅱ1 organic matter and Ro mostly of 0.6%1.2%, the Chang 7 mudstones and shales are confirmed to be the principal source of the widely distributed oils in the Ordos Basin. In order to discuss the adsorption characteristics of Chang 7 organicrich shales, we analyzed the test results of isothermal adsorption, the content of organic carbon, XRD, thermal maturity and liquid nitrogen adsorption of 16 shale samples chosen from the study area. It turns out that the Langmuir volume of the Chang 7 shales varies between 0.56 m3/t and 4.43 m3/t. With the increase of pressure, the Langmuir volume rises correspondingly until the pressure reaches a certain level. The Langmuir volume has a positive correlation with the organic carbon content, clay mineral content, specific surface area and pore volume. In contrast to many marine shales, the Langmuir volume of the Chang 7 lacustrine shale has a negative correlation with quartz content, which is probably due to the fact that the source of the lakedeposited quartz is of terrestrial detrital origin. The relation of the Langmuir volume and thermal maturity (Ro) of the studied shales is negative when the Ro is less than 0.9% or so but becomes positive as the Ro is more than 0.9%. Based on the comprehensive evaluation, it is found that the TOC content and clay mineral content have the most significant effect on adsorption characteristics, while the specific surface area is a more direct factor.

Key words: Triassic, Yanchang Formation, continental shale, adsorption characteristic, controlling factors, Ordos Basin