Genesis Types and Sources of Mesozoic Lacustrine Shale Gas in the Southern Ordos Basin, NW China

Abstract

The continental lacustrine-fluvial facies are the main sedimentary environment of the late Triassic Yanchang Formation in the Ordos Basin of China. The 7 th Member and 9 th Member of the Yanchang Formation (Chang 7 and Chang 9 for short) are shale members that contain an abundance of rich organic matter, in which, natural gas has been found. Ascertaining the genetic types and sources of the shale gas is of considerable significance to the evaluation of the exploratory potential and to reduce the exploratory risk in the Yangchang Formation. The analysis of the geochemistry characteristics of the shale gas, shale desorbed gas and dissolved gas of crude oil in the Yanchang Formation, of the study area, has have been performed. The result shows that the three types of gases are dominated by methane and that the dryness is less than 0.92 and ranges from 0.64 to 0.8. The three types of gases in the Triassic Yanchang Formation have similar gas components but are completely different from the gas components in Palaeozoic coal-derived type gas. The δ 13 C 1 values of the shale gas, shale desorbed gas and crude oil dissolved gas vary between -41.6‰ and -51.6‰, and the average value is -47.8‰. The δ 13 C 2 varies between -40.1‰ and -33.6‰, and the δ 13 C 3 is between -38.6‰ and -30.8‰. The shale gas, shale desorbed gas and crude oil dissolved gas are all oil-type pyrolysis gases, which originate from the shale of Chang 7 and Chang 9. The results indicate that all of the gases have components of lighter methane carbon isotopes, which is obviously inconsistent with the low thermal maturity (0.7%≤Ro≤1.3%) of the shale in the study area. The experiments demonstrate that the isotope fractionation during the process of gas adsorption and desorption is the primary cause of the inconsistence.