Geochemical characteristics of Carboniferous-Permian hydrocarbon source rocks of Xiangtan 9 well in Ejin Banner, western Inner Mongolia

The Carboniferous-Permian lithologic association of Xiangtan 9 well in Ejin Banner is composed of volcanic rocks and clastic rocks, the concordia ages of volcanic zircon are from 302.2Ma±7.8Ma to 319.9Ma±4.5Ma, which shows that the stratigraphic age is late Carboniferous. The lithologic association is consistent with that of Ganquan Formation along Dahulishan section, so the formation of Xiangtan 9 well should be Ganquan Formation. The Carboniferous-Permian strata in the drill hole is 1027.4m (nonpenetrated) in thickness. In this huge thickness, there are 20 layers of dark mudstones (silty mudstone) with the thickness of each layer more than 4m, and the total thickness of these layers is over 288.5m, accounting for 28.1% of the total thickness. The analytical results of geochemical characteristics of some drill hole cores indicate that the organic matter abundance is middle-higher, the kerogen type is Ⅰ—Ⅱ1, and the organic matter has developed into the mature-high mature period. All these characteristics show that the hydrocarbon source rocks have good hydrocarbon-generating conditions. Compared with the analytical results of Ganquan Formation hydrocarbon source rocks in adjacent Dhulishan section, the organic matter abundance of Xiangtan 9 well is obviously higher than that of the surface samples, 1.8 times that of surface samples for TOC, 295.3 times that of surface samples for potential hydrocarbon generation amount(S1+S2), 32.8 times that of surface samples for chloroform asphalt "A" and 42 times that of surface samples for total hydrocarbon. The kerogen type evaluation shows that H/C-O/C of the core samples can better reflect kerogen type, but the kerogen type is not effectively evaluated by H/C-O/C of the surface samples due to the decrease of H resulting from the weathering effect. All of the above data show that weathering has exerted great influence on the abundance and type of hydrocarbon source rocks.