Geochemical characteristics and genesis of pre-salt gases in the Ordos Basin, China

The geochemical characteristics, genesis and accumulation mechanism of the pre-salt gases of Ordos Basin have been rarely studied in China. Components and carbon isotope values of pre-salt gases are investigated and compared with those of the Upper Paleozoic and post-salt gases. Homogenization temperatures and carbon isotope values of gases of the pre-salt fluid inclusions were also measured. Based on the geochemical parameters, combined with the geological conditions, the genesis and accumulation mechanism of pre-salt gases are studied. The components, carbon isotope values and the first two homogenization temperature ranges (90-120 degrees C and 120-160 degrees C) of the pre-salt gases contain those of the Upper Paleozoic and post-salt gases. The pre-salt inclusions have an extra homogenization temperature range (160-200 degrees C) and the pre-salt rocks also have some hydrocarbon generation capacity, which means the pre-salt rocks may also generate some gases. The pre-salt gases are mixed gases generated from the Upper Paleozoic coal strata, the post-salt and pre-salt Lower Paleozoic Majiagou Formation, which is confirmed by the geochemical characteristics and the geological settings.