The Geochemistry Characteristics Near the Earth Surface in Deep Basin Gas Reservoirs of Northern Ordos Basin

The upper Palaeozoic reservoirs in the northern Ordos Basin are deep basin gas reservoirs. It can be divided into gas reservoirs and water areas from the south to north. Gas areas distribute in the south near source and hydrocarbon accumulation are controlled by the distribution and physical properties of the sandstones, pressure in the gas reservoirs indicates negative abnormity without marginal water and bottom water. The upward and microscopic diffusion of hydrocarbon is weak, near-surface geochemistry background field is low, the water area is distributed in the north, which is far from the source rocks, and its hydrocarbon accumulation is controlled by the structure, pressure in the gas reservoirs is normal with marginal water and bottom water. Hydrocarbon is strongly diffused upward, where near-surface geochemistry background field is relatively weaker. Based on the characters stated above, the geochemistry characteristics and abnormal patterns between gas and water areas are established for gas-water interface of deep basin reservoirs. The abnormal pattern is that the steep background area is the gas-water interface of the deep basin gas reservoirs, and the gas reservoirs in the water areas are mainly the circular and semi-circular abnormal patterns.