Geochemical analysis and origin of gas in volcanic reservoirs in the Songliao Basin

Abstract

Volcanic reservoirs are extensive in the Songliao Basin and mainly include intermediate-basic rocks in the northern part, intermediate-acidic rocks in Xujiaweizi in the southern part, and acidic rocks in the Jinglin block. The natural gas in the volcanic reservoirs of the Songliao Basin has a wide range of compositions, with alkanes being dominant in most cases, although carbon dioxide is dominant in some wells. Generally, the gas in the volcanic rocks near deep faults has high contents of carbon dioxide, whereas the natural gas in volcanic rocks far from faults has low carbon dioxide contents. The gas in the volcanic reservoirs is of multiple origins, including abiogenic gas of probable mantle origin (generally found in wells with high carbon dioxide contents) and organic gas mainly derived from organic matter in the basin. The abiogenic alkanes have δ 13 C values in the order of δ 13 C 1 > δ 13 C 2 > δ 13 C 3 > δ 13 C 4 , which is opposite that of alkanes of organic origin. The 3 He/ 4 He ratios of the fluid inclusions from the volcanic reservoirs range from 0.286 × 10 −6 to 7.33 × 10 −6 , with an average of 2.48 × 10 −6 , and the R/Ra ratios range from 0.26 to 5.24, with most values being greater than 1.0, indicating mixed origins of noble gases from the crust and the mantle. The gas in fluid inclusions from the volcanic reservoirs has δ 13 C 1 values ranging from −17.1 to −28.7‰ (PDB), δ 13 C 2 values ranging from −23.4 to −32.4‰ (mostly approximately −25‰), and δ 13 Cco 2 values ranging from −10.97 to −21.73‰, which are significantly different from the isotopic compositions of the gas in the present reservoirs, suggesting that some abiogenic alkanes may have been charged into the reservoirs during the geologic history of the basin. The early charged CO 2 is mainly organic in origin, while the abiogenic CO 2 was charged during the main accumulation period, producing a mix of origins for the gas in the volcanic reservoirs of the Songliao Basin. The abiogenic alkanes, He, and CO 2 in the natural gas indicate the addition of some abiogenic gas to the gas. According to the relationship between the distribution and attitude of volcanic rocks and faults, we found that the abiogenic gas reservoirs are located near fault zones, whereas the organic and mixed gas reservoirs are located far from fault zones. The geochemical study of natural gas is helpful in determining the origin and spatial distribution patterns of gas in deep volcanic reservoirs and for directing further gas exploration in the Songliao Basin.