Target geochemical exploration for oil and gas on the south central Laoshan uplift of the South Yellow Sea Basin: seabed oil-gas seepage and double halo-type geochemical anomaly

Geochemical samples of 163 sites are collected from the south central Laoshan uplift of the South Yellow Sea Basin and analyzed for oil and gas geochemical indices including hydrocarbon gases and aromatic hydrocarbons. From the analysis results, geochemical anomalies are extracted, the relationship of seabed geochemical anomalies with the seepage of oil and gas revealed, and the model of geochemical anomalies established. The geochemical anomalies of adsorbed methane, adsorbed heavy hydrocarbon gas, headspace methane, headspace heavy hydrocarbon gas, total aromatic hydrocarbons and their derivatives (260 nm) and total polycyclic aromatic hydrocarbons (320 nm) are circularly distributed around the Gaoshi 3 structure, and the hydrocarbon gas anomalies are closer to the outside of the trap than the aromatic hydrocarbon anomalies. The horizontal and vertical distribution patterns of geochemical anomalies along major seismic profiles suggest that the formation of hydrocarbon geochemical anomalies may be related to the Gaoshi 3 structure and vertical leakage of deep reservoirs. Based on the observation, a double halo geochemical anomaly model around the Gaoshi 3 structure is established. The inner halo anomalies are mainly aromatic hydrocarbon index anomalies, which may be caused by the seepage of residual reservoirs in the upper part of the Gaoshi 3 structure, while the outer halo anomalies are mainly hydrocarbon gas anomalies, caused by the seepage of primary gas reservoirs in the deep part of the Gaoshi 3 structure. The relationship between surface geochemical anomalies and marginal faults of the Gaoshi 3 structure shows that both inner and outer halo anomalies are closely related to the margin faults of the Gaoshi 3 structure in space, indicating that these marginal faults may be the main migration path for deep oil and gas vertical leakage, at the same time, also meaning that the Gaoshi 3 structure is the first choice for drilling well to explore the Mesozoic-Paleozoic marine oil and gas on the Laoshan uplift..