Geological survey and rock-physics seismic interpretation of an ultra-shallow gas reservoir in the deep-water Qiongdongnan Basin, China

Ultra-shallow gas and hydrate-bearing systems in deep-water Quaternary strata are important exploration targets, but their geological controls and seismic responses remain difficult to quantify because the reservoirs are weakly consolidated, laterally heterogeneous, and commonly occur near the base of the gas-hydrate stability zone. This study presents an integrated geological and rock-physics seismic interpretation of the Lingshui 36-1 (LS36-1) ultra-shallow gas reservoir in the Qiongdongnan Basin. Regional geology, seismic interpretation, gas-source geochemistry, drilled sediment samples, and rock-physics modeling were combined into a case-specific interpretation workflow. The results indicate that shallow gas enrichment is controlled by the coupling of a Quaternary submarine-fan sand body, a fault-gas chimney-microfracture migration system, and three sealing elements: deep-water mud, mass-transport deposits, and hydrate-bearing intervals. The gas is methane-dominated dry gas related to mature to over-mature coal-measure source rocks of the Paleogene Yacheng Formation. Samples showed that the reservoirs are very weakly consolidated and highly porous; Ledong submarine-fan sandstone had porosities of 38.4%–42.5% and permeabilities of 48.4–598.4 mD, whereas a broader shallow-reservoir sample set showed porosities of 28.4%–43.6%. Laboratory velocity measurements were interpreted as trend-calibration data rather than pressure-equivalent reservoir velocities. In free-gas-bearing samples, replacing gas with water increased P- and S-wave velocities; in hydrate-bearing samples, decreasing water saturation increased hydrate saturation and therefore increased stiffness. A Hertz–Mindlin–Hashin–Shtrikman–Gassmann workflow, constrained by well logs and calibrated mineral/fluid parameters, predicted high P-wave impedance at the top of hydrate-bearing intervals and low P-wave impedance at the top of free-gas intervals. We therefore used P-wave impedance to identify sand-prone reservoirs, while the P-wave velocity ratio ranked high gas-saturation intervals. The inversion is consistent with LS36-1 well data but lacks independent blind-well validation because no additional verification well is available. The workflow provides a case-specific, uncertainty-aware approach for evaluating deep-water ultra-shallow gas and hydrate-associated reservoirs.