Determining 3D seismic characteristics of the conduit system of the Changchang sag, Qiongdongnan Basin

Although some giant gas fields found in the deepwater area of the Qiongdongnan Basin (QDNB), China, are often associated with mud diapirs and/or gas chimneys, no comprehensive 3D work has been undertaken to characterize them. We have conducted a 3D seismic investigation using the root-mean square, coherence, and instantaneous frequency attributes to provide a better understanding of the conduit systems in the QDNB. The results show that the conduit system that we investigated can be separated vertically into four zones in the following order: (1) a structurally diapiric weak zone at the base, followed by (2) an injected or reinjected sandstones zone, (3) a gas chimney zone, and (4) a mud volcanic zone at the top. The morphology of the structurally weak zone is elliptical, formed by the intersection of northwest-southeast- and nearly east-west-trending tectonic faults. We infer that this zone provides pathways for the ascent of the diapiric mud that was probably sourced by the underlying overpressured mudstones. The injected or reinjected sandstones zone is characterized by high amplitude anomalies, and it was probably fed by the lobes of underlying submarine fans. The gas chimney zone, which is characterized by low frequencies and weak amplitudes, is probably composed of a mixture of uprising mud and free gas formed from the underlying overpressured mudstones, whereas the mud volcano with a Christmas-tree pattern and is composed of a central crater, the southern flank of which is a mudflow, formed when the uprising mud migrating upward through faults got to the paleo seafloor. Finally, we have developed schematic illustrations that would aid in understanding the different stages of the formation and internal architecture of this conduit system.