Gas migration mode for the central canyon in deep-water Qiongdongnan Basin

Exploration results showed that the gas in the shallow Neogene central canyon reservoirs of the Qiongdongnan Basin mainly came from the deep Paleogene source rocks in the deep-water area. However, given that the late tectonic activity in the basin was weak, how did the gas in the deep formations migrate to the Neogene reservoirs? Using seismic frequency division technology, the resolution of 3D seismic data was improved. A fracture system below the central canyon was identified, and a gas transport mode was established by combining this interpretation with the development characteristics of regional faults. There were four stages of fault activity in the deep-water area of the basin, with pronounced periodicity. The activity and stationary periods happened alternately. Based on this, this paper proposed a concept of fracture cycle, which includes a shorter activity period and a longer stationary period. A fracture activity corresponds to a fracture cycle, and multiple fracture activities can be superimposed to form a stacking pattern of fracture cycles. Then, studying the difference of the migration mode for fractures in different periods, this paper proposed a fast inflow migration mode during the fault activity period and an effective seepage migration mode during the stationary period, and pointed out that it could enhance the efficiency of gas migration in a overpressure rift basin, if the inflow migration mode and seepage migration mode occurred alternately. In the Lingshui Depression, a series of ladder-like faults developed, connecting source rocks in the deep formations with many high-angle small faults in the shallow formations, which allowed deep natural gas migrate to shallow reservoirs. Thus, a relayed transport mode of “multi-stage fracturing and multiple faults” for gas migration in the central canyon was proposed.