Origin of a giant fuzzy reflection zone and its implication for natural gas exploration in the southwestern Qiongdongnan Basin of the South China Sea

The southwestern depression of the Qiongdongnan Basin (QDNB) hosts thick Cenozoic sediments and awaits major hydrocarbon discovery. Multichannel seismic (MCS) profile CFT2011 across the southwestern QDNB reveals a similar to 60-km-wide fuzzy reflection zone (FRZ) within the sediments, but its origin and distribution remain unclear. Here ocean bottom seismometer (OBS) data of Line CFT2011 are processed with focus on the velocity structures by traveltime tomography inversion and analyzed together with the coincident and adjacent MCS profiles. The OBS velocity results show that the giant FRZ features lower velocity with difference up to 1.5 km/s and smaller vertical velocity gradient than the surrounding sedimentary sequences at the same depth, likely resulting from enhanced fluid infilling. The MCS profile exhibits that the giant FRZ is about 3-9-km thick and extends from the Paleogene strata rich in organic matters upward to the lower Pleistocene sediments. Within the shallow overlying sediments, multiple bright spots with reverse polarity are imaged and their reflection amplitudes increase with offset, consistent with the features of gas-charged sediments. They are probably shallow gas reservoirs with gases sourced from the deep FRZ. Therefore, the FRZ is proposed to be a giant gas-charged zone, which probably contains lots of hydrocarbon gases migrated vertically from the deep Paleogene source rocks through the boundary faults of the depressions and the minor fractures generated under overpressure. This FRZ is also imaged on the adjacent MCS profiles MCS-L1 and MCS-L2 with the width of about 40 km and 68 km, respectively. It is roughly estimated to cover an area of similar to 1 900 km(2) and host a volume of similar to 11 400 km(3) assuming an average thickness of 6 km, implying huge natural gas potential in the sedimentary depression of the southwestern QDNB of the South China Sea.