Reconciling some ideas on stratigraphic correlation to understanding the occurrence of gas hydrates in the Shenhu Area, northern continental slope of the South China Sea

The Shenhu Area on the northern South China Sea was the first place in China from which marine gas hydrate samples were obtained. Currently, two schemes of stratigraphic correlation based on biostratigraphy (SI) and sequence... [ view full abstract ]

The Shenhu Area on the northern South China Sea was the first place in China from which marine gas hydrate samples were obtained. Currently, two schemes of stratigraphic correlation based on biostratigraphy (SI) and sequence stratigraphy (SII) were co-existed in the hydrate drilling area. The major stratigraphic boundaries since Late Miocene from these two contrasting schemes were oversimplified co-applied in the Shenhu Area. In this study, these two schemes and related stratigraphic boundaries were reviewed from previous works and described in detail on the basis of obtained discontinuous core samples and regional covered seismic data (high-resolution 2D seismic data and pseudo-3D seismic data). The comparisons between these two schemes were conducted focused on thicknesses of strata and sedimentation rates with significant conflicts. The comprehensive comparable analyses implied SII might be more suitable than SI in the drilling area. Coincident with the regional sequence stratigraphic correlation, some truncation features and erosional characters from various channels since Late Miocene were used in the study area to identify and trace the stratigraphic boundaries T1 and T2 of SII. Moreover, two sedimentary units above BSRs were recognized, fine-grained turbidites with thin-bedded chaotic reflectors at the bottom and fine-grained sediment failures with thick continuous reflectors at the top. That is to say, though the lithological features and grain sizes were similar from the recovered discontinuous core samples, sediments recorded from the drilling sites located at the ridges of submarine canyons were constituted by two sedimentary units with different forming mechanisms. During this study, the dating results of calcareous nannofossil events from the discontinuous core samples were suggested not to be used directly to define the stratigraphic boundaries due to the insufficiently continuous geological information and the lack of regional comparisons. Especially, the identifications of fine-grained turbidites and fine-grained sediment failures might indicate the ages from discontinuous core samples were represented the results of allochthonous deposits. Reconciling these two schemes of stratigraphic correlation could help reestablishing sequence stratigraphic framework in the study area, constituting of three systems tracts, small-scale channels and fine-grained turbidites during LST, drape layers in TST, and shelf-edge delta and clinoform, sediment failures and submarine canyons in HST. Subsequently, a possibly model for the formation and occurrences of hydrates under the conditions of low upward methane flux and two different sedimentary units were proposed, which might give some new ideas to explain the heterogeneous distribution of hydrates in the Shenhu Area.