Sedimentary and petrophysical characteristics of various turbidites at IODP Sites U1499 and U1500 in the northern South China Sea

The deep-sea turbidite is an important target for petroleum and gas hydrate exploration, and is of significance to paleoceanographic, paleoenvironmental, paleotectonic and paleoclimatic researches. Turbidites have been investigated mostly in a sedimentological and less in a petrophysical perspective. In this paper, cores and thin sections (and /or smear slides) are used to study the sedimentary characteristics and petrophysical responses of the turbidites at the IODP Sites of U1499 and U1500 in the northern part of the South China Sea (SCS). Three types of turbidites are identified, i.e. the calcareous, terrigenous, and terrigenous-calcareous mixed turbidites. The later two types dominate. Each type of turbidites has its own petrophysical characteristics. The calcareous turbidites are characterized by low magnetic susceptibility, high brightness in color reflectance spectrometry, variable density, and low natural gamma radiation; the terrigenous turbidites characterized by low magnetic susceptibility, low to medium brightness, medium to high density, and medium to low natural gamma radiation; and the mixed turbidites characterized by variable magnetic susceptibility, brightness and natural gamma radiation and medium to high density. The turbidites mainly occur in the late Miocene and the middle-late Pleistocene sequences, followed by the early Pleistocene, the Pliocene, and the early-middle Miocene. The euatatic lowstand periods in the late Miocene and middle-late Pleistocene could be favorable for the deposition of the turbidites in the SCS. The gradual decrease in the abundance of calcarous turbidites since late Miocene could be associated with the gradual deepening of the oceanic basin and the shrinking of carbonate depositional area in the SCS.