Deep-Water Oligocene In The South China Sea: Sedimentologic Characteristics And Hydrocarbon Potential

In the past decade, many oil companies have shifted their exploration from the continental shelf to deep-water area. Inspirited by the giant reservoirs discovered in the Cenozoic passive margin submarine fans, particularly in the Gulf of Mexico and Angola, oil companies focus on the exploration in the turbidity and related systems.The paper has dealt with the Oligocene nannofossil clay in the ODP Site 1148 by means of scanning electronic microscopy, X-ray diffraction, paleontological analysis, gas chromatograph-mass spectrometry, Rock-Eval pyrolysis and stable isotopic analysis, with a purpose to understand the sedimentological characteristics of the deep-water Oligocene that was discovered for the first time in China Seas, and to discuss its hydrocarbon potential.ODP Site 1148, located on the northern slope of the South China Sea (18°50.17'N, 116°33.94'E, water-depth 3294 m, and core length 859.45 m), reveals a late Cenozoic sequence from the earliest Oligocene (ca. 32.8 Ma) to the Holocene (Wang et al. 2000). The upper boundary of the Oligocene is at the depth of 460 m. The micropaleontological results indicate consistent deep-water condition at Site 1148 even since the earliest Oligocene. Based on the data of lithology, fossil assemblages, sedimentation and accumulation rates, bulk rock stable isotope and geochemistry, the Oligocene depositional history can be subdivided into＜32.8～32 Ma (859.5-710 m), 32～25.5 Ma (710-475 m) and 25.5～24.0 Ma (460-475m) three stages. Stage 1 represents the incipient spreading of the South China Sea Basin with rapid accumulation and distal fine-grained turbidites, stage 2 the relative stable spreading with lower accumulation and homogenous nannofossil clay, in its earliest and middle period (32-28.5 M), and the later stage 2 and stage 3 the accelerated spreading with lowest accumulation, convolute bedding and slumping.There exist two negative excursions in bulk carbon isotope at 475 m (～25.5 Ma) and 710 m (～32 Ma) respectively, that are coincident with the statigraphic boundaries and seismic reflection profiles. Based on the comparison of the Oligocene sequences between Site 1148 and the Pearl Mouth Basin, the age of the upper boundaries of Zhuhai Formation and Enping Formation are suggested as～25.5 Ma and～32 Ma B.P. respectively.The Oligocene oxygen isotope of at Site 1148 shifts lighter during the diagenesis. It is opposite to the diagenetic effect happened in the low latitude open sea where the oxygen isotope shifts heavier. Our results also show that the carbon isotope of carbonate test shifts heavier, that differs from the previous work in which the carbon isotope is considered to be lesser sensitive to the diagenetic effects.X-ray diffraction and scanning electron microscopy analyses were performed and the resultsshow that the opal-A of siliceous fossils has been transformed into opal-CT in the interval between 475 m and 600 m. With the depth increasing, the fragmentation of siliceous fossils increased, the X-ray peaks of authigenic quartz became more prominent and the X-ray peaks of opal-CT reduced in intensity, indicating the occurrence of the opal-CT in quartz transformation in this interval. The opal-CT intensity peak disappears in samples below 600 m, suggesting that the opal-CT had been completely transformed into quartz. The abrupt decrease in core recovery from nearly 100% above 470 m to 8% in the interval of 470 m～485 m at Site 1148 is likely to be related to the silica diagenesis, particularly to the transformation of opal-A to opal-CT. The opal-A/opal-CT boundary, however, has not been determined in this work mainly due to the poor core recovery.The Oligocene sediments at Site 1148 contain a relatively abundant organic matter, with a content more than 0.5% below depth 670 m and more than 0.59% below depth 710 m. Rock-Eval pyrolysis and observation under microscope reveal that the typesⅡandⅢof Kerogen is mainly from terrestrial higher plants, and partly from the aquatic. However, the pyrolysis Tmax values and GC/MS data indicate the gross thermal immaturity. Nevertheless, the 400 m-thick Oligocene and the 100 m-thick sediments with TOC＞0.5% are undoubted facts from that the deep-water Oligocene of the northern South China Sea is considered as a higher potential in the hydrocarbon generation.