Geochemical characteristics and their significances of rare-earth elements in deep-water well core at the Lingnan Low Uplift Area of the Qiongdongnan Basin
“…The lithology of the Southern Uplift and Hainan Island could have been similar to explain the difficulty in discriminating between these two provenances based on zircon age patterns. Proxies such as heavy mineral assemblages and rare earth elements in borehole LS33 [ Sun et al ., ; Liu et al ., ] also fail to trace the proper sediment sources from the Southern Uplift.…”
Section: Discussionmentioning
confidence: 99%
“…It is thus reasonable to assume a significant sediment contribution from Hainan Island to the Qiongdongnan Basin during the Early Oligocene. Although the two Lower Oligocene samples LS33 and Y19‐2 have a comparable age pattern to that of Hainan Island (Figure ), a sediment supply from the nearby Southern Uplift appears to be more plausible based on seismic data [ Ren et al ., ; Liu et al ., ] (Figure b), as well as evidence of a short transport path by coarse‐grained sediments in LS33 [ Sun et al ., ] and lack of rounded zircons in Y19‐2 (Figure ). The lithology of the Southern Uplift and Hainan Island could have been similar to explain the difficulty in discriminating between these two provenances based on zircon age patterns.…”
The early rift sedimentation history of the South China Sea is still not well understood due to restricted borehole coverage of the Paleogene strata and lack of reliable stratigraphic dating. We use detrital zircon U‐Pb geochronology to explore the source‐to‐sink characteristics of syn‐rift sequences in the northern South China Sea. The results reveal significant intrabasinal provenances in addition to the well‐perceived terrigenous supply from the north. The Dongsha Uplift is considered to account for the dominance of the Early Cretaceous zircons in the Eocene samples. The Lower Oligocene sediments in the Qiongdongnan Basin could have been sourced from Hainan Island and local uplifts, but their distinction cannot be confirmed by the U‐Pb age spectra. Contemporary sediments in the northern Pearl River Mouth Basin were most likely transported from southeastern South China with well‐rounded zircon grains showing U‐Pb age similarity to those from the northeastern tributaries of the Pearl River. By contrast, intrabasinal sources from the west and east are suggested to have contributed the infill of the southern part of the Pearl River Mouth Basin based on generally euhedral zircon shapes. These sedimentary source patterns appear to change very little in the Oligocene northern South China Sea. However, the newly detected Neoproterozoic zircons in the Upper Oligocene sediments from borehole L21 tend to indicate a southern source. The episodic and diachronic nature of rifting and erosion processes in the early South China Sea is the cause of complex patterns in the Paleogene provenance history.
“…The lithology of the Southern Uplift and Hainan Island could have been similar to explain the difficulty in discriminating between these two provenances based on zircon age patterns. Proxies such as heavy mineral assemblages and rare earth elements in borehole LS33 [ Sun et al ., ; Liu et al ., ] also fail to trace the proper sediment sources from the Southern Uplift.…”
Section: Discussionmentioning
confidence: 99%
“…It is thus reasonable to assume a significant sediment contribution from Hainan Island to the Qiongdongnan Basin during the Early Oligocene. Although the two Lower Oligocene samples LS33 and Y19‐2 have a comparable age pattern to that of Hainan Island (Figure ), a sediment supply from the nearby Southern Uplift appears to be more plausible based on seismic data [ Ren et al ., ; Liu et al ., ] (Figure b), as well as evidence of a short transport path by coarse‐grained sediments in LS33 [ Sun et al ., ] and lack of rounded zircons in Y19‐2 (Figure ). The lithology of the Southern Uplift and Hainan Island could have been similar to explain the difficulty in discriminating between these two provenances based on zircon age patterns.…”
The early rift sedimentation history of the South China Sea is still not well understood due to restricted borehole coverage of the Paleogene strata and lack of reliable stratigraphic dating. We use detrital zircon U‐Pb geochronology to explore the source‐to‐sink characteristics of syn‐rift sequences in the northern South China Sea. The results reveal significant intrabasinal provenances in addition to the well‐perceived terrigenous supply from the north. The Dongsha Uplift is considered to account for the dominance of the Early Cretaceous zircons in the Eocene samples. The Lower Oligocene sediments in the Qiongdongnan Basin could have been sourced from Hainan Island and local uplifts, but their distinction cannot be confirmed by the U‐Pb age spectra. Contemporary sediments in the northern Pearl River Mouth Basin were most likely transported from southeastern South China with well‐rounded zircon grains showing U‐Pb age similarity to those from the northeastern tributaries of the Pearl River. By contrast, intrabasinal sources from the west and east are suggested to have contributed the infill of the southern part of the Pearl River Mouth Basin based on generally euhedral zircon shapes. These sedimentary source patterns appear to change very little in the Oligocene northern South China Sea. However, the newly detected Neoproterozoic zircons in the Upper Oligocene sediments from borehole L21 tend to indicate a southern source. The episodic and diachronic nature of rifting and erosion processes in the early South China Sea is the cause of complex patterns in the Paleogene provenance history.
“…Sea level fluctuated greatly during Miocene, and sedimentation was characterized by littoral and deltaic facies to neritic facies 7 , 8 , 30 . During the rapid subsidence sub-stage, the two basins experienced the similar subsidence rates, and the sedimentation was characterized by neritic-bathyal facies 8 , 29 , 33 , 34 . Figure 2 Cenozoic stratigraphic table of the Yinggehai and Qiongdongnan basins with lithostratigraphy, relative sea level curve, sedimentation rate, and regional tectonic events 8 , 14 , 26 .…”
Together, the Yinggehai and Qiongdongnan basins have received a large amount of terrigenous sediments, but the provenance evolution of Cenozoic sediments in the two basins remains disputable. Combined with previous studies in the Yinggehai and Qiongdongnan basins, the elemental geochemistry of Oligocene to Pliocene sediment samples in the junction area of the two basins were analyzed to explore the tectonic implications, parent rock characteristics, and provenance evolution of the two basins during the Cenozoic. The results reveal that all the sediment samples were derived from continental island arc to passive continental margin settings. The light REE enrichment and stable content of heavy REE with large negative Eu anomalies indicate that they were probably derived from Hainan Island. The reconstructed provenance evolution model showed that the Red River Source (RRS) provided sedimentary materials for the Central Depression of Yinggehai Basin from the Oligocene to the Pliocene, and Hainan Island Source (HIS) was also one of the sources for sediments deposited in the Central Depression of Yinggehai Basin during the Miocene. However, most of the sediments preserved in the Yingdong Slope and Qiongdongnan Basin were derived from the HIS from the Oligocene to the Pliocene, and sediments deposited in the Yingdong Slope were also derived from the RRS during the Miocene. Furthermore, the junction area of the two basins had a mixed source of the RRS and HIS during the Cenozoic.
“…Exploration of the Qiongdongnan Basin (QDNB) over the past two decades has focused on the Miocene central canyon oil and gas belt and Oligocene reef limestone and sandy conglomerates [27][28][29][30][31][32], while little attention has been paid to buried-hill reservoirs [21,33,34]. The amount of research on granite reservoirs in the QDNB is much less compared to those mentioned above [35,36]. Recent drillings of the Songnan Low Uplift (SNLU) show that the thickness of the Mesozoic granite reservoir is more than 100 m and the natural gas reserves amount to more than 1 × 10 8 m 3 , indicating huge exploration potential [37].…”
Recently, a large-scale gas reservoir was discovered in granitic buried hills of the Songnan Low Uplift in the Qiongdongnan Basin. However, the strong heterogeneity of granite reservoirs limits further exploration and evaluation. Based on observations of sixty core samples and sixty thin sections, mineral composition, zircon dating, apatite fission tracks, physical properties, image logs, outcrop surveys and seismic interpretations, the characteristics of granite weathering crust of the Songnan Low Uplift are analyzed, and its controlling factors and evolution process are evaluated. The results show that weathered granite in the study area can be divided into several zones, from top to bottom: eluvium–slope zone, sandy zone, weathered fracture zone and horizontal undercurrent vuggy zone. The reservoirs in the eluvium–slope zone are dominated by microfissures and intergranular dissolution pores and have an average porosity of 4.68% and permeability of 2.34 md; the reservoirs in the sandy zone are composed of intergranular and intragranular dissolution pores and have an average porosity of 11.46% and permeability of 4.99 md; the reservoirs in the weathered fracture zone consist of various fractures and have an average porosity of 3.91% and permeability of 2.5 md; the reservoirs in the horizontal undercurrent vuggy zone are subhorizontal fractures and vugs and have an average porosity of 2.7% and permeability of 0.23 md. The development of granite reservoirs is jointly influenced by petrology and minerals, long-term exposure in a warm humid paleoclimate, faults, diverse topographies and shallow buried depth. Based on the above, our study establishes a development model of weathering crust and suggests that only the gentle slope and platform remain strongly weathered zones. After undergoing a complex evolution process of formation–destruction/denudation–regeneration–preservation, the current weathering crust of the Songnan Low Uplift is finally established. The results of this study have important theoretical and application value for the hydrocarbon exploration of buried hills in the Qiongdongnan Basin and provide a reference example for other granite reservoirs worldwide.
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