Provenance Discrimination Of Upper Miocene To Pleistocene Reservoirs In The Yinggehai Basin: Constrains From Detrital Zircon U-Pb Geochronology And Geochemistry Of Sedimentary Rocks

The Yinggehai-Song Hong Basin, located in the passive continental margin of the northern South China Sea, is a Cenozoic petroliferous Basin. Tectonically, the basin situated in the intersection of the Pacific, India-Australia and Eurasian plates, and be affected by the interaction of these tectonic units since Cenozoic, and with a complex tectonic evolution history. Up to now, several shallow and deep oil and gas fields have been discovered in the basin, indicates a good exploration prospect. Generally, provenance analysis is an important content in the study of sedimentary basins, because the position, characters, range and distance of provenance can directly affect the generation, migration and preservation conditions of gas, which is also the foundation of the reservoir prediction and evaluation. Because there were multiple potential source areas with complex rock types, coupled with the research methods used here were traditional and ordinary, makes the provenance studies still based on the hypothesis and speculation, which largely limits the accuracy of provenance analysis. This study, based on the previous studies, using the comprehensive methods of detrital zircon U-Pb geochronology, Lu-Hf isotopes and whole-rock geochemical and Sr-Nd isotopic data to establish detrital zircon age spectrum and signature of different source areas, to identify the provenance of upper Miocene Huangliu Formation, Pliocene Yinggehai Formation and Pleistocene Ledong Formation, and to discuss the influence of each source area in spatially and temporally since Late Miocene. Finally, we use the sediments from the modern rivers to constrain the tectonic evolution of each source area. The main conclusions are as listed as following:1. The signatures of the three potential source areas have been established.The study of detrital zircon U-Pb ages and Lu-Hf isotopes from the modern rivers of the potential source areas shows that 1) Hainan has age peaks of 1280¹600 Ma, 447 Ma, 236²56 Ma, 159 Ma, 99 Ma, and scattered with Paleoproterozoic and Archean ages. The modern rivers in western Hainan mainly provide Yinggehai Basin with Indosinian and Yanshanian detrital material. The ages around 100 Ma are the signature of Hainan source. The εHf（t） values show negative and suggest that the sediments were produced by remelting of the old crust. 2) Red River source area has age peaks or interval of 2100²580 Ma, 1550¹980 Ma, 1550 Ma, 416 Ma, 241 Ma and 30 Ma. The modern rivers mainly provide Jinningian, Caledonian, Indosinian and Himalayan detritus to the basin. Cenozoic（ca. 30 Ma） and Neoproterozoic（ca. 752 Ma） ages are the signatures of Red River. These zircons show a wide range of εHf（t） values, suggests a complex source characteristics. 3) Central Vietnam produced zircon age peaks at 2452 Ma, 951 Ma, 428 Ma, 242 Ma and 24 Ma. The modern rivers mainly supply the basin of Caledonian and Indosinian clastic sediments. There are very little Yanshanian ages in these rivers, and the εHf（t） values of the Indosinian zircon are lower than the similar ages in Red River and Hainan.2. The spatial distribution of provenance in the Yinggehai Basin since late Miocece has been identified.（1） During the early stage of late Miocene, Haikou and Dongfang structures were given priority to Red River source, and Dongfang might be influenced by central Vietnam and Hainan. Lianhua structure has two sources from central Vietnam and Red River, and mainly from the former. The detrital material of Lingtou structure was mainly derived from Hainan with part of which from Red River. The position and provenance of the samples present some regularities, suggests that the ages of detrital zircon were clearly controlled by source areas. Most structures were influenced by the Red River in different degree, indicates that Red River has significant contribution to the basin at this time.（2） During the late stage of late Miocene, the provenance of Haikou structure was still derived from Red River, and there was a small amount of material derived from Hainan. The provenance of Dongfang is similar to Haikou, belongs to the mixed source area, the source was mainly from Red River, and small parts of sediments were derived from Hainan and central Vietnam. Comparing with the early stage of late Miocene, the provenance of Lianhua structure has no change, mainly derived from Truong Son Belt and Kontum Massif in central Vietnam. However, some changes occur in Lingtou structure, the L112 was mainly derived from Hainan while L262 was given priority to Red River source.（3） During the early stage of Pliocene, the source of Haikou structure was mainly derived from Hainan, a small amount of material was from Red River. The provenance of Dongfang structure was not changed compared to late Miocene, and mainly derived from Red River. Lianhua structure mainly from central Vietnam, and affected by the Red River in a certain extent. Lingtou structure was from Hainan, and under the influence of Red River at L112. Ledong structure is a mixed source located in southern Yinggehai Basin, and all of the potential areas contributed the clastic to the structure at this time.（4） During the late stage of Pliocene, the provenance of Dongfang structure was complex with Red River, Hainan and central Vietnam source areas. Lianhua structure was still a mixed source of Red River and central Vietnam. Lingtou structure mainly derived from Hainan. Ledong structure shows the source characteristics of both Hainan and Red River.（5） During the Pleistocene, all the structures in the basin have been influenced by Red River, suggests that Red River on the largest contribution to the basin. In addition to Red River source, a part of detrital material of Lianhua structure possible derived from central Vietnam. The source of Lingtou structure was mainly from Hainan. In Ledong structure, there might some detrital material joined from central Vietnam in addition to the Red River and Hainan sources.3. The temporal provenance distribution characteristics of the Yinggehai Basin since late Miocece have been recognized.The study shows that the provenance of Yinggehai Basin has no obvious changed since late Miocene, but some localized changes occur in different structures by time. From late Miocene to Pleistocene, the source of Dongfang structure has always been rich and had no significant changed, which was mainly derived from Red River, and the Hainan and central Vietnam are complementary. Lianhua structure has a relative stable source since late Miocene, and been influenced by both central Vietnam and Red River. The contributions of these two sources were changed in different sedimentary period. The sediment of Lingtou structure was mainly derived from Hainan and Red River in northern Vietnam. The age spectrums have changed to be more complex from late Miocene to Pliocene, while the provenance changed from Hainan to Red River and Hainan at the same time. Finally, the provenance was given priority to Red River during Pleistocene.4. The geochemical characteristics of different strata have been identified and used to trace the sourceThe contents of SiO₂ and Al₂O₃ is the highest oxide in Yinggehai Basin, which are 1.53% 78.55% and 3.26% 78.55%, respectively, followed by Fe₂O₃ T and CaO of 3.86% 17.61% and 0.77% 17.61%, respectively. The contents of K2 O, Mg O and Na2 O are between 0.62% and 4.03%, whereas Mn O, P₂O₅, Ti O2 are the lowest oxide, ranging from 0.05% to 0.91%. The sedimentary rock samples are mainly poor maturity of greywacke and middle maturity of litharenite and arkose. Comparing with PAAS, the sedimentary rocks enriched with Cr, Sr, Cs, Cu, Zn, Ba and Pb, and show depletion of Eu and Hf. The contents of rare earth elements show some changes, between 73.6×10^-6 and 268.7×10^-6, with a mean of 195.8×10^-6. Most of the samples show obvious negative Eu anomaly, with patterns of light rare earth concentration and heavy rare earth flat, showing a typical terrigenous characteristic. The Ce shows no anomaly, suggests that the degree of oxidation was not very high during sedimentary process. The CIA values of samples are ranging from 19.0 to 66.0, with an average of 46.2, indicating a middle intensity of weathering. Geochemical diagrams show that sediments of Yinggehai Basin mainly from the felsic rocks in upper crust. The detrital material affected by the enrichment of zircon in some extent. 87Sr/86 Sr values have a variation ranging from 0.711169 to 0.722365. The 143Nd/144 Nd values change little, ranging from 0.512014⁰.512096, with corresponding Nd（0） values between-12.17^-10.57. The values of 143Nd/144 Nd and Nd（0） obviously controlled by source, which have been changed relating to the position of samples. Overall, the Sr-Nd isotopes of Yinggehai Basin are similar to Red River in northern Vietnam, suggests that Red River was a continuous source area and the biggest contributor supply detritus to the basin since late Miocene.5. The possible time of the capture of Red River has been constrainedThe detrital zircon U-Pb ages of three drillings in Dongfang structure of central Yinggehai Basin show similar age spectrums, indicates that the region has a relative stable source supply since late Miocene, there was no obvious source change happened. Trace element ratios in different location show stable and no obvious abnormality occur and sustain. Although we can’t provide the precise time for river capture, but the relative stability of the geochemical and geochronological data suggest that the detrital material of Red River was no change since the late Miocene. If the capture event does exist, it may occur before the late Miocene（¹0.5 Ma）.6. The tectonic evolution of each source area has been revealed（1） An important Mesoproterozoic（¹400 Ma） tectono-magmatic event happened in Hainan, which may be related to an episodic rifting event of the Rodinia. The Caledonian movement in Hainan might be the important part of south China Caledonian orogeny, and the formation of the magma was derived from the remelting of Paleoproterozoic crust. The strong Hercynian-Indosinian tectono-magmatic activity charactered by widely distributed Permian island arc-type volcanic rock, collision-type granite and Triassic extension-type granite. Ancient crustal remelting was the main mechanism of the magmatism. These igneous rocks might be the east extension part of the Sanjiang magmatic belt, and its formation may be related to the close, subduction and collision of paleo Tethys Ocean. In middle Jurassic to early stage of late Cretaceous, Hainan developed A-type granitic and volcanic rocks, alkaline rock, bimodal volcanic rock, mafic dike and adakite, these magmatic activitives might be related to the west subduction of the Pacific plate.（2） Yangtze Block has an Archean-Paleoproterozoic basement, the relevant ages have exposed scatteredly in south China, and the magmatism mainly remelting from the Archean crust. Yangtze Block had a relatively strong metamorphism which corresponding to the convergence of the Columbia supercontinent at ca. 2.0 Ga. Paleoproterozoic experienced a intensive metamorphism, caused arc-continental collision and the cratonization of Yangtze. Global Grenville Orogeny（¹000 Ma） in the Yangtze characterized by strong tectonic activity, which made the Yangteze and Cathysia block together. Another tectonic event related to Rodinia supercontinent took place at ca. 800 Ma. During Neoproterozoic, large-scale tectonic events happened due to the break up of Rodinia. Caledonian magmatism is one of the most important magmatic activities in South China, corresponding to the strong orogeny and magmation at the late stage of early Paleozoic, and the crust-mantle migmatization may be the important mechanism of magmatic rocks. Affected by the collision between South China and Indochina, the strong Indosinian activity could be discovered in Red River Fault Zone and expressed by well-developed magmatic rocks. Because of the the closing of the paleo Tethys Ocean during this period, collision and convergence among these small plates in South China, and formed strong tectono-magmatic activity. Due to west subduction of Pacific, eastern South China formed the multiphase magmatism under the influence of multiple tectonisms during middle-late Jurassic. A series of Cenozoic granites and metamorphic rocks related to the collision between India and Eurasia have been discovered near the Red River Fault Zone since Cenozoic.（3） Archaean basement formed at ca. 2.5 Ga in Indochina, and the first magma lithification process occured in the Kontum Massif. The second large-scale magmatism in eastern Indochina might appear in Kontum Massif and west of Red River during Paleoproterozoic, Paleoproterozoic granite and migmatite have been found in these areas. The Kontum Massif becomes stable since the Neoproterozoic. The Indochina might located between South China and Sibumasu blocks during Late Ordovician to Silurian, and squeezed by these two blocks, formed the continent-arc collision between Truong Son Belt and Kontum Massif, which caused the development of large-scale Caledonian tectono-magmatic activity. The eastern Indochina strongly affected by the Indosinian movement which formed a series of magmatic rocks and metamorphic rocks. Indosinian tectonic and magma activities might relate to the closing of paleo Tethys Ocean on south margin of Yangtze and the subduction and collision between Indochina and South China blocks.