| The Fushan Depression, located in the southeast of Beibuwan Basin, South China Sea, is a "dustpan" shape depression featured by zonary distribution in E-W direction and blocking distribution in N-S direction. In this study, using various equipment including high-speed centrifuge, thin section, XRD and SEM, we successfully separated authigenic clay minerals from sedimentary rocks (sandstones and mudstones) collected from drilling cores in Fushan Depression. Then trace elements analysis as well as Rb-Sr isochron and 40Ar-39Ar dating methods were conducted for these authigenic clay samples in order to reveal the tectonic geothermal history of the Fushan Depression. The volcanic rocks collected from Bailian Sag (in the east of the Fushan Depression) at the depth of 3450 m were examined trace elements concentrations, Sr-Nd-Pb-Hf isotopic compositions and zircon U-Pb ages. The correlation between geothermal event ages reconstructed by authigenic clays and the zircon ages of volcanic rocks was also discussed in order to reveal the impact effect of short-lived volcanic eruption on maturity of hydrocarbon source rocks in the Liushagang Formation.In addition, the distribution rule of hydrocarbon resources was revealed through statistics of 250 exploration boreholes within sequence stratigraphic framework. Combined with the identification of transfer zone in the central area, this study examined the relationship between the development of the transfer zone and distribution characteristics of hydrocarbon resources. The controlling effect of the transfer zone on sequence stacking pattern and facies distribution was also discussed in this study. The following conclusions can be drawn from the present study:1. The sedimentary rocks collected from 16 drilling cores in the Fushan Depression are feldspar lithic sandstone and mudstones. The authigenic clay minerals were successfully identified and separated from these sedimentary rocks using various instruments including high-speed centrifuge, thin section, XRD and SEM. The clay minerals are characterized by high content of pure illite. The illite existing in the pores of sandstones is characterized by hair-shape, silk-shape and plate-shape, indicating that its origin is by direct precipitation from K-bearing fluids in permeable environments. The plate-shaped illite existiing close to kaolinite shows typical structures of authigenic illite, suggesting that this type of illite is the product of illitization of kaolinite through K-uptake in less permeable environments. The SEM results are consistent with these findings, showing that the sedimentary rocks in Fushan Depression are enriched in kaolinite and illite.XRD analysis results show that the authigenic clay minerals are characterized by fine-grained size (most of them <0.5 μm) and high purity of illite. The sandstones in the western area contain a small amount of feldspar, making it difficult to separate pure illite. As a result, most of the clay samples in the western area are less than 0.2μm. There is no shift and change of air-dried XRD peak after glycolation, and such illlite usually precipitates at high temperature (> 160 degree).2. The chondrite-normalized rare earth element (REE) patterns of studied clay samples exhibit serious enrichment of light rare earth element (LREE) and slight enrichment or depletment in heavy rare earth element (HREE) with intense fractionation of REE. The finer grain size of clays is usually correlated with lower contents of REE concentrations. Upper-continental curst (UCC)-normalized REE patterns show that large grain size (>0.2μm) clay samples are characterized by obvious enrichment in REE, while fine grain size (<0.1μm) clay samples exhibit slight depletion in LREE, indicating that chemical composition of fluid are not uniform during the period of illite formation. This may be related to the result of fluid-rock interactions. The Eu anomaly is common in the eastern clay samples, but is rarely observed in the western area, indicating that the chemical composition of fluids in the western area is different from that in the eastern area. UCC-normalized trace element patterns exhibit that all the studied clay samples are depleted in HFSE (e.g. Zr, Hf, Nb, Ta). The clay samples in the eastern area are characterized by enrichment in Ba, Pb, Sn, Li, Cu and Co, while the clay samples in the western area are only enriched in Sn and Li.3. The high-precision 87Rb/86Sr ratios of the studied clay samples were identified by ICP-MS, and then 42 clay samples characterized by large range of 87Rb/86Sr ratios were selected for Sr isotopic analysis using TIMS. Five parallel Rb-Sr isochrons were obtained in the western area, providing the isochron ages ranging from 35 to 38 Ma, with the most representative age of 35.4±2.8Ma. The tectonic geothermal event recorded by the clay samples in the eastern area is recorded by five parallel Rb-Sr isochrones, which provide the ages between 30 to 31 Ma, with the most representative age of 31.4±1.6Ma. To test the reliability of the above isochron ages, the clay samples characterized by different grain sizes but from the same drilling core were used to calculate the corresponding Rb-Sr isochron ages, which match well with the above isochron data. This good match further confirms that that the isochron ages obtained in this study are reliable. In addition, the initial 87Sr/86Sr isotopic values range from 0.711 to 0.715 in the western clay samples, but vary between 0.715 and 0.719 in the eastern clay samples, indicating the existence of multiple stages of fluid events.Three clay samples were selected for 40Ar-39Ar dating at the University of Michigan. The 40Ar-39Ar dating results show that there is a serious loss of 40Ar resulting in unreliable 40Ar-39Ar ages. This should be closely related to the fine grain size of illite (<0.1μm). Such fine illite is easy to be disturbed by external circumstances, resulting in previously 40Ar lost in nature. In addition, it is worth noting that Hainan Uplift is characterized by high thermal flow, which may be the other significant factor responsible for 40Ar loss. Although this study does not provide reliable 40Ar-39Ar ages of illite samples, the following inspirations can be reached:(1) In contrast to 40Ar-39Ar dating method, Rb-Sr isochron dating method is more reliable for dating fine-grain size illite.40Ar-39Ar dating method commonly provides less precise age data due to natural 40Ar loss and/or loss of Ar during irradiation; (2) the fine-grain size illite from the regions with high thermal flow is not suitable for 40Ar-39Ar dating.4. A set of igneous rocks was revealed in the second member of Liushagang Formation (SQEls2) from the Bailian region, with a maximum depth of 200 m. Five volcanic samples were collected from two drilling cores, showing that they are mainly composed of pyroxene, plagioclase, feldspar and mica. The main lithology of these samples is diabase. The geochemical analysis shows that they are enriched in both REE and LILE contents, but depleted in HFSE contents, indicating they are the products of upwelling mantle magma of garnet peridotite. Sr-Nd-Pb-Hf isotopic analysis further confirms that theu are a mixture of two end-members. One end-member is DMM from lithospheric mantle or asthenosphere mantle, the other is EM2 probably from Hainan mantle plume activity. The zircon LA-LCP-MS U-Pb ages of basic diabase can be divided into two groups. One group age ranges from 238 Ma to 271 Ma, and the other group varies between 32 Ma and 37 Ma.5. Using Rb-Sr isochron dating method, this study provides the record of two stages of tectonic geothermal events in the Fushan Depression with the ages ranging from 37 Ma to 30 Ma. The first tectonic event occurring ～35.4 Ma matches well with apatite fission track (AFT) data and (U-Th-Sm)/He ages reported by Shi et al.,2011 and Leichao,2012. This age is also consistent with the uplift timing of Hainan Island from late Eocene through Oligocene, indicating that this age records the tectonic uplifting event during late Eocene. The other tectonic event (～ 31.4 Ma) in the eastern area matches well the youngest zircon U-Pb age of volcanic rocks. This magmatic intrusion may be related to Hainan seafloor spreading. Considering that no magmatic intrusion event related to Hainan spreading has been reported yet, this set of igneous rocks is of great significance for academic research.6. The distribution characteristics of hydrocarbon resources have been studied through statistics of 250 boreholes within sequence stratigraphic framework. The eastern area is enriched in gas with gas-oil ratio over 20000, indicating that the source rocks in this area are in high-over maturation. The statistic results show that hydrocarbon resources of Fushan Sag mainly distribute in SQEls3HST, SQE1s2LST and SQEls1HST. In the highstand system tract (HST) stratas, hydrocarbon resources are enriched in the central region, followed by the western region. In contrast, in the lowstand system tract (LST) stratas, hydrocarbon resources are enriched in the eastern region, followed by the western region.7. There is a close relationship between tectonic geothermal event and maturity of hydrocarbon source rocks. It appears that the magmatic intrusion event in the eastern region contributed significantly to the maturity of hydrocarbon source rocks. Compared with the other regions in Fushan Depression, the magmatic intrusion area is characterized by extremely high maturity of hydrocarbon source rocks, extremely high Ro anomaly, extremely low residual TOC content, and extremely low chloroform bitumen "A" content. Therefore, it is most likely that the temperature increase in this region associated with this magmatic intrusion event, causing the regional over-maturation of hydrocarbon source rocks and consequently, the formation of gas. Therefore, the traditional concept, which assumes that the maximum temperature in a basin is achieved at the time of maximum burial, is not appropriate as a sole mechanism in the reconstruction of thermal histories of many sedimentary basins. As such, when reconstructing the thermal history of a basin, it is important to consider not only sedimentation and stepwise burial, but also the geotectonic evolution of the region. Hyperthermal anomalies may occur before or after maximum burial, and can be a major cause of maximum paleotemperatures that lead to mineral reactions and organic maturation. More attention should be paied to the influence of short-term paleothermal events that are characteristic of rift systems and arc/back-arc areas8. The transfer zone in the central region has a controlling effect for not only facies distribution but also hydrocarbon migration. Although the formation mechanism of these transfer faults is not clear, the space relationship between depositional systems and transfer fault locations indicates the transfer zone together with NW trending transfer faults may have a close relationship with depositional facies distribution during the highstand stage. During the highstand stages, sediment continued to enter the strike ramp, developing successively large-scale fan-delta systems which are aligned parallel to the transfer faults and the ramp axis. This resulted in greater channel sediment deposits developing in the area adjacent to the NW trending transfer faults, suggesting an influence of the transfer zone on sedimentary routes. By contrast, during the lowstand stages, large-scale sublacustrine fan systems spilled over delta fronts and was rapidly flew down the flexure slope into the deep lake towards northeast, rather than parallel to ramp axis. This suggests that the transfer zone did not contribute significantly to the low-stand fan distribution, and that slope gradient (palaeogeomorphology) was the primary control in this instance.The identification of the transfer zone in the central zone give a reasonable answer to the fact that over 60% of hydrocarbons found in this depression have been in the transfer zone area, mainly from tilted fault blocks, horst blocks and sublacustrine fan reservoirs. The transfer zone area has many unique advantages, including high tectonic position, abundant source supply, high-quality source rocks, two hydrocarbon resources and the deep antithetic fault system. Under the effect of high tectonic position, the hydrocarbons from both Huangtong Sag and Bailian Sag migrated upwards to the transfer zone region.9. There is a good response relation among tectonic setting, sequence stacking pattern, depositional filling pattern and hydrocarbon distribution. Different tectonic settings usually develop different sequence stacking patterns that are correlated with unique hydrocarbon traps. Three types of structural slope break belts, which controlled the sequence architecture and sediment filling, are evident in the Late Eocene strata of the Fushan Depression:multi-level step-fault belt in the western area, flexure slope belt in the central area and gentle slope belt in the eastern area. The exploration of hydrocarbon traps in the Fushan Depression should be conducted at different levels. The central area is recommended as the key region for exploration. A variety of different kinds of traps have developed in the central area of the Fushan Depression, including structural traps (horst traps and tilted traps) on the top, up-dip pinch-out traps on the flanks and sublacustrine fan traps in the deep lake.The western area can be regarded as another significant exploration region. The structural traps should be the main exploration targets in this region. The sublacustrine fan distributed in the Huangtong Sag can be identified as a high-risk exploration area requiring further research. In the eastern area, the structural traps associated with the deep antithetic fault system should be the main exploration targets in SQEls3, while up-dip pinch-out traps are the main exploration targets in SQElsl.In the eastern area, "self-generation, self-reservoir and self-coverage" lithologic reservoir should be the most significant exploration targets due to excellent source-reservoir-seal assemblage. To achieve good exploration results, it is of great importance to determine the distribution range of sand bodies of sublacustrine fans. |
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