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The lower limits of petrophysical properties for an effective reservoir are among the key parameters for assessing hydrocarbon reserves and are therefore directly related to hydrocarbon exploration and development strategies. However, the lower limits for marine sandstone gas reservoirs are still not clear and the impact factors also remain to be discussed. This study analysed the lower petrophysical property limits of an effective sandstone reservoir in the Qiongdongnan Basin using porosity, permeability and gas testing. The results showed that the lower porosity and permeability limits of effective reservoirs developed in the deltas are 8.9% and 1.2 × 10−3 μm2, respectively, and 11.3% and 4.0 × 10−3 μm2 in the submarine canyons and fans, respectively. Sedimentary facies, sediment transport distance, grain size and burial depth of sandstone significantly influence the lower physical property limits. The lower porosity and permeability limits increase with the increase in sediment transport distance as well as the decrease in sandstone grain size and burial depth. Sediment sources and sedimentary facies determine whether sandstone can become an effective reservoir in the Qiongdongnan Basin. Specifically, the sediment source dramatically influences the petrophysical properties of sandstone. The sandstone sourced from the Red River has higher porosity and permeability, followed by the sandstone sourced from the Hainan Uplift, and the sandstone sourced from the palaeo-uplift within the basin has the lowest porosity and permeability. The feldspar dissolution by CO2 and organic acid is the primary formation mechanism of the effective reservoir in the Lingshui Formation, whereas the dissolution of glauconite is more common in the sandstone reservoirs of the Sanya and Meishan formations. [ABSTRACT FROM AUTHOR]

Qiongdongnan Basin has a tectonic geological background of high temperature and high pressure in a deep reservoir setting, with mantle‐derived CO2. A water‐rock reaction device was used under high temperature and high pressure conditions, in conjunction with scanning electron microscope (SEM) observations, to carry out an experimental study of the diagenetic reaction between sandstone at depth and CO2‐rich fluid, which is of great significance for revealing the dissolution of deep clastic rock reservoirs and the developmental mechanism of secondary pores, promoting deep oil and gas exploration. In this study, the experimental scheme of the water‐rock reaction system was designed according to the parameters of the diagenetic background of the deep sandstone reservoir in the Qiongdongnan Basin. Three groups of single mineral samples were prepared in this experiment, including K‐feldspar samples, albite samples and calcite samples. Using CO2 as a reaction solution, a series of diagenetic reaction simulation experiments were carried out in a semi‐closed high temperature and high pressure simulation system. A field emission scanning electron microscope (SEM) was used to observe the microscopic appearance of the mineral samples after the water‐rock reaction, the characteristics of dissolution under high temperature and high pressure, as well as the development of secondary pores. The experimental results showed that the CO2‐rich fluid has an obvious dissolution effect on K‐feldspar, albite and calcite under high temperature and high pressure. For the three minerals, the main temperature and pressure window for dissolution ranged from 150°C to 300°C and 45 MPa to 60 MPa. Scanning electron microscope observations revealed that the dissolution effect of K‐feldspar is most obvious under conditions of 150°C and 45 MPa, in contrast to conditions of 200°C and 50 MPa for albite and calcite. Through the comparative analysis of experimental conditions and procedures, a coupling effect occurred between the temperature and pressure change and the dissolution strength of K‐feldspar, albite and calcite. Under high temperature and high pressure, pressure changed the solubility of CO2, furthermore, the dissolution effect and strength of the sandstone components were also affected. The experiment revealed that high temperature and high pressure conditions with CO2‐rich fluid has a significant dissolution effect on aluminosilicate minerals and is conducive to the formation of secondary pores and effective reservoirs. Going forward with the above understanding has important implications for the promotion of deep oil and gas exploration. [ABSTRACT FROM AUTHOR]

The natural gas generation process is simulated by heating source rocks of the Yacheng Formation, including the onshore-offshore mudstone and coal with kerogens of Type II2-III in the Qiongdongnan Basin. The aim is to quantify the natural gas generation from the Yacheng Formation and to evaluate the geological prediction and kinetic parameters using an optimization procedure based on the basin modeling of the shallow-water area. For this, the hydrocarbons produced have been grouped into four classes (C C C and C). The results show that the onset temperature of methane generation is predicted to occur at 110°C during the thermal history of sediments since 5.3 Ma by using data extrapolation. The hydrocarbon potential for ethane, propane and heavy gaseous hydrocarbons (C) is found to be almost exhausted at geological temperature of 200°C when the transformation ratio (TR) is over 0.8, but for which methane is determined to be about 0.5 in the shallow-water area. In contrast, the end temperature of the methane generation in the deep-water area was over 300°C with a TR over 0.8. It plays an important role in the natural gas exploration of the deep-water basin and other basins in the broad ocean areas of China. Therefore, the natural gas exploration for the deep-water area in the Qiongdongnan Basin shall first aim at the structural traps in the Ledong, Lingshui and Beijiao sags, and in the forward direction of the structure around the sags, and then gradually develop toward the non-structural trap in the deep-water area basin of the broad ocean areas of China. [ABSTRACT FROM AUTHOR]

The Qiongdongnan Basin (QDNB) is a petroleum-bearing basin in the northern margin of the South China Sea (SCS). Provenance analysis is an important tool in petroleum exploration and is helpful in predicting the distribution of good reservoirs in areas with less drilling. However, the provenance of the QDNB remains controversial, and further research is needed. Here, we apply whole-rock geochemistry, petrography, and heavy minerals combined with published detrital zircon U-Pb ages to constrain the provenance of Oligocene to middle Miocene sandstones in the QDNB and the tectonic environment of their deposition. The Oligocene to middle Miocene sandstones from the QDNB are geochemically immature. These sandstones were derived from a weakly weathered source area characterized by dominantly felsic igneous rocks, with a particular slight addition from recycled sedimentary sources. And the Lingshui Formation underwent less weathering than the upper and lower strata. The tectonic setting of the sandstones in the basin belongs to a continental island arc (CIA) or an active continental margin (ACM). Heavy mineral assemblages and detrital zircon U-Pb ages reveal sediment input from multiple sources. Hainan Island was a major and continuous source area contributing to the basin from the early Oligocene to the middle Miocene. In addition, intrabasinal uplift regions could have supplied material to local areas. While these sedimentary source patterns appear to have changed very little in the late Oligocene, a provenance from central Vietnam was explicitly discerned in the western QDNB in the late Oligocene. It is possible that the paleo-Pearl River supplied large volumes of sediment to the eastern QDNB from the late Oligocene to the early Miocene. • The sandstones from the QDNB are generally considered chemically and texturally immature to submature and they experienced a low degree of weathering in their source areas. The Lingshui Formation also underwent less weathering than the upper and lower strata. • These sediments were largely from felsic igneous source rocks, with a particular slight addition from recycled sedimentary sources, which originated in a continental island arc (CIA) and active continental margin (ACM) setting. • Hainan Island is likely to have been the dominant and stable source of the detritus from the early Oligocene to the middle Miocene, a provenance from central Vietnam is explicitly discerned in the western QDNB in the late Oligocene. It is possible that a dominant sedimentary contribution from the paleo-Pearl River to the eastern QDNB occurred during the late Oligocene–early Miocene. [ABSTRACT FROM AUTHOR]

Analyses of high-resolution seismic reflection, drilling, and well-log data were combined to identify and investigate deepwater fans developed in the eastern part of the Qiongdongnan Basin, South China Sea, during the Middle Miocene and their relationship to slope morphology. We identified three types of deepwater fans that developed on the northern continental margin of the South China Sea during the Middle Miocene: (1) those consisting of small canyons and depositional lobes; (2) those consisting of large canyons and depositional lobes; and (3) those consisting of distributary channels without depositional lobes. Sourced from deltaic sediments deposited on the northern continental shelf margin, the lithologies of the sediments in all fans are dominated by siltstone. The scale of development and the internal features are mainly controlled by the slope topography. The deepwater fans consisting of small canyons and depositional lobes developed on slopes with low gradient, caused by the continuous progradation of the shelf-edge towards the basin centre due to sediment transport in high-speed flows. The canyon heads developed in this setting are located below the shelf-break; these canyons are narrower, shallower, and shorter than those of the deepwater fans consisting of large canyons. Thus, the scale of the depositional lobes deposited by gravity flow through such canyons is smaller than that of deepwater fans with large depositional lobes. Deepwater fans consisting of large canyons and depositional lobes developed on steep narrow slopes, caused by rising shelf-edge trajectories due to the equilibrium between the sediment supply and subsidence rates. The canyon heads developed in this setting are located upon the shelf-break and extend toward the toe of the slope, incising the shelf-break; these canyons are wider, deeper, and longer than those of the deepwater fans consisting of small canyons. The scale of the depositional lobes deposited by gravity flows transported through such canyons is correspondingly larger than that of deepwater fans with small depositional lobes. Deepwater fans consisting of distributary channels without depositional lobes developed on gentler and wider slopes than slopes where small canyons and depositional lobes formed, owing to the low sediment supply. The deepwater distributary channels have a long transportation distance and wider distribution area, enabling the formation of large-scale deepwater fans. Depositional models for all three types of deepwater fans were postulated. • Three types of Middle Miocene deepwater fans in the Qiongdongnan Basin were identified. • The slope topography determines the internal features of these deepwater fans. • Sea-level fluctuations have a significant impact on the development of these deepwater fans. [ABSTRACT FROM AUTHOR]

Abstract The Ledong Submarine Fan in the western Qiongdongnan Basin runs parallel to the continental slope, with an area of over 10,000 km2 and maximum thickness of greater than 2000 m. Using bulk rare earth element (REE) compositions of the sediments and detrital zircon U-Pb age data, we analysed the provenance of the Ledong Submarine Fan and its regional tectonic implications. The zircon grains in the Ledong Submarine Fan show a wide range of shapes, from prismatic crystals to oval grains with mostly rounded corners, indicating the long transport distance of these zircon grains prior to deposition. The zircon ages have a wide range from 27 to 3006 Ma, with five major age peaks at ca. 27 Ma, 88 Ma, 256 Ma, 415 Ma and 785 Ma, along with a number of subordinate age peaks at ca. 1024 Ma, 1376 Ma, 1879 Ma and 2374 Ma. These age distribution patterns are similar to those from the Red River at Yen Bai. The total REE content range is between 87.72 and 210.38 mg/g in the Ledong Submarine Fan. The chondrite-normalized REE patterns show light REE (LREE) enrichment and weakly negative Eu anomalies, which are similar to those in the Red River at Hanoi. The evidence from zircon U-Pb ages and REE geochemistry indicates that the provenance of the Ledong Submarine Fan is the Red River System. The development of the Ledong Submarine Fan indicates that a rapid subsidence occurred in the western Qiongdongnan Basin during the late Miocene, and the strike slip reversal of the Red River Fault from sinistral to dextral motion was the key factor that induced gravity flow to form the Ledong Submarine Fan. Highlights • Detrital zircons and REE from the sediments in the continental slope-parallel Ledong Submarine Fan were examined. • The source of the Ledong Submarine Fan sediments was from the Red River System. • The development of the Ledong Submarine Fan has implication for the tectonic activity of the Red River Fault. [ABSTRACT FROM AUTHOR]

The “source-conduit-sink” model is crucial for studying deep-water sedimentary systems along a continental margin. Using seismic data, bulk rare earth element compositions of sediments and zircon U-Pb age data, we examined the supply and deposition (i.e., the source and sink) of the sediments in the Central Canyon of the South China Sea. Five phases of secondary canyon fill are present in the Central Canyon. The natural levees developed at the head of phase 1 of the secondary canyon deposits indicate that the Central Canyon initially developed at 10.5 Ma. The sediments in the Central Canyon were supplied by the Ledong submarine fan, and the provenance of the material in the Ledong submarine fan and Central Canyon was eastern Vietnam. Large amounts of sediments were transported through the Central Canyon to the Shuangfeng Basin and deposited during four phases of submarine fan development. Phases 1–3 of the Shuangfeng submarine fans are composed of deep-water branching channel and inter-channel sediments. Phase 4 of the Shuangfeng submarine fan consists of deep-water channel and lobe sediments. Tectonic events, including the broad uplift of the Tibetan Plateau and central-southern Vietnam during the late Miocene, reversal of the strike-slip Red River Fault, and rapid subsidence in the Qiongdongnan Basin at approximately 5.5 Ma, provided favourable conditions for the growth of the Ledong submarine fan, Central Canyon and Shuangfeng submarine fan system. [ABSTRACT FROM AUTHOR]

Abstract: Vitrinite reflectance (VR, Ro%) measurements from residual kerogen of pyrolysis experiments were performed on immature Maoming Oil Shale substituted the samples for the gas-prone source rocks of Yacheng formation of the Qiongdongnan Basin in the South China Sea. The work was focused on determination an upper limit of maturity for gas generation (ULMGG) or “the deadline of natural gas generation”. Ro values at given temperatures increase with increasing temperature and prolonged heating time, but ΔRo-value, given a definition of the difference of all values for VR related to higher temperature and adjacent lower temperature in open-system non-isothermal experiment at the heating rate of 20°C/min, is better than VR. And representative examples are presented in this paper. It indicates that the range of natural gas generation for Ro in the main gas generation period is from 0.96% to 2.74%, in which ΔRo is in concordance with the stage for the onset and end of the main gas generation period corresponding to 0.02% up to 0.30% and from 0.30% up to 0.80%, respectively. After the main gas generation period of 0.96–2.74%, the evolution of VR approach to the ULMGG of the whole rock for type II kerogen. It is equal to 4.38% of VR, where the gas generation rates change little with the increase of maturation, ΔRo is the maximum of 0.83% corresponding to VR of 4.38%Ro, and the source rock does not nearly occur in the end process of hydrocarbon gas generation while Ro is over 4.38%. It shows that it is the same the ULMGG from the whole rock for type II kerogen as the method with both comparison and kinetics. By comparing to both the conclusions of pyrolysis experiments and the data of VR from the source rock of Yacheng formation on a series of selected eight wells in the shallow-water continental shelf area, it indicate that the most hydrocarbon source rock is still far from reaching ULMGG from the whole rock for type II kerogen. The source rock of Yacheng formation in the local areas of the deepwater continental slope basin have still preferable natural gas generative potential, especially in the local along the central depression belt (namely the Ledong, Lingshui, Songnan and Baodao sags from southwest to northeast) from the depocenter to both the margin and its adjacent areas. It help to evaluate the resource potential for oil and gas of the hydrocarbon source rock in the deepwater continental slope of the Qiongdongnan Basin or other basins with lower exploration in the northern of the South China Sea and to reduce the risk in exploration. [Copyright &y& Elsevier]

The provenance of Upper Oligocene to Upper Miocene sandstones in the western Qiongdongnan Basin was studied by zircon U–Pb ages. More than 200 zircon grains were separated from the sandstone of the Upper Oligocene Lingshui, Lower Miocene Sanya, and Upper Miocene Huangliu formations, and spot analysed for U–Pb dating. Most zircon grains from the Upper Oligocene Lingshui and Upper Miocene Huangliu formations had rounded corners, suggesting that the grains were transported a relatively long distance before deposition, while grains from the Lower Miocene Sanya Formation were usually angular in shape, implying a shorter transport distance from their provenance. The Lingshui Formation contained exclusively Cenozoic, Mesozoic, Palaeozoic, and Proterozoic zircon ages, and the age spectrum showed two major peaks at ca. 247 and 441 Ma, along with nine subordinate age peaks at ca. 35, 305, 517, 623, 811, 852, 911, 1158, and 1905 Ma. This indicates that Upper Oligocene sediments in the northwestern area of the Qiongdongnan Basin may have originated from the Red River system, which had four major peaks at 254, 418, 751, and 1848 Ma, and five subordinate peaks at ca. 32, 81, 514, 606, and 967 Ma. Results suggest that sediments from the Red River system entered the Qiongdongnan Basin as early as the Late Oligocene. The zircon age spectrum of the Sanya Formation had two major peaks at 99 and 237 Ma, indicating that Lower Miocene sandstones in the northwestern area of the Qiongdongnan Basin were mainly from Hainan Island, with ages ranging from 110 to 90 to 280–220 Ma. This study infers that the sandstones may be the paleo-Red River capture by the Yangze River, which resulted in a large decrease of sediment supply from the paleo-Red River system, such that the sediments of the paleo-Red River did not develop in the western area of the Qiongdongnan Basin during the Early Miocene. Results indicate that if the Red River capture existed, it would have predated the Early Miocene. The zircon age spectrum from Upper Miocene sediments of the western Qiongdongnan Basin showed five major age peaks at ca. 27, 88, 256, 415, and 785 Ma, indicating that its provenance was the Red River system. The Late Miocene provenance change from Hainan Island to the Red River may have been in response to a major phase of the Tibetan Plateau uplift. • Detrital zircon populations from the Upper Oligocene to Upper Miocene sediments were examined. • The age spectra of the Upper Oligocene, Lower Miocene, and Upper Miocene sediments show a striking difference. • If the Red River capture ever existed it should predate the Early Miocene. • The provenance change during the Late Miocene may correspond to the uplift of the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Submarine canyons have been the subject of intense studies in recent years because of their academic significance to studies of climate signals, eustasy and tectonic movement of the neighbouring area and their economic significance for hydrocarbon exploration in deep-water areas. The Central Canyon in the Qiongdongnan Basin runs parallel to the continental slope, and has a length, width and depth of 520 km, 2–15 km and 270–800 m. Using seismic and well log data, we analyses the sedimentary differences and controlling factors of the sediments in the different segments of the Central Canyon. The sediments within the Central Canyon can be subdivided into five phases of the secondary canyon filling. The most typical features of each phase of the secondary canyon are high- and low-density turbidity currents developed in the phase 1–2 of the secondary canyon, the muddy debris flows, turbidity currents and mass transport deposits in the phase 3, the high-density turbidity currents in the phase 4, the hemi-pelagic mudstone and mass transport deposits in the phase 5. There are four depositional models corresponding to the head area, upstream segment, middle reaches and downstream segment of the Central Canyon were established. It is characterized by muddy matrix-supported conglomerates and thickly massive sandstones with the feature of upward fining and thinning deposited in the head area, the thickly high net to gross sandstone infilled in the upstream segment, the thinly interbedded sandstones and mudstones and muddy mass transport deposits developed in the middle reaches, and the extensive hemi-pelagic mudstone infilled in the downstream segment. The transformation of the debris flows into turbidity currents and high-density turbidity currents into low-density turbidity in the head and upstream segment, supersaturated filling in the upstream segment, extensive development of mass transport deposits in the middle reaches, under-compensation filling and widely development of canyon wall collapse in the downstream segment are the main reasons contributing to the sedimentary differences between different segments of the Central Canyon. [ABSTRACT FROM AUTHOR]

The Yinggehai and Qiongdongnan basins in the northwestern South China Sea preserve a large volume of Cenozoic sediments. However, their sources are still remain controversial and need a further research. This paper uses discriminant diagrams and bivariate plots of major, trace and rare earth elements, combined with heavy mineral data and detrital zircon U-Pb ages to determine the provenance, source area weathering and tectonic setting of the Upper Miocene to Pliocene sediments in the Yinggehai and Qiongdongnan basins, offshore South China Sea. The sandstone samples used in this study are characterized by four features: (i) The studied sandstones are first-cycle deposits, no recycling processes are recorded in these sediments, and there is a low degree of weathering conditions in the source areas. (ii) The sandstones from the DF fan, LD fan and Central Canyon System may have a similar source, being derived from an old upper continental crust mainly composed of felsic igneous source rocks. (iii) Detrital zircon U-Pb ages suggest that Central Vietnam is likely to be the dominant source of the DF fan, LD fan and Central Canyon System. (iv) The tectonic setting of the sandstones in the DF fan, LD fan and Central Canyon System belongs to the continental island arc (CIA) or the active continental margin (ACM) fields. [ABSTRACT FROM AUTHOR]

• With the increase of sediment transport distance in the Central Canyon, there are higher quality reservoirs. • The early hydrocarbon charge is beneficial to the formation of high quality reservoirs. • The formation of high quality reservoir is controlled by the diagenesis process. The Central Canyon sandstones, located in the Ledong–Lingshui Depression, Qiongdongnan Basin, South China Sea, are the main reservoirs of the deep-water area. The present study aimed to examine the diagenetic processes and reservoir quality of these sandstones. Using an integrated approach of examination of casting thin sections, X-ray diffraction, porosity and permeability measurements, scanning electron microscopy, carbon and oxygen stable isotope analyses, fluid inclusion analyses and electron microprobe analysis, we analysed the diagenetic features of the sandstones and the reservoir quality of the LD30-1 and LS17-2 sandstones, respectively. The results showed that the sandstones consist of lithic arkoses and sublitharenites, the main pore systems consist of primary intergranular pores and secondary pores, and the main cements consist of authigenic quartz, clay minerals, and carbonates. By combining the analyses of burial history and hydrocarbon charging history, we discovered that the LD30-1 sandstone reservoirs and the LS17-2 sandstone reservoirs are quite different. A significant negative shift in δ13C and δ18O (ranging from −4.88‰ to −2.27‰ and from −11.83‰ to −5.96‰, respectively) is evident in the carbonate cements of the LD30-1 sandstones, and oxidation of organic matter had an important role in the occurrence of the carbonate cements. However, relatively high and concentrated δ13C and δ18O values in the LS17-2 sandstones, ranging from −1.76‰ to −0.48‰ and −5.04‰ to −2.95‰, respectively, are closer to the original isotopic compositions. Based on casting thin sections, the abundance of quartz overgrowths in the LD30-1 sandstones is higher than that in the LS17-2 sandstones. The development of each clay mineral in the LD30-1 and LS17-2 sandstones is quite different. The LS17-2 sandstones have higher reservoir quality than the LD30-1 sandstones. Early hydrocarbon charge is an important factor causing the differences of diagenetic minerals and reservoir quality between the LS17-2 sandstones and the LD30-1 sandstones. [ABSTRACT FROM AUTHOR]