陆相湖盆海侵事件对烃源岩发育影响机制

海侵是影响湖盆烃源岩发育的重要事件之一,通过调研国内外相关文献,述评了这一领域的研究现状。海侵事件广泛分布于中国的松辽盆地、渤海湾盆地和苏北盆地以及国外多个盆地,海侵可通过古遗迹标志、矿物岩石学标志以及地球化学特征等识别。其次海侵烃源岩一般具有有机质丰度较高、有机质类型较好的特征,主要归因于海侵事件明显促进烃源岩发育,海侵作用一方面可能为湖水带来了大量的营养物质,显著提高了古湖泊生产力,另外一方面,盐度可能有利于形成稳定的水体分层,在水底形成强还原环境,促进有机质保存。以西非裂谷盆地（Termit盆地）为例,总结了海侵影响湖相烃源岩发育的模式。     

Charging of oil fields surrounding the Shaleitian uplift from multiple source rock intervals and generative kitchens,Bohai Bay basin,China

This paper discusses origin and charging directions of oil fields on the Shaleitian Uplift, Bohai Bay basin. The Shaleitian Uplift is a footwall uplift surrounded by three sags containing mature source rocks. The origins of the four oil fields on the Shaleitian Uplift, both in terms of source rock intervals and in terms of generative kitchens, were studied using biomarker distributions for 61 source rock samples and 27 oil samples. Hierarchical cluster analysis using 12 parameters known to be effective indicators of organic matter input and/or depositional conditions allowed the identification of six oil types or classes. These six oil classes could then be linked to three distinct source rock intervals ranging in age from 43.0 Ma to 30.3 Ma. The third member (43.0–38.0 Ma in age) and first member (35.8–32.8 Ma) of the Eocene Shahejie Formation, and the third member of the Oligocene Dongying Formation (32.8–30.3 Ma) each sourced one class of oil. The other three classes represent mixtures of oil generated from multiple source rock intervals. Traps on the Shaleitian Uplift were charged in the east by oil generated from the Eocene Shahejie Formation in the Bozhong Sag, in the southeast by oil generated from the Eocene Shahejie and then Oligocene Dongying formations in the southwestern part of the Bozhong Sag and/or in the eastern part of the Shanan Sag, and in the southwest by oil generated from the Eocene Shahejie Formation in the western part of the Shanan Sag. The estimated migration distances range from less than 5 km to about 20 km. The compositional heterogeneity within fields and multiple-parameter comparisons between oils from nearby wells in different fields have proven to be a powerful tool to determine the in-filling histories of oil fields in cases where multiple source rock intervals and multiple generative kitchens exist.     

Regional structure and tectonic history of the obliquely colliding Columbus foreland basin, offshore Trinidad and Venezuela

Cenozoic eastward migration of the Caribbean plate relative to the South American plate is recorded by an 1100-km-long Venezuela-Trinidad foreland basin which is oldest in western Venezuela (65-55 Ma), of intermediate age in eastern Venezuela (34-20 Ma) and youngest beneath the shelf and slope area of eastern offshore Trinidad (submarine Columbus basin, 15.0 Ma-Recent). In this study of the regional structure, fault families, and chronology of faulting and tectonic events affecting the hydrocarbon-rich Columbus foreland basin of eastern offshore Trinidad, we have integrated approximately 775 km of deep-penetration 2D seismic lines acquired by the 2004 Broadband Ocean-Land Investigations of Venezuela and the Antilles arc Region (BOLIVAR) survey, 325 km of vintage GULFREX seismic data collected by Gulf Oil Company in 1974, and published industry well data that can be tied to some of the seismic reflection lines. Top Cretaceous depth structure maps in the Columbus basin made from integration of all available seismic and well data define for the first time the elongate subsurface geometry of the 11-15 km thick and highly asymmetrical middle Miocene-Recent depocenter of the Columbus basin. The main depocenter located 150-200 km east of Trinidad and now the object of deepwater hydrocarbon exploration is completely filled by shelf and deepwater sediments derived mainly from the Orinoco delta. The submarine Darien ridge exhibits moderate (20-140 m) seafloor relief, forms the steep (12°-24°), northern structural boundary of the Columbus basin, and is known from industry wells to be composed of 0.5-4.5 km thick, folded and thrust-imbricated, hydrocarbon-bearing section of Cretaceous and early Tertiary limestones and clastic rocks. The eastern and southern boundaries of the basin are formed by the gently (1.7°-4.5°), northward-dipping Cretaceous-Paleogene passive margin of South America that is in turn underlain by Precambrian rocks of the Guyana shield.Interpretation of seismic sections tied to wells reveals the following fault chronology: (1) middle Miocene thrusting along the Darien ridge related to highly oblique convergence between the Caribbean plate and the passive margin of northern South America; continuing thrusting and transpression in an oblique foreland basin setting through the early Pleistocene; (2) early Pliocene-recent low-angle normal faults along the top of the Cretaceous passive margin; these faults were triggered by oversteepening related to formation of the downdip, structurally and bathymetrically deeper, and more seaward Columbus basin; large transfer faults with dominantly strike-slip displacements connect gravity-driven normal faults that cluster near the modern shelf-slope break and trend in the downslope direction; to the south no normal faults are present because the top Cretaceous horizon has not been oversteepened as it is adjacent to the foreland basin; (3) early Pliocene-Recent strike-slip faults parallel the trend of the Darien ridge and accommodate present-day plate motions.     

Barbados: maturation,source rock potential and burial history within a Cenozoic accretionary complex

A number of diagenetic properties were studied in sedimentary rocks exposed along a 25 km across-strike transect on Barbados, the crest of the Barbados accretionary complex. The island of Barbados consists of three structural levels: a lower (mostly Eocene or undated) basal complex composed of quartzose turbidites, hemipelagites and melange; an intermediate level (Eocene to Miocene) of nappes composed mostly of calcareous pelagite; and a Pleistocene coral cap. Diagenetic studies concentrated on the basal complex although preliminary data from the nappes are also presented. The following subjects were studied: (1) clay mineralogy (% illite interlayers in mixed-layer clays); (2) vitrinite reflectance; (3) organic type and maturation, by Rock-Eval pyrolysis; (4) thermal alteration index of spores and pollen; (5) dry bulk density; (6) silica mineralogy. Taken together, the data can be interpreted to indicate maximum temperatures attained during diagenesis of less than 80°C. Based on a uniform palaeogeothermal gradient of 15°C km^?1, and a temperature at the sediment/water interface of 10°C, maximum burial depths of basement complex rocks were about 2–5 km. Minimum possible burial depths are < 1 km. No across-strike changes in maturation are observed indicating that these trench-associated rocks were uplifted uniformly. Mudstones are typically rich in total organic carbon (> 1 %), and are immature. Organic matter was derived mostly from terrigenous or oxidized sources (type III kerogens), although some marine planktonic sources are indicated locally (type II kerogens). Organic matter is therefore primarily gas-prone.     

Source rock deposition controlled by tectonic subsidence and climate in the western Pearl River Mouth Basin,China: Evidence from organic and inorganic geochemistry

Interest in factors controlling lacustrine source rock deposition has increased over the last few decades because this type of deposits contain significant petroleum resources. Generally, tectonic subsidence and climate are the two root causes as they control the accommodation potential, water column properties and sources of organic matter. In this study, coupling organic geochemical and elemental geochemical data, two potential source rocks, i.e., the Eocene Wenchang Formation (E₂w) and Oligocene Enping Formation (E₃e) were investigated. Two models were finally raised to explain deposition of the two set of source rocks according to their paleoclimatic and tectonic properties. The source rock potential shows a strong heterogeneity. The second member of the Eocene Wenchang Formation (E₂w₂) is characterized by high organic matter content and oil-prone kerogen type. In contrast, the first member of the Eocene Wenchang Formation (E₂w₁) and the Oligocene Enping formation (E₃e) are characterized by low organic matter content and gas-prone kerogen type. The primary productivity and depositional environment exhibit notable differences between the two potential source rocks horizons and show an obvious variation from the depocenter to the slope and can be best explained by the coevolution of tectonic subsidence and climate. During the E₂w depositional stage, low sediment supply led to mudstone deposited in deep lacustrine environment and resulted in underfilled lake basin. The low water inflow provided little terrigenous organic matter (low bicadinane, perylene and floranthene contents) and oxygen. Besides, the low area/depth ratio impeded the water circulation, thus resulted in shallow thermocline and anoxic-suboxic bottom environment (abundant dibenzothiophene and high C₃₅/C₃₁22S hopane ratios). Therefore abundant algae, which contributed to the high amorphous organic matter (AOM) content, can be preserved. The warm and wet climate (high Mn/Mg ratios) gave birth to autochthonous organism, such as dinoflagellates and Pavlova gyrans (abundant 4-methyl sterane). During the E₃e depositional stage, the sufficient sedimentary supply resulted in expanding, shallow lacustrine and swamp environment. The higher area/depth ratio and high sediment supply made environment unstable and can be strongly influenced by external environment (broader range of Mn/Mg ratios). Enough terrigenous organic matter (TOM) was transported to the slope but little to the depocenter. The slightly hot and dry climate (low Mn/Mg ratios) led to decreasing autochthonous organism and evaporation environment. The shallow water depth and relative dry climate resulted in saline, suboxic-dysoxic acid bottom environment. The co-variation of organic and inorganic indexes indicates the combination is a valid method in reconstructing source rock depositional models.     

东海丽水凹陷新生代基底岩体锆石 U-Pb年龄及其构造意义

丽水凹陷东部边界 W Z27-1井为东海陆架盆地钻穿新生代的少数钻井之一,依据录井资料显示其前新生代钻遇花岗 岩。通过对其底部岩屑样品岩石薄片研究发现,矿物成分主要为石英和长石,呈半自形或它形粒状分布,具有典型的花岗岩 结构;结合对样品的肉眼观察,以及地震剖面和测井解释资料,进一步确认该井底部所钻遇地层为浅层中酸性花岗岩,或具 有一定规模的分布。同时运用锆石 U-Pb定年方法,厘定该花岗岩的形成时代,结果显示该岩体主体形成于晚三叠世-早侏 罗世 （ 194~243 Ma依 ） ,大致可分为 205 Ma依和 220 Ma依两期。上述结果,首次揭示在东海陆架盆地丽水凹陷内发育有印支期 基底花岗岩体,具有重要的构造指示意义。结合我国东南陆域及琉球岛弧同期岩体发育特征及中生代沉积岩分布特点,认为 东南陆域与东海陆架盆地在中生代处于同一基底之上,经历三期相同构造背景的转换。     

Thermal history and source rock characterization of a Paleozoic section in the Awbari Trough,Murzuq Basin,SW Libya

In some areas of the Murzuq Basin, SW Libya, the lower Silurian member comprises highly radioactive shales (Hot Shale), which are regarded as the most significant factor controlling petroleum generation in the basin. In this context, it was the goal of our project to study the distribution and maturity of the Hot Shale in the framework of basin evolution. Organic geochemical, organic petrological and basin modelling methods were used to obtain a more thorough understanding. Four wells from the northern and central part of the Awbari Trough have been selected for this study.     

Relating petroleum system and play development to basin evolution: West African South Atlantic basins

Sedimentary basins can be classified according to their structural genesis and evolutionary history and the latter can be linked to petroleum system and play development. We propose an approach in which we use the established concepts in a new way: breaking basins down into their natural basin cycle division, then defining the characteristics of each basin cycle (including the type of petroleum systems and plays they may contain) and comparing them with similar basin cycles in other basins, thereby providing a means to learn through a greater population of (perhaps not immediately obvious) analogues. Furthermore, we introduce the use of the trajectory plot as a new tool in such an analysis. This methodology has been applied to the West African South Atlantic marginal basins between Cameroon and Angola, and we demonstrate that the similar tectonostratigraphic evolution of the individual basins along this margin has led to the development of similar types of petroleum systems and play (level)s. Consequently, we can make analogue comparisons among these basins in order to evaluate and predict the presence of potential, yet undiscovered, hydrocarbon accumulations in less well explored parts of the margin.     

Thermal maturity,source rock potential and kinetics of hydrocarbon generation in Permian shales from the Damodar Valley basin,Eastern India

This study investigates the source rock characteristics of Permian shales from the Jharia sub-basin of Damodar Valley in Eastern India. Borehole shales from the Raniganj, Barren Measure and Barakar Formations were subjected to bulk and quantitative pyrolysis, carbon isotope measurements, mineral identification and organic petrography. The results obtained were used to predict the abundance, source and maturity of kerogen, along with kinetic parameters for its thermal breakdown into simpler hydrocarbons.The shales are characterized by a high TOC (>3.4%), mature to post-mature, heterogeneous Type II–III kerogen. Raniganj and Barren Measure shales are in mature, late oil generation stage (Rr%_Raniganj = 0.99–1.22; Rr%_{Barren Measure} = 1.1–1.41). Vitrinite is the dominant maceral in these shales. Barakar shows a post-mature kerogen in gas generation stage (Rr%_Barakar = 1.11–2.0) and consist mainly of inertinite and vitrinite. The δ¹³C_org value of kerogen concentrate from Barren Measure shale indicates a lacustrine/marine origin (−24.6–−30.84‰ vs. VPDB) and that of Raniganj and Barakar (−22.72–−25.03‰ vs. VPDB) show the organic provenance to be continental. The δ¹³C ratio of thermo-labile hydrocarbons (C₁–C₃) in Barren Measure suggests a thermogenic source.Discrete bulk kinetic parameters indicate that Raniganj has lower activation energies (ΔE = 42–62 kcal/mol) compared to Barren Measure and Barakar (ΔE = 44–68 kcal/mol). Temperature for onset (10%), middle (50%) and end (90%) of kerogen transformation is least for Raniganj, followed by Barren Measure and Barakar. Mineral content is dominated by quartz (42–63%), siderite (9–15%) and clay (14–29%). Permian shales, in particular the Barren Measure, as inferred from the results of our study, demonstrate excellent properties of a potential shale gas system.     

Tracing of deeply-buried source rock: A case study of the WC9-2 petroleum pool in the Pearl River Mouth Basin, South China Sea

The identification of a deeply-buried petroleum-source rock, owing to the difficulty in sample collection, has become a difficult task for establishing its relationship with discovered petroleum pools and evaluating its exploration potential in a petroleum-bearing basin. This paper proposes an approach to trace a deeply-buried source rock. The essential points include: determination of the petroleum-charging time of a reservoir, reconstruction of the petroleum generation history of its possible source rocks, establishment of the spatial connection between the source rocks and the reservoir over its geological history, identification of its effective source rock and the petroleum system from source to trap, and evaluation of petroleum potential from the deeply-buried source rock. A case study of the W9-2 petroleum pool in the Wenchang A sag of the Pearl River Mouth Basin, South China Sea was conducted using this approach. The W9-2 reservoir produces condensate oil and gas, sourced from deeply-buried source rocks. The reservoir consists of a few sets of sandstone in the Zhuhai Formation, and the possible source rocks include an early Oligocene Enping Formation mudstone and a late Eocene Wenchang Formation mudstone, with a current burial depth from 5000 to 9000 m. The fluid inclusion data from the reservoir rock indicate the oil and the gas charged the reservoir about 18–3.5 Ma and after 4.5 Ma, respectively. The kinetic modeling results show that the main stages of oil generation of the Wenchang mudstone and the Enping mudstone occurred during 28–20 Ma and 20–12 Ma, respectively, and that the δ¹³C₁ value of the gas generated from the Enping mudstone has a better match with that of the reservoir gas than the gas from the Wenchang mudstone. Results from a 2D basin modeling further indicate that the petroleum from the Enping mudstone migrated upward along the well-developed syn-sedimentary faults in the central area of the sag into the reservoir, but that the petroleum from the Wenchang mudstone migrated laterally first toward the marginal faults of the sag and then migrated upward along the faults into shallow strata. The present results suggest that the trap structure in the central area of the sag is a favorable place for the accumulation of the Enping mudstone-derived petroleum, and that the Wenchang mudstone-derived petroleum would have a contribution to the structures along the deep faults as well as in the uplifted area around the sag.     

Physical modelling of chemical compaction,overpressure development,hydraulic fracturing and thrust detachments in organic-rich source rock

Geological evidence for overpressure is common worldwide, especially in petroleum-rich sedimentary basins. As a result of an increasing emphasis on unconventional resources, new data are becoming available for source rocks. Abnormally high values of pore fluid pressure are especially common within mature source rock, probably as a result of chemical compaction and increases in volume during hydrocarbon generation. To investigate processes of chemical compaction, overpressure development and hydraulic fracturing, we have developed new techniques of physical modelling in a closed system. During the early stages of our work, we built and deformed models in a small rectangular box (40 × 40 × 10 cm), which rested on an electric flatbed heater; but more recently, in order to accommodate large amounts of horizontal shortening, we used a wider box (77 × 75 × 10 cm). Models consisted of horizontal layers of two materials: (1) a mixture of equal initial volumes of silica powder and beeswax micro-spheres, representing source rock, and (2) pure silica powder, representing overburden. By submerging these materials in water, we avoided the high surface tensions, which otherwise develop within pores containing both air and liquids. Also we were able to measure pore fluid pressure in a model well. During heating, the basal temperature of the model surpassed the melting point of beeswax (∼62 °C), reaching a maximum of 90 °C. To investigate tectonic contexts of compression or extension, we used a piston to apply horizontal displacements.In experiments where the piston was static, rapid melting led to vertical compaction of the source layer, under the weight of overburden, and to high fluid overpressure (lithostatic or greater). Cross-sections of the models, after cooling, revealed that molten wax had migrated through pore space and into open hydraulic fractures (sills). Most of these sills were horizontal and their roofs bulged upwards, as far as the free surface, presumably in response to internal overpressure and loss of strength of the mixture. We also found that sills were less numerous towards the sides of the box, presumably as a result of boundary effects. In other experiments, in which the piston moved inward, causing compression of the model, sills also formed. However, these were thicker than in static models and some of them were subject to folding or faulting. For experiments, in which we imposed some horizontal shortening, before the wax had started to melt, fore-thrusts and back-thrusts developed across all of the layers near the piston, producing a high-angle prism. In contrast, as soon as the wax melted, overpressure developed within the source layer and a basal detachment appeared beneath it. As a result, thin-skinned thrusts propagated further into the model, producing a low-angle prism. In some experiments, bodies of wax formed imbricate zones within the source layer.Thus, in these experiments, it was the transformation, from solid wax to liquid wax, which led to chemical compaction, overpressure development and hydraulic fracturing, all within a closed system. According to the measurements of overpressure, load transfer was the main mechanism, but volume changes also contributed, producing supra-lithostatic overpressure and therefore tensile failure of the mixture.     

Geochemical characteristics of Tertiary coal-bearing source rocks in Xihu depression,East China Sea basin

This study aims at investigating hydrocarbon generation potential and biological organic source for the Tertiary coal-bearing source rocks of Pinghu Formation (middle-upper Eocene) in Xihu depression, East China Sea shelf basin. Another goal is to differentiate coal and mudstone with respect to their geochemical properties. The coal-bearing sequence has a variable organofacies and is mainly gas-prone. The coals and carbonaceous mudstones, in comparison with mudstones, have a higher liquid hydrocarbon generation potential, as reflected by evidently higher HI values (averaging 286 mg HC/g C) and H/C atomic ratios (round 0.9). The molecular composition in the coal-bearing sequence is commonly characterized by unusually abundant diterpenoid alkanes, dominant C₂₉ sterane over C₂₇ and C₂₈ homologues and high amount of terrigenous-related aromatic biomarkers such as retene, cadalene and 1, 7-dimethylphenanthrene, indicating a predominantly terrigenous organic source. The source rocks show high Pr/Ph ratios ranging mostly from 3.5 to 8.5 and low MDBTs/MDBFs ratios (<1.0), indicating deposition in an oxic swamp-lacustrine environment. The coals and carbonaceous mudstones could be differentiated from the grey mudstones by facies-dependent biomarker parameters such as relative sterane concentration and gammacerane index and carbon isotope composition. Isotope and biomarker analysis indicate the genetic correlation between the Pinghu source rocks and the oils found in Xihu depression. Moreover, most oils seem to be derived from the coal as well as carbonaceous mudstone.     

Characteristics and petroleum origin of the Carboniferous volcanic rock reservoirs in the Shixi Bulge of Junggar Basin,western China

Shixi Bulge of the central Junggar Basin in western China is a unique region that provides insight into the geological and geochemical characteristics of large-scale petroleum reservoirs in volcanic rocks of the western Central Asian Orogenic Belt. Carboniferous volcanic rocks in the Shixi Bulge mainly consist of striped lava and agglomerate, as well as breccia lava and tight tuff. Volcanic rocks differ in porosity and permeability. Striped lava exhibits the highest porosity (average: 14.2%) but the lowest permeability (average: 0.67 × 10⁻¹⁵ m) among the rock types. Primary gas pores are widely developed and mostly filled. Secondary dissolution pores and fractures are two major reservoir storage spaces. Capillary pressure curves suggest the existence of four pore structure types of reservoir rocks. Several factors, namely, lithology, pore structure, and various diagenesis, govern the physical properties of volcanic rocks. The oil is characterized by a high concentration of tricyclic terpane, a terpane distribution of C₂₃ < C₂₁ > C₂₀, and sterane distributions of C₂₇ < C₂₈ < C₂₉ and C₂₇ > C₂₈ < C₂₉. Oil and gas geochemistry revealed that the oil is a mixture derived primarily from P₂w source rock and secondarily from P₁j source rock in the sag west of Pen-1 Well. The gases are likely gas mixtures of humic and sapropelic organic origins, with the sapropelic gas type dominant in the mixture. The gas mixture is most likely cracked from kerogen rather than oils. The Carboniferous volcanic reservoirs in Shixi Bulge share some unique characteristics that may provide useful insights into the various roles of different volcanic reservoir types in old volcanic provinces. The presence of these reservoirs will undoubtedly encourage future petroleum exploration in volcanic rocks up to the deep parts of sedimentary basins.     

Integrated gravity and magnetic study on patterns of petroleum basin occurrence in the China seas and adjacent areas

The China seas and adjacent areas contain numerous petroleum basins. One of the main challenges for future oil and gas exploration is to identify the inherent patterns of petroleum basin distribution. The formation and evolution of petroleum basins along with the migration and accumulation of oil and gas are often closely related to the tectonic environment. The gravity and magnetic fields with high lateral resolution and wide coverage provide important data for regional tectonic research. Based on the gravity data in the Global Satellite Gravity Anomaly Database (V31.1) and magnetic data from the Earth Magnetic Anomaly Grid (2-arc-minute resolution) (V2), this study uses integrated gravity and magnetic field technique to obtain integrated gravity and magnetic field result for the China seas and adjacent areas, and then adopts the normalized vertical derivative of the total horizontal derivative technique to conduct partition. Finally, it identifies the relationship between the partition characteristics and tectonics as well as the patterns of petroleum basin occurrence. The research shows that the partition of gravity and magnetic field integrated result has a good correlation with the Neo-Cathaysian tectonic system and tectonic units. The petroleum basins are characterized according to three blocks arranged from north to south and four zones arranged from east to west. The north?south block structure causes the uneven distribution of oil and gas resources in the mainland area and the differences in the hydrocarbon-bearing strata. Petroleum basins are more abundant in the north than in the south. The ages of the main oil- and gas-bearing strata are “Paleozoic–Mesozoic, Paleozoic–Mesozoic–Cenozoic, and Paleozoic–Mesozoic”, in order from north to south. The difference in the overall type of oil and gas resources in all basins is controlled by the east–west zonation. From east to west, the oil and gas resource type exhibits a wave-like pattern of “oil and gas, gas, oil and gas, gas”. The vertical distribution is characterized by an upper oil (Mesozoic–Cenozoic) and lower gas (Mesozoic–Paleozoic) structure. Within the study area, the Paleozoic marine strata should be the main strata of future natural gas exploration.     

Structural patterns and tectonic history of the Bauer microplate, Eastern Tropical Pacific

The Bauer microplate was an independent slab of oceanic lithosphere that from 17 Ma to 6 Ma grew from 1.4 × 10⁵ km² to 1.2 × 10⁶ km² between the rapidly diverging Pacific and Nazca plates. Growth was by accretion at the lengthening and overlapping axes of the (Bauer-Nazca) Galapagos Rise (GR) and the (Pacific-Bauer) East Pacific Rise (EPR). EPR and GR axial propagation to create and rapidly grow the counter-clockwise spinning microplate occurred in two phases: (1) 17–15Ma, when the EPR axis propagated north and the GR axis propagated south around a narrow (100- to 200-km-wide) core of older lithosphere; and (2) 8–6 Ma, when rapid northward propagation of the EPR axis resumed, overlapping ∼400 km of the fast-spreading Pacific-Nazca rise-crest and appending a large (200- to 400-km-wide) area of the west flank of that rise as a ‘northern annex’ to the microplate. Between 15 and 8 Ma the microplate grew principally by crustal accretion at the crest of its rises. The microplate was captured by the Nazca plate and the Galapagos Rise axis became extinct soon after 6 Ma, when the south end of the Pacific-Bauer EPR axis became aligned with the southern Pacific-Nazca EPR axis and its north end was linked by the Quebrada Transform to the northern Pacific-Nazca EPR axis. Incomplete multibeam bathymetry of the microplate margins, and of both flanks of the Pacific-Bauer and Bauer-Nazca Rises, together with archival magnetic and satellite altimetry data, clarifies the growth and (counter-clockwise) rotation of the microplate, and tests tectonic models derived from studies of the still active, much smaller, Easter and Juan Fernandez microplates. Our interpretations differ from model predictions in that Euler poles were not located on the microplate boundary, propagation in the 15–8 Ma phase of growth was not toward these poles, and microplate rotation rates were small (5°/m.y.) for much of its history, when long, bounding transform faults reduced coupling to Nazca plate motion. Some structures of the Bauer microplate boundary, such as deep rift valleys and a broad zone of thrust-faulted lithosphere, are, however, similar to those observed around the smaller, active microplates. Analysis of how the Bauer microplate was captured when coupling to the Pacific plate was reduced invites speculation on why risecrest microplates eventually lose their independence.     

Structures of the northeasternmost South China Sea continental margin and ocean basin: geophysical constraints and tectonic implications

The northeastern part of the South China Sea is a special region in many aspects of its tectonics. Both recent drilling into the Mesozoic and new reflection seismic surveys in the area provide a huge amount of data, fostering new understanding of the continental margin basins and regional tectonic evolution. At least four half-grabens are developed within the Northern Depression of the Tainan Basin, and all are bounded on their southern edges by northwestward-dipping faults. One of the largest half-grabens is located immediately to the north of the Central Uplift and shows episodic uplift from the late Oligocene to late Miocene. Also during that period, the Central Uplift served in part as a material source to the Southern Depression of the Tainan Basin. The Southern Depression of the Tainan Basin is a trough structure with deep basement (up to 9 km below sealevel or 6 km beneath the sea bottom) and thick Cenozoic sedimentation (>6 km thick). Beneath the Southern Depression we identified a strong landward dipping reflector within the crustal layer that represents a significant crustal fault. This reflector coincides with a sharp boundary in crustal thicknesses and Moho depths. We show that the northeasternmost South China Sea basin, which may have undergone unique evolution since the late Mesozoic, is markedly different from the central South China Sea basin and the Huatung Basin, both geologically and geophysically. The Cenozoic evolution of the region was largely influenced by pre-existing weaknesses due to tectonic inheritance and transition. The South China Sea experienced multiple stages of Cenozoic extension.     

Charging of the Penglai 9-1 oil field,Bohai Bay basin,China: Functions of the delta on accumulating petroleum

The Penglai 9-1 (PL9-1) oil field, which contains China's third largest offshore oil accumulation (in-place reserves greater than 2.28 × 10⁸ ton or 1.49 × 10⁹ bbl), was found in shallow reservoirs (700–1700 m, 2297–5577 ft) within the most active fault zone in east China. The PL9-1 field contains two oil-bearing series, the granite intrusions in Mesozoic (Mz) and both the sandstone reservoirs in Neogene Guantao (Ng) and Neogene Minghuazhen (Nm) Formation. The origins of the PL9-1 field, both in terms of source rock intervals and generative kitchens, were determined by analyzing biomarker distributions for 61 source rock samples and 33 oil samples. The Mesozoic granite intrusions, which hold more than 80% of the oil reserves in the field, were charged in the west by oil generated from the third member (Es₃) of the Shahejie Formation in the Bodong depression. The Neogene reservoirs of the PL9-1 field were charged in the west by oil generated from the third member (Es₃) of the Shahejie Formation in the Bodong depression and in the south by oil generated from the first member (Es₁) of the Shahejie Formation in the Miaoxibei depression. Interactive contact between the large fan delta and the mature source rocks residing in the Es₃ Formation of the Bodong depression resulted in a high expulsion efficiency from the source rocks and rapid oil accumulation in the PL9-1 field, which probably explains how can this large oil field accumulate and preserve within the largest and most active fault zone in east China.     

Seismic stratigraphy of the western South Korea Plateau,East Sea: implications for tectonic history and sequence development during back-arc evolution

The western South Korea Plateau in the East Sea (Sea of Japan) is occupied by rifted continental fragments formed in association with the early phase of back-arc opening. The present study focuses on the seismic stratigraphy of the sedimentary succession and the underlying acoustic basement in this region, based on closely spaced multichannel seismic reflection profiles. The sedimentary succession occurs mainly within a series of subparallel basement troughs (grabens or half grabens) bounded by faulted continental blocks (horsts) or volcanic ridges, and commonly floored by extrusive volcanic rocks showing hyperbolic reflectors. These features are strongly suggestive of continental rifting accompanied by normal faulting, volcanic activity and high rates of basin subsidence. The sedimentary succession can be subdivided into four seismic units. Unit 1 is characterized by short and irregular high-amplitude reflectors and interpreted as a syn-rift deposit consisting of a non-marine volcanics/sediment complex in topographic lows. Units 2 and 3 formed in an open marine environment during the Middle Miocene to Early Pliocene, characterized by an onlap-fill and later draping marine sedimentary succession dominantly composed of hemipelagic sediments and turbidites with frequent intercalation of mass-flow deposits. Along the western margin of the plateau, these units were deformed under a compressional regime in the Early Pliocene, associated with the back-arc closing phase. Unit 4 (deposited since the Early Pliocene) comprises hemipelagic sediments and turbidites with evidence of sporadic slides/slumps.     

Geological structure and petroleum resource potential of the North Sea basin

The North Sea basin occupies a spacious depression almost isometric in shape. In the west and northwest, the basin is bordered by the continental crust consolidated during the Precambrian, Caledonian, and Hercynian orogenic epochs, which now forms epiplatformal orogenic structures. They are represented by the London-Brabant uplift and the Arden massif in the southwest and south and the Baltic Shield in the east and northeast. The North Sea basin may be considered as an ancient aulacogen that was transformed in the Early Mesozoic into a complex system of continental rifts and grabens. The sedimentary cover of the basin is represented by a thick (8.5?C12.5 km) Ordovician-Quaternary sequence. Oil and gas generation in the sedimentary cover of the basin is likely connected with four main productive sequences: the coaliferous Upper Carboniferous (Westphalian), the subsalt Zechstein, the Jurassic-Lower Cretaceous (Lotharingian, Toarcian, Kimmeridgian, and Weldian bituminose shales), and the shaly Cenozoic. The large oil and gas reserves in the North Sea??s sedimentary cover (over 280 fields) implies that the above-mentioned sequences have realized their oil-generating potential. The present-day position of the main oil and gas generation zones in the sedimentary section of the North Sea explains the distribution of the oil and gas fields through the basin from the genetic standpoint. The petroleum resource potential of the basin is still significant. In this regard, most promising are the spacious shelf areas, turbidite sediments, deep Paleozoic sequences, and continental slopes in the northern part of the basin, which remains insufficiently investigated.     

The formation and tectonic evolution of Philippine Sea Plate and KPR

The Philippine Sea Plate has an extremely special tectonic background. As an oceanic plate, it is almost entirely surrounded by subduction zones with complex internal tectonic features. On the basis of enormous published literature, this paper offers a comprehensive overview of the tectonic and evolution history of the Philippine Basin and the Kyushu-Palau Ridge (KPR) in the Philippine Sea Plate, and discusses the geological features of KPR. Referring to relevant definitions of various "ridges" stipulated in United Nations Convention on the Law of the Sea, so the KPR is believed to be a remnant arc formed during the opening of the Parece Vela and Shikoku Basins in the Philippine Sea Plate. It is a submarine ridge on oceanic plate rather than a submarine elevation. And thus, it is not a natural component of the Japan continental margin.