Hydrocarbon accumulation model for Neogene traps in the Chengdao area,Bohai Bay Basin,China

Chengdao is an offshore area in the Bohai Bay Basin that contains approximately 25.7 × 10⁸ bbl of oil and gas reserves within the sandstone reservoirs in Neogene strata. However, previous predictions of hydrocarbon accumulation in Neogene traps are inaccurate, resulting in a current failure rate of 50% when drilling for hydrocarbons in this area. To build an improved exploration model for Neogene traps, we select 92 traps from Neogene strata in the Chengdao area to quantify the filling degree, which is an indicator of hydrocarbon accumulation efficiency. The quantified filling degree is based on actual geological and exploration data and differs significantly among various trap types. The filling degree of traps also varies significantly with their structural locations and decreases generally from the northwest to the southeast along the Chengbei Fault zone. Vertically, the filling degree is highly heterogeneous, initially increasing from the bottom to the middle of Neogene strata and then decreasing towards the top of the strata. These Neogene hydrocarbon reservoirs are sourced from the Paleogene, and as they lay vertically away from the source rocks, their hydrocarbon enrichment is constrained largely by hydrocarbon migration distance and vertical migration pathways. The sealing capacity of faults and cap rocks, sandbody orientation and reservoir sedimentary facies determine the maximum column height, which in turn affects the amount of hydrocarbon accumulation within these traps. A scatter plot analysis of individual controls and volumetric filling for each trap type is compiled using multivariate linear regression analysis to quantify controls and the dominant control of hydrocarbon accumulation is determined.     

Hydrocarbon migration and accumulation of the Suqiao buried-hill zone in Wen'an Slope,Jizhong Subbasin,Bohai Bay Basin,China

The hydrocarbon migration and accumulation of the Suqiao deep buried-hill zone, in the Jizhong Subbasin, the Bohai Bay Basin, eastern China, was investigated from the perspective of paleo-fluid evidence by using fluid inclusions, quantitative fluorescence techniques (QGF), total scanning fluorescence method (TSF) and organic geochemical analysis. Results show that the current condensate oil-gas reservoirs in the study area once were paleo-oil reservoirs. In addition, the reservoirs have experienced at least two stages of hydrocarbon charge from different sources and/or maturities. During the deposition of the Oligocene Dongying Formation (Ed), the deep Ordovician reservoirs were first charged by mature oils sourced from the lacustrine shale source rocks in the fourth member of Shahejie and Kongdian Formations (Es₄+Ek), and then adjusted at the end of Ed period subsequently by virtue of the tectonic movement. Since the deposition of the Neogene Minghuazhen Formation (Nm), the reservoirs were mainly charged by the gas that consisted of moderate to high-maturity condensate and wet gas sourced from the Es₄+Ek lacustrine shale source rocks and mature coal-derived gas sourced from the Carboniferous-Permian (C-P) coal-bearing source rocks. Meanwhile, the early charged oil was subjected to gas flushing and deasphalting by the late intrusion of gas. The widely distributed hydrocarbon inclusions, the higher QGF Index, and FOI (the frequency of oil inclusions) values in both gas-oil and water zone, are indicative of early oil charge. In addition, combined with the homogenization temperatures of the fluid inclusions (<160 °C) and the existence of solid-bitumen bearing inclusions, significant loss of the n-alkanes with low carbon numbers, enrichments of heavier components in crude oils, and the precipitation of asphaltene in the residual pores suggest that gas flushing may have played an important role in the reservoir formation.     

Mineralogical and chemical distribution of the Es3L oil shale in the Jiyang Depression,Bohai Bay Basin (E China): Implications for paleoenvironmental reconstruction and organic matter accumulation

The Es₃^L (lower sub-member of the third member of the Eocene Shahejie Formation) shale in the Jiyang Depression is a set of relatively thick and widely deposited lacustrine sediments with elevated organic carbon, and is considered to be one of the most important source rocks in East China. We can determine the mineralogy, organic and inorganic geochemistry of the Es₃^L shale and calculate paleoclimate indexes by using multiple geochemical proxies based on organic chemistry (total organic carbon [TOC] and Rock-Eval pyrolysis), major and trace elements, X-Ray diffraction, and carbon and oxygen isotope data from key wells alongside ECS (Elemental Capture Spectroscopy) well log data. These indicators can be used to analyze the evolution of the paleoenvironment and provide a mechanism of organic matter (OM) accumulation. The Es₃^L oil shale has high TOC abundance (most samples >3.0%) and is dominated by Type I kerogens. Additionally, the organic-rich shale is rich in CaO and enrichment in some trace metals is present, such as Sr, Ba and U. The positive δ¹³C and negative δ¹⁸O values, high Sr/Ba, B/Ga and Ca/Ca + Fe ratios and low C/S ratios indicate that the Es₃^L shales were mainly deposited in a semi-closed freshwater-brackish water lacustrine environment. The consistently low Ti/Al and Si/Al ratios reflect a restricted but rather homogeneous nature for the detrital supply. Many redox indicators, including the Th/U, V/(V + Ni), and δU ratios, pyrite morphology and TOC-TS-Fe diagrams suggest deposition under dysoxic to suboxic conditions. Subsequently, the brackish saline bottom water evolved into an anoxic water body under a relatively arid environment, during which organic-lean marls were deposited in the early stage. Later, an enhanced warm-humid climate provided an abundant mineral nutrient supply and promoted the accumulation of algal material. OM input from algal blooms reached a maximum during the deposition of the organic-rich calcareous shale with seasonal laminations. High P/Ti ratios and a strongly positive relationship between the P and TOC contents indicate that OM accumulation in the oil shale was mainly controlled by the high primary productivity of surface waters with help from a less stratified water column. Factors such as the physical protection of clay minerals and the dilution of detrital influx show less influence on OM enrichment.     

Evaluation of geological factors in characterizing fault connectivity during hydrocarbon migration: Application to the Bohai Bay Basin

Faults play an intricate role in hydrocarbon migration and accumulation since they can serve either as a conduit or a seal. Quantitative evaluation of fault opening/sealing properties requires the selection of valid and optimal parameters among numerous geological factors to characterize the hydraulic behaviors of faults. The present study focuses on the Chengbei Step-Fault Area in the Qikou Depression, Bohai Bay Basin, NE China, because hydrocarbon migration and accumulation in this area occurred in a relatively short period so that accumulated hydrocarbons can be used as an indicator to deduce hydraulic connectivity of a fault zone between two sites. Various geological parameters pertinent to a fault, such as burial depth, dip angle, throw, strike, percentage of sandstone of faulted intervals, fluid pressure in faulted mudstone, stress normal to the fault plane, and shale gouge ratio, are analyzed to assess their effectiveness in characterizing fault connectivity. An index, the fault-connectivity probability (N_p), is proposed to evaluate the possibility that a fault has been once serving as a migration pathway. The statistical relationship between N_p and any a geological parameter may be used to indicate the effectiveness of this parameter in characterizing the connectivity of a fault during hydrocarbon migration. The correlation coefficient of a relationship is a good indicator of the effectiveness; and the results are generally in agreement with qualitative assessments. Parameters representing a single geological factor are generally ineffective, whereas those representing implicitly or explicitly two or more factors, such as shale gouge ratio, stress normal to the fault plane, and fault opening index, are more effective.     

渤海湾盆地垦利A油田石油地质特征及成藏模式

针对垦利A油田油气成藏条件和成藏模式认识不清的问题,结合研究区构造特征和沉积背景,综合运用钻井、测井、分析化验等资料,对垦利A油田油气成藏条件和富集规律进行深入剖析。结果表明：莱州湾凹陷北洼和南次洼沙河街组沙三中段、沙四段均发育优质烃源岩,油源供给充足;储集层储层物性好,盖层区域分布稳定,形成3套有利储盖组合;油源断层、渗透性砂体和不整合面构成良好的油气输导系统;油气富集层位具有“西浅东深”特征。基于上述认识,建立了3种油气成藏模式：构造东部为近源“侧向式”成藏模式,在古近系沙河街组和中生界潜山成藏;构造中部为远源“阶梯式”成藏模式,具复式成藏特征;构造西部为远源“网毯式”成藏模式,在新近系馆陶组成藏。     

Study on the Cretaceous turbidite and reservoir features in the Qingshankou Formation in northern Songliao Basin,NE China

Based on analysis of well and drilling data, cores, sediment grains and 3D seismic data, four types of turbidites–slope fan, channelized, laminated and sublacustrine fan turbidite–are identified in Members 1 and 2 of the Qingshankou Formation in northern Songliao Basin. The slope fan turbidite is located in Members 1 and 2 of the Qingshankou Formation. It is dominated by silt and fine sand and is distributed in an SN-trending ribbon zone along the slope break at delta front in the western part of the basin. The channelized turbidite is located at the bottom of Member 1 of the Qingshankou Formation. It is dominated by silt and fine sand and is distributed in an SN-trending strip-shaped zone along the Qijia-Gulong sag, with funnel-shaped sublacustrine fans at the end. The laminated turbidite body is located in Member 2 of the Qingshankou Formation. It is dominated by siltstone and argillaceous siltstone and is distributed continuously in a tongue-shaped zone along the northern delta front towards the lacustrine region, with belt-like distributaries at the central part and sublacustrine fans at the end. Low-permeability and low-yield lithologic reservoirs are formed near the delta front within the slope fan turbidite and channelized turbidite. There are “sweet spots” in local regions, where reservoir reform techniques are required to attain high industrial yields. Laminated turbidite and sublacustrine fans can form unconventional and continuous reservoirs that generally have no natural productivity; industrial production is impossible until horizontal drilling and multistage volume fracturing are employed. Therefore, the research results are important to the exploration of unconventional oil and gas reservoirs in northern Songliao Basin.     

Present temperature field and Cenozoic thermal history in the Dongpu depression,Bohai Bay Basin,North China

The Dongpu depression is located in the southern Bohai Bay Basin, North China, and it has abundant oil and gas reserves. There has been no systematic documentation of this depression's temperature field and thermal history. In this article, the present geothermal gradient and heat flow were calculated for 68 wells on the basis of 892 formation-testing data from 523 wells. Moreover, the Cenozoic thermal history was reconstructed using 466 vitrinite reflectance data from 105 wells. The results show that the Dongpu depression is characterized by a medium-temperature field between stable and active tectonic areas, with an average geothermal gradient of 34.8 °C/km and an average heat flow of 66.8 mW/m². The temperature field in the Dongpu depression is significantly controlled by the Changyuan, Huanghe, and Lanliao basement faults and thin lithosphere thickness. The geothermal gradient twice experienced high peaks. One peak was during the Shahejie 3 Formation depositional period, ranging from 45 °C/km to 48 °C/km, and the second peak was in the middle and late of the Dongying Formation depositional period, ranging from 39 °C/km to 40 °C/km, revealing that the Dongpu depression experienced two strong tectonic rifts during the geothermal gradient high peak periods. The geothermal gradient began to decrease from the Neogene, and the geothermal gradient is 31–34 °C/km at the present day. In addition, these results reveal that source rock thermal evolution is controlled by the paleo temperature field of the Dongying Formation depositional period in the Dongpu depression. This study may provide a geothermal basis for deep oil and gas resource evaluation in the Dongpu depression.     

渤海湾盆地南堡凹陷断裂控藏特征研究

南堡凹陷为渤海湾盆地的一个新生代富生烃凹陷,内部断层十分发育,大量的油气藏都围绕断裂分布。根据断裂平面及剖面组合特征,断裂平面组合样式可分为4种：平行状、交织状、梳状和帚状;剖面组合分为3种：复“y”形组合、花状组合及阶梯式组合。根据断裂、砂体及油气藏的分布特征,发现二级和三级生长断层凹面储集厚度较大、且砂体孔渗性较好,是油气运移的主要方向。反向旋转断层组合的断层下盘和同向旋转断层组合的断层上盘是油气聚集的有利部位。但由于断层带的分带性,上诱导裂缝带相对下诱导裂缝带渗透性好。上诱导裂缝带利于油气运移,下诱导裂缝带利于封堵油气,油气的最有利聚集部位为反向旋转断层组合的下盘。     

Mechanisms of petroleum accumulation in the Bozhong sub-basin, Bohai Bay Basin, China. Part 1: Origin and occurrence of crude oils

The origin of the fourteen major oil fields in the Bozhong sub-basin, Bohai Bay basin was studied based on the results of Rock-Eval pyrolysis on more than 700 samples and biomarker analysis on 61 source rock samples and 87 oil samples. The three possible source rock intervals have different biomarker assemblages and were deposited in different environments. The third member of the Oligocene Dongying Formation (E₃d₃, 32.8–30.3 Ma in age) is characterized mainly by high C₁₉/C₂₃ tricyclic terpane (>0.75), high C₂₄ tetracyclic terpane/C₂₆ tricyclic terpane (>2.5), low gammacerane/αβ C₃₀ hopane (<0.15) and low 4-methyl steranes/ΣC₂₉ steranes (<0.15) ratios, and was deposited in sub-oxic to anoxic environments with significant terrigenous organic matter input. The first (E₂s₁, 35.8–32.8 Ma) and third (E₂s₃, 43.0–38.0 Ma) members of the Eocene Shahejie Formation have low C₁₉/C₂₃ tricyclic terpane and low C₂₄ tetracyclic terpane/C₂₆ tricyclic terpane ratios and were deposited in anoxic environments with minor terrestrial organic matter input, but have different abundances of 4-methyl steranes and gammacerane. The hydrocarbon-generating potential and biomarker associations of these three source rock intervals were controlled by tectonic evolution of the sub-basin and climate changes. Three oil families derived from E₂s₃, E₂s₁ and E₃d, respectively, and three types of mixed oils have been identified. All large oil fields in the Bozhong sub-basin display considerable heterogeneities in biomarker compositions and originated from more than one source rock interval, which suggests that mixing of oils derived from multiple source rock intervals or multiple generative kitchens, and/or focusing of oils originated from a large area of a generative kitchen, is essential for the formation of large oil fields in the Bozhong sub-basin. E₂s₃- and E₂s₁-derived oils experienced relatively long-distance lateral migration and accumulated in traps away from the generative kitchen. E₃d₃-derived oils had migrated short distances and accumulated in traps closer to the generative kitchen. Such a petroleum distribution pattern has important implications for future exploration. There is considerable exploration potential for Dongying-derived oils in the Bozhong sub-basin, and traps close to or within the generative kitchens have better chance to contain oils generated from the Dongying Formation.     

东海西部台北坳陷中生界油气成藏模式研究

东海陆架盆地是在元古界、古生界复杂基底上发展形成的中—新生代叠合含油气盆地,蕴藏丰富的油气资源,对中国的能源战略意义重大,东海陆架盆地中生界将是未来中国东部油气勘探新区及重要领域。台北坳陷位于东海陆架盆地西部坳陷带南部,中生界陆相河湖沉积体系发育福州组泥岩、石门潭组泥岩两套主力烃源岩。根据台北坳陷烃源岩、储盖组合、输导体系等油气藏形成的静态要素为基础,结合油气来源和成藏期次分析,研究其成藏过程,明确了台北坳陷以福州组、石门潭组为主力烃源,福州组、石门潭组及始新世瓯江组砂体为主要储层,油气沿着凹陷边缘的砂体、活动的断层运移,砂体尖灭和断层封闭时期,油气早期成藏;始新世末－渐新世的全盆性挤压反转,油气重新调整聚集成藏。从而建立起台北坳陷中生界早期成藏,后期调整的成藏模式。     

渤中凹陷北部古近系底部不整合发育特征及其与油气成藏关系

不整合是油气输导体系中的重要组成部分,也是油气成藏的有利场所。以渤海湾盆地渤中凹陷北部为例,综合利用三维地震资料、钻测井资料和岩心薄片资料,对古近系底部不整合发育特征及其对油气成藏影响进行分析。结果表明,渤中凹陷北部古近系底部不整合包括5种接触类型和7种结构类型。不整合通道类型划分为双运移通道I型、单运移通道Ⅱ1型和Ⅱ2型以及封堵型Ⅲ型,单运移通道Ⅱ1型是研究区主要的不整合油气成藏类型。运聚类型包括侧向型、圈闭+侧向型、垂向+侧向型和封堵型4种。不整合之上砂岩输导油气的能力与砂岩微观特征及裂缝倾角相关,半风化岩层对油气的输导储集能力受控于原始孔渗良好且不易形成风化黏土层的火山岩。     

Depositional and diagenetic controls on deeply-buried Eocene sublacustrine fan reservoirs in the Dongying Depression,Bohai Bay Basin,China

This paper investigates the reservoir potential of deeply-buried Eocene sublacustrine fan sandstones in the Bohai Bay Basin, China by evaluating the link between depositional lithofacies that controlled primary sediment compositions, and diagenetic processes that involved dissolution, precipitation and transformation of minerals. This petrographic, mineralogical, and geochemical study recognizes a complex diagenetic history which reflects both the depositional and burial history of the sandstones. Eogenetic alterations of the sandstones include: 1) mechanical compaction; and 2) partial to extensive non-ferroan carbonate and gypsum cementation. Typical mesogenetic alterations include: (1) dissolution of feldspar, non-ferroan carbonate cements, gypsum and anhydrite; (2) precipitation of quartz, kaolinite and ferroan carbonate cements; (3) transformation of smectite and kaolinite to illite and conversion of gypsum to anhydrite. This study demonstrates that: 1) depositional lithofacies critically influenced diagenesis, which resulted in good reservoir quality of the better-sorted, middle-fan, but poor reservoir quality in the inner- and outer-fan lithofacies; 2) formation of secondary porosity was spatially associated with other mineral reactions that caused precipitation of cements within sandstone reservoirs and did not greatly enhance reservoir quality; and 3) oil emplacement during early mesodiagenesis (temperatures > 70 °C) protected reservoirs from cementation and compaction.     

Lacustrine massive mudrock in the Eocene Jiyang Depression,Bohai Bay Basin,China: Nature,origin and significance

Massive mudrock refers to mudrock with internally homogeneous characteristics and an absence of laminae. Previous studies were primarily conducted in the marine environment, while notably few studies have investigated lacustrine massive mudrock. Based on core observation in the lacustrine environment of the Jiyang Depression, Bohai Bay Basin, China, massive mudrock is a common deep water fine-grained sedimentary rock. There are two types of massive mudrock. Both types are sharply delineated at the bottom and top contacts, abundant in angular terrigenous debris, and associated with oxygen-rich (higher than 2 ml O₂/L H₂O) but lower water salinities in comparison to adjacent black shales. In addition, type 1 is laterally isolated and contains abundant sand injections and contorted layers formed in the depositional process, but type 2 exactly distributes in the distal part of deep water gravity-driven sandstone units, and shows scoured bases, high-angle mineral crytsals, and fining-upward trend. It is suggested that type 1 is a muddy mass transport deposit (MMTD) formed by slide, slump, and/or debris flow, and type 2 is a turbiditic mudrock deposited by settling from dilute turbidity currents. A warm and humid climate and high subsidence rate are two main triggering events. Because of its mass movement nature, MMTD preserves the mineralogic composition and organic matter characteristics of the source sediment. By contrast, dilute turbidity currents are able to greatly entrain biochemically-formed micrite and planktonic organisms from the water column, and deposit them in the turbiditic mudrock. Because of their different ability to deposit organic matter, MMTD have poor or fair source rock potential, but the turbiditic mudrock is able to be a potentially effective source rock. The minerals in the massive mudrock are disorganized and chaotic, which cause fractures to develop in various directions, thereby, enhancing the vertical migration of oil and gas molecules to horizontal wellbore in shale reservoir exploitation.     

Salt structures in the Laizhouwan depression, offshore Bohai Bay basin, eastern China: New insights from 3D seismic data

Using the new high-quality 3D seismic data, this paper addresses the salt structures in the KL11 area of the Laizhouwan depression in the southern offshore Bohai Bay basin. In the study area, the salt in the Sha-4 Member of the Paleogene Shahejie Formation thickened, and then formed an S–N trending salt wall, which changes shape regularly along its trend from salt diapir to salt pillow. The change in thickness of the suprasalt layers record five growth phases of the salt wall from the Eocene to the Quaternary: (1) early diapirism, (2) active diapirism, (3) passive diapirism, (4) relative structural quiescence, and (5) arching. The evolution of the salt structures was mostly governed by the multi-phase compression induced by the dextral strike-slip of the Tan–Lu fault, which formed a restraining bend in the study area. There was an original passive stock in the south, which was later tectonically squeezed by E–W compression and became a diapir. As the shortening propagated to the north from the original stock, the salt pillow was created in the north. Relative structural quiescence then followed until the next phase of compression, which arched the thick roof of the salt wall.     

Alteration and multi-stage accumulation of oil and gas in the Ordovician of the Tabei Uplift,Tarim Basin,NW China: Implications for genetic origin of the diverse hydrocarbons

The Ordovician is the most important exploration target in the Tabei Uplift of the Tarim Basin, which contains a range of petroleum types including solid bitumen, heavy oil, light oil, condensate, wet gas and dry gas. The density of the black oils ranges from 0.81 g/cm³ to 1.01 g/cm³ (20 °C) and gas oil ratio (GOR) ranges from 4 m³/m³ to 9300 m³/m³. Oil-source correlations established that most of the oils were derived from the Mid-Upper Ordovician marine shale and carbonate and that the difference in oil properties is mainly attributed to hydrocarbon alteration and multi-stage accumulation. In the Tabei Uplift, there were three main periods of hydrocarbon accumulation in the late Caledonian stage (ca. 450–430 Ma), late Hercynian stage (ca. 293–255 Ma) and the late Himalayan stage (ca. 12–2 Ma). The oil charging events mainly occurred in the late Caledonian and late Hercynian stage, while gas charging occurred in the late Hercynian stage. During the late Caledonian stage, petroleum charged the reservoirs lying east of the uplift. However, due to a crustal uplifting episode in the early Hercynian (ca. 386–372 Ma), most of the hydrocarbons were transformed by processes such as biodegradation, resulting in residual solid bitumen in the fractures of the reservoirs. During the late Hercynian Stage, a major episode of oil charging into Ordovician reservoirs took place. Subsequent crustal uplift and severe alteration by biodegradation in the west-central Basin resulted in heavy oil formation. Since the late Himalayan stage when rapid subsidence of the crust occurred, the oil residing in reservoirs was exposed to high temperature cracking conditions resulting in the production of gas and charged from the southeast further altering the pre-existing oils in the eastern reservoirs. A suite of representative samples of various crude oils including condensates, lights oils and heavy oils have been collected for detailed analysis to investigate the mechanism of formation. Based on the research it was concluded that the diversity of hydrocarbon physical and chemical properties in the Tabei Uplift was mainly attributable to the processes of biodegradation and gas washing. The understanding of the processes is very helpful to predict the spatial distribution of hydrocarbon in the Tabei Uplift and provides a reference case study for other areas.     

Effects of oil cracking on fluorescence color,homogenization temperature and trapping pressure reconstruction of oil inclusions from deeply buried reservoirs in the northern Dongying Depression,Bohai Bay Basin,China

The effects of oil cracking on fluorescence color, homogenization temperature (T_ho) and trapping pressure (P_t) of oil inclusions from deeply buried reservoirs (DBRs) (3672–4359 m) in the northern Dongying Depression were determined based on fluorescence spectroscopy and homogenization temperatures of oil inclusions, kinetic modeling of crude oil cracking, and petroleum inclusion thermodynamics modeling.The modeling results demonstrate that fluorescence color, T_ho and predicted P_t have strong relationships with the transformation rate via cracking of oil to gas (T_r), and the formation temperature (T_f) that the inclusions experienced. The fluorescence color is hardly influenced at all during the initial stages of oil cracking (T_r < 13%, T_f < 160 °C), but fluorescence color begins to shift toward shorter wavelengths (blue shift) during progressive oil cracking (T_r < 24%, T_f < 190 °C). With further oil cracking, the fluorescence color may either experience no change or continue its blue shift. Eventually the fluorescence color will disappear as the aromatic compounds are completely cracked. The T_ho increases at first (T_r < 24%, T_f < 190 °C), but then decreases or even becomes negative during major oil cracking. The reconstructed P_t values show a corresponding reverse trend.Oil inclusions from DBRs and other shallow reservoirs in the Dongying Depression show an obvious blue shift in fluorescence color at a depth of approximate 4000 m (T_f = 160 °C) and generally contain solid bitumen below 4000 m, supporting the effect of oil cracking on fluorescence variation, consistent with the modeling result. The T_ho from DBRs in the Minfeng area increases with increasing burial depth (T_f < 190 °C), which is also consistent with the modeling results. However, the T_ho of oil inclusions with blue-white fluorescence from DBRs in the Shengtuo area did not show such a trend. Recent trapping, high trapping pressure and higher-maturity oil may have led to a low-degree of oil cracking, and thus less modification of T_ho in the Shengtuo area.Oil cracking results in consistent volume ratios of pyrobitumen to oil inclusions (F_vpy) in the same fluid inclusion assemblage, and the F_vpy value increases with oil cracking level, which can be used to recognize if oil cracking has occurred in oil inclusions and what level of oil cracking they have experienced.As the oil cracking model used in this study did not account for the role of pressure, it is more applicable for oil inclusions that were trapped under normally pressured conditions. Oil inclusions trapped under overpressured conditions will be less influenced by oil cracking.     

Organic material accumulation of Carnian mudstones in the North Qiangtang Depression,eastern Tethys: Controlled by the paleoclimate,paleoenvironment, and provenance

The Late Triassic mudstones are considered to be the most significant hydrocarbon source rocks (TOC: 0.54%–3.29%) in the North Qiangtang Depression, eastern Tethys. Here, we present geochemical data from the Woruo Mountain Carnian mudstones, in order to investigate their paleoclimate, paleoenvironment, and provenance and to analyze the mechanism of organic material accumulation. The paleoclimate condition was warm and humid during the Carnian mudstones deposition, as indicated by moderate chemical index of alteration (CIA; 73–76), which may be connected with the Late Triassic Carnian stage global climate event in the Tethys. The low U/Th (0.17–0.25) and C_org/P_tot (7–33) ratio values and moderate manganese contents, reflect the oxidizing conditions during the Carnian mudstones deposition. The relatively high primary productivity in this study is supported by the relatively high P concentrations. The Al₂O₃–(CaO* + Na₂O)–K₂O ternary plot and Th/Sc–Zr/Sc crossplot reflect that the source areas have undergone a medium chemical weathering with weak sedimentary recycling. The TiO₂–Zr, Co/Th–La/Sc, La/Th–Hf, and La/Yb–∑REE bivariate diagrams indicate that the provenance of Carnian mudstones was primarily from felsic igneous rocks. The collision setting has been identified based on the multi-major elements discriminate plots in the present study. The Riwanchaka and Mayigangri masses to the southwest and south of the study area consisting mainly of Middle-Triassic granodiorite and Late-Triassic granite are likely responsible for supplying provenance to the Woruo Mountain Carnian mudstones, which have similar REE patterns. The relatively high TOC contents of Carnian mudstones are related to high paleoproductivity and fast sedimentation rates, which will lead to preservation of some organic matter even when bottom waters are completely oxidizing. The detrital input during the Carnian mudstones deposition would result in dilution of organic matter.     

Field Observation and Analysis of Wave-Current-Sediment Movement in Caofeidian Sea Area in the Bohai Bay, China

In order to study the mechanism of flow-sediment movement, it is essential to obtain measured data of water hydrodynamic and sediment concentration process with high spatial and temporal resolution in the bottom boundary layer (BBL). Field observations were carried out in the northwest Caofeidian sea area in the Bohai Bay. Near 2 m isobath (under the lowest tidal level), a tripod system was installed with AWAC (Acoustic Wave And Current), ADCP (Acoustic Doppler Current Profilers), OBS-3A (Optical Backscatter Point Sensor), ADV (Acoustic Doppler Velocimeters), etc. The accurate measurement of the bottom boundary layer during a single tidal period was carried out, together with a long-term sediment concentration measurement under different hydrological conditions. All the measured data were used to analyze the characteristics of wave-current-sediment movement and the BBL. Analysis was performed on flow structure, shear stress, roughness, eddy viscosity and other parameters of the BBL. Two major findings were made. Firstly, from the measured data, the three-layer distribution model of the velocity profiles and eddy viscosities in the wave-current BBL are proposed in the observed sea area; secondly, the sediment movement is related closely to wind-waves in the muddy coast area where sediment is clayey silt: 1) The observed suspended sediment concentration under light wind conditions is very low, with the peak value generally smaller than 0.1 kg/m3 and the average value being 0.03 kg/m3; 2) The sediment concentration increases continuously under the gales over 6-7 in Beaufort scale, under a sustained wind action. The measured peak sediment concentration at 0.4 m above the seabed is 0.15-0.32 kg/m3, and the average sediment concentration during wind-wave action is 0.08-0.18 kg/m3, which is about 3-6 times the value under light wind conditions. The critical wave height signaling remarkable changes of sediment concentration is 0.5 m. The results show that the suspended load sediment concentration is mainly influenced by wave-induced sediment suspension.     

Geochemistry,origin and accumulation of natural gases in the deepwater area of the Qiongdongnan Basin,South China Sea

The Qiongdongnan Basin, South China Sea has received huge thickness (>12 km) of Tertiary-Quaternary sediments in the deepwater area to which great attention has been paid due to the recent discoveries of the SS22-1 and the SS17-2 commercial gas fields in the Pliocene-Upper Miocene submarine canyon system with water depth over 1300 m. In this study, the geochemistry, origin and accumulation models of these gases were investigated. The results reveal that the gases are predominated by hydrocarbon gases (98%–99% by volume), with the ratio of C₁/C_1-5 ranging from 0.92 to 0.94, and they are characterized by relatively heavy δ¹³C₁ (−36.8‰ to −39.4‰) and δD_CH4 values (−144‰ to −147‰), similar to the thermogenic gases discovered in the shallow water area of the basin. The C_5-7 light hydrocarbons associated with these gases are dominated by isoparaffins (35%–65%), implying an origin from higher plants. For the associated condensates, carbon isotopic compositions and high abundance of oleanane and presence of bicadinanes show close affinity with those from the YC13-1 gas field in the shallow water area. All these geochemical characteristics correlate well with those found in the shales of the Oligocene Yacheng Formation in the Qiongdongnan Basin. The Yacheng Formation in the deepwater area has TOC values in the range of 0.4–21% and contains type II_b–III gas-prone kerogens, indicating an excellent gas source rock. The kinetic modeling results show that the δ¹³C₁ values of the gas generated from the Yacheng source rock since 3 or 4 Ma are well matched with those of the reservoir gases, indicating that the gas pool is young and likely formed after 4 Ma. The geologic and geochemical data show that the mud diapirs and faults provide the main pathways for the upward migration of gases from the deep gas kitchen into the shallow, normally pressured reservoirs, and that the deep overpressure is the key driving force for the vertical and lateral migration of gas. This gas migration pattern implies that the South Low Uplift and the No.2 Fault zone near the deepwater area are also favorable for gas accumulation because they are located in the pathway of gas migration, and therefore more attention should be paid to them in the future.     

Field observation and analysis of wave-current-sediment movement in Caofeidian Sea area in the Bohai Bay,China

In order to study the mechanism of flow-sediment movement, it is essential to obtain measured data of water hydrodynamic and sediment concentration process with high spatial and temporal resolution in the bottom boundary layer （BBL）. Field observations were carried out in the northwest Caofeidian sea area in the Bohai Bay. Near 2 m isobath （under the lowest tidal level）, a tripod system was installed with AWAC （Acoustic Wave And Current）, ADCP （Acoustic Doppler Current Profilers）, OBS-3A （Optical Backscatter Point Sensor）, ADV （Acoustic Doppler Velocimeters）, etc. The accurate measurement of the bottom boundary layer during a single tidal period was carried out, together with a long-term sediment concentration measurement under different hydrological conditions. All the measured data were used to analyze the characteristics of wave-current-sediment movement and the BBL. Analysis was performed on flow structure, shear stress, roughness, eddy viscosity and other parameters of the BBL. Two major findings were made. Firstly, from the measured data, the three-layer distribution model of the velocity profiles and eddy viscosities in the wave-current BBL are proposed in the observed sea area; secondly, the sediment movement is related closely to wind-waves in the muddy coast area where sediment is clayey silt： 1） The observed suspended sediment concentration under light wind conditions is very low, with the peak value generally smaller than 0.1 kg/m^3 and the average value being 0.03 kg/m^3; 2） The sediment concentration increases continuously under the gales over 6-7 in Beaufort scale, under a sustained wind action. The measured peak sediment concentration at 0.4 m above the seabed is 0.15-0.32 kg/m^3, and the average sediment concentration during wind-wave action is 0.08-0.18 kg/m^3, which is about 3-6 times the value under light wind conditions. The critical wave height signaling remarkable changes of sediment concentration is 0.5 m. The results show that the suspended load sediment concentration is mainly influenced by wave-induced sediment suspension.