Discovery and inspiration of large-and medium-sized glutenite-rich oil and gas fields in the eastern South China Sea:An example from Paleogene Enping Formation in Huizhou 26 subsag,Pearl River Mouth Basin

Based on the practice of oil and gas exploration in the Huizhou Sag of the Pearl River Mouth Basin,the geochemical indexes of source rocks were measured,the reservoir development morphology was restored,the rocks and minerals were characterized microscopically,the measured trap sealing indexes were compared,the biomarker compounds of crude oil were extracted,the genesis of condensate gas was identified,and the reservoir-forming conditions were examined.On this basis,the Paleogene Enping Formation in the Huizhou 26 subsag was systematically analyzed for the potential of oil and gas resources,the development characteristics of large-scale high-quality conglomerate reservoirs,the trapping effectiveness of faults,the hydrocarbon migration and accumulation model,and the formation conditions and exploration targets of large-and medium-sized glutenite-rich oil and gas fields.The research results were obtained in four aspects.First,the Paleogene Wenchang Formation in the Huizhou 26 subsag develops extensive and thick high-quality source rocks of semi-deep to deep lacustrine subfacies,which have typical hydrocarbon expulsion characteristics of"great oil generation in the early stage and huge gas expulsion in the late stage",providing a sufficient material basis for hydrocarbon accumulation in the Enping Formation.Second,under the joint control of the steep slope zone and transition zone of the fault within the sag,the large-scale near-source glutenite reservoirs are highly heterogeneous,with the development scale dominated hierarchically by three factors(favorable facies zone,particle component,and microfracture).The(subaqueous)distributary channels near the fault system,with equal grains,a low mud content(<5%),and a high content of feldspar composition,are conducive to the development of sweet spot reservoirs.Third,the strike-slip pressurization trap covered by stable lake flooding mudstone is a necessary condition for oil and gas preservation,and the NE and nearly EW faults obliquely to the principal stress have the best control on traps.Fourth,the spatiotemporal configuration of high-quality source rocks,fault transport/sealing,and glutenite reservoirs controls the degree of hydrocarbon enrichment.From top to bottom,three hydrocarbon accumulation units,i.e.low-fill zone,transition zone,and high-fill zone,are recognized.The main area of the channel in the nearly pressurized source-connecting fault zone is favorable for large-scale hydrocarbon enrichment.The research results suggest a new direction for the exploration of large-scale glutenite-rich reservoirs in the Enping Formation of the Pearl River Mouth Basin,and present a major breakthrough in oil and gas exploration.

Tectonic-thermal history and hydrocarbon potential of the Pearl River Mouth Basin,northern South China Sea:Insights from borehole apatite fission-track thermochronology

The Pearl River Mouth Basin(PRMB)is one of the most petroliferous basins on the northern margin of the South China Sea.Knowledge of the thermal history of the PRMB is significant for understanding its tectonic evolution and for unraveling its poorly studied source-rock maturation history.Our investigations in this study are based on apatite fission-track(AFT)thermochronology analysis of 12 cutting samples from 4 boreholes.Both AFT ages and length data suggested that the PRMB has experienced quite complicated thermal evolution.Thermal history modeling results unraveled four successive events of heating separated by three stages of cooling since the early Middle Eocene.The cooling events occurred approximately in the Late Eocene,early Oligocene,and the Late Miocene,possibly attributed to the Zhuqiong II Event,Nanhai Event,and Dongsha Event,respectively.The erosion amount during the first cooling stage is roughly estimated to be about 455-712 m,with an erosion rate of 0.08-0.12 mm/a.The second erosion-driven cooling is stronger than the first one,with an erosion amount of about 747-814 m and an erosion rate between about 0.13-0.21 mm/a.The erosion amount calculated related to the third cooling event varies from 800 m to 3419 m,which is speculative due to the possible influence of the magmatic activity.

Petroleum Migration Characteristics in the Northeastern Part of the Baiyun Depression,Pearl River Mouth Basin,South China Sea

This paper investigates the origin and migration characteristics of petroleum in the northeastern part of the Baiyun Depression,Pearl River Mouth Basin(PRMB).The discovered petroleum in the study area is mainly located in the Lower Zhujiang Member(N_(1)z^(2))and mainly originated from the Enping Formation source rocks in the eastern sag.Active faults(vertical migration)and N_(1)z^(2)sandstones(lateral migration)acted as the petroleum migration systems.The fault activities in the Dongsha event controlled the episodic petroleum migration.Fractures in the fault zones provided effective conduits,and overpressure was the driving force.The vertical migration could not cross the fault zones laterally.The petroleum injection areas in the carrier beds were the contact zones of petroleum-migration faults and carrier beds.The lateral migration was steady-state migration,and buoyancy was the driving force.The migration pathways in the carrier beds were controlled by the structural morphology.Secondary petroleum migration in the study area could be divided into two parts:vertical migration along the fractures in the fault zones and lateral migration through preferential petroleum migration pathways(PPMPs)in the carrier beds.The petroleum migration behaviors,including migrating direction,driving force,and migration pattern,in the faults and sandstone carrier beds were quite different.This study provides a typical example for comprehending secondary migration processes and has great importance for determining future exploration targets in the deep-water area of the PRMB.

Microbial Community Composition and Function in Sediments from the Pearl River Mouth Basin

This study was conducted to characterize the diversity and function of microbial communities in marine sediments of the Pearl River Mouth Basin(PRMB)in the South China Sea.The results showed that the bacterial and archaeal communities varied greatly with depth.Proteobacteria in bacterial communities and Nitrososphaeria and Woesearchaeota in archaeal communities were dominant in the shallow sediments(1-40 cm),while Chloroflexi in bacterial communities and Bathyarchaeia in archaeal communities were dominant in the deep sediments(50-200 cm).Regarding ecological functions based on the metatranscriptomic data,genes involved in various pathways of nitrogen metabolism and sulfur metabolism were observed in the tested sediment samples.Metagenomic analysis revealed that Proteobacteria contribute the most to nearly all genes involved in nitrogen and sulfur metabolism.Moreover,Thaumarchaeota contribute the most to certain genes involved in nitrification,denitrification and assimilatory sulfate reduction pathways.The most abundant bacterial genus,Candidatus Scalindua,is crucial for nitrification,dissimilatory nitrate reduction,denitrification and assimilatory sulfate reduction pathways.

Structural Characteristics and its Significances on Hydrocarbon Accumulation in the Yunkai Low Uplift,Pearl River Mouth Basin

The Yunkai low uplift with low exploration degree is close to the Baiyun sag,and has hydrocarbon exploration potential in the deepwater area of the Pearl River Mouth Basin.Based on seismic and drilling data,balanced profiles and growth strata,this paper mainly discusses geological structures and formation processes of the Yunkai low uplift,and also analyzes the characteristics of fault system and their influence on hydrocarbon migration and accumulation.The EWtrending basement faults divide the Yunkai low uplift into two parts,i.e.the southern sector and the northern sector.The northern sector is a relatively wide and gentle uplift,while the southern sector is composed of two secondary half-grabens with faulting in the south and overlapping in the north.The Yunkai low uplift experienced three major formation stages,including the rapid uplifting stage during the deposition period of the Eocene Wenchang Formation,the slow uplifting stage during the deposition period of the Late Eocene-Middle Miocene Enping-Hanjiang formations,and the whole burial stage from the Middle Miocene to present.The extensional faults in the Yunkai low uplift and its adjacent areas strike mainly along the NW,NWW and near-EW directions.Also,the strikes of faults present a clockwise rotation from the deep to the shallow strata.According to effects of faults on hydrocarbon accumulation the faults in the Yunkai low uplift and its adjacent areas can be divided into trap-controlled faults and source-controlled faults.The trap-controlled faults control trap development and can effectively seal oil and gas.The source-controlled faults connect directly source rocks and reservoirs,which are highly active during the rifting stage and weakly active since the Miocene.This activity features of the source-controlled faults is beneficial to migration of the early crude oil from the Baiyun sag to the high part of the Yunkai low uplift,but is not good for migration of the late natural gas.In the Yunkai low uplift and its adjacent areas,the traps in the deep Zhuhai and Enping formations that are close to source rocks in the Baiyun sag should be the favorable exploration objectives.

Huizhou Movement and its significance in Pearl River Mouth Basin, China

The Huizhou Movement refers to the Middle Eocene tectonic transition from the early to the late Wenchang Rifting stage(about 43 Ma ago) in the Pearl River Mouth Basin. Based on seismic reflection, drilling, logging and geological data, fault characteristic analysis, denudation thickness recovery, magmatism statistics, regional tectonic dynamics comparison and other methods are used to reveal the characteristics, properties and dynamic mechanism of the Huizhou Movement. The Huizhou Movement mainly shows the North-South transition of rifting and the migration along the faults, basement uplift, magmatic diapir and stratigraphic denudation. It is believed that the Huizhou Movement is a comprehensive reflection of plate interaction and lithospheric thinning process in the Pearl River Mouth Basin, which is closely related to the transition of lithosphere from initial rifting to rapid thinning, the India-Eurasia hard collision and the change of subduction direction of the Pacific plate. The Huizhou Movement has significant influence and control on the Paleogene hydrocarbon-generating sags and the development of hydrocarbon source rocks, sedimentary system and deep high-quality reservoir, hydrocarbon migration and accumulation in the Pearl River Mouth Basin.

Structural Variability and Rifting Process of the Segmented Cenozoic Pearl River Mouth Basin,Northern Continental Margin of the South China Sea

The Pearl River Mouth Basin(PRMB)is an important area for studying the evolution of continental marginal basins in the northern South China Sea(SCS),but the structural variability and spatiotemporal rifting process remains poorly understood.This study investigates the differential structural features of the eastern,middle and western PRMB,as well as the extensional deformation laws in operation during the rifting stage,according to an integrated analysis of geometric characteristics and kinematic parameters,i.e.,horizontal displacement and stretching factors of basin and crust.The PRMB underwent at least three phases of intense extension,which varied in time and space.(1)During the middle Eocene,most sags in the PRMB were intensely stretched and high-angle planar to listric boundary faults controlled the wedge-shaped stratigraphic geometry.(2)During the late Eocene-to-early Oligocene,the stratigraphic geometry of the sags was slightly wedge-shaped and continuously controlled by boundary faults,however,the extensional strength decreased relatively in the Northern depression zone,but increased in the Southern depression zone.(3)During the late Oligocene,the extension was extremely weak in the northeast PRMB,but relatively strong in the southwest PRMB,leading to tabular stratigraphic geometry in the northeast PRMB,but localized slightly wedge-shaped stratigraphic geometry in the southwest.The southwest PRMB still underwent relatively strong extension during the early Miocene.The southwest PRMB that was induced by a small-scale localized mantle convection system constantly rifted during the late Oligocene,controlled by the weak lithosphere,westward(southwestward)diachronous opening and southward jump of the ocean ridge.The applied quantitative parameters and spatiotemporal rifting process may be used as a reference with which to study the segmented continental margin rifts.

Chlorite cement and its effect on the reservoir quality of sandstones from the Panyu low-uplift,Pearl River Mouth Basin

Based on porosity and permeability measurements,mercury porosimetry measurements,thin section analyses,SEM observations,X-ray diffraction(XRD) analysis and granulometric analyses,diagenetic features of reservoir sandstones taken from the Zhuhai formation in the Panyu low-uplift of the Pear River Mouth Basin were examined.This study shows that chlorite cements are one of the most important diagenetic features of reservoir sandstones.The precipitation of chlorite was controlled by multiple factors and its development occurred early in eo-diagenesis and continued till Stage A of middle diagenesis.The precipitation of chlorite at the early stage was mainly affected by the sedimentary environment and provenance.Abundant Fe-and Mg-rich materials were supplied during the deposition of distributary channel sediments in the deltaic front setting and mainly in alkaline conditions.With the burial depth increasing,smectite and kaolinite tended to be transformed into chlorite.Smectite cements were completely transformed into chlorite in sandstones of the studied area.Volcanic lithics rich in Fe and Mg materials were dissolved and released Fe2+ and Mg 2+ into the pore water.These cations precipitated as chlorite cements in middle diagenesis in an alkaline diagenetic environment.Chlorite coatings acted as porosity and permeability,thus helping preserve cements in the chlorite cemented sandstones.The reservoir quality of chlorite cemented sandstones is much better than sandstones without chlorite cements.Chlorite cements play an important role in the reservoir evolution that was mainly characterized by preserving intergranular porosity and forming better pore-throat structures of sandstones.

Genetic pattern of belt-wide petroliferous phenomenon in the eastern Pearl River Mouth Basin and its practical application

The Pearl River Mouth Basin(PRMB) covers an area of approximately 20×104 km2. However, oil-gas fields detected in this area thus far are highly concentrated and controlled predominantly by second-order structural belts, the seven largest of which aggregate proved oil reserves of 7.7×108 m3, accounting for 86% of the total discovered reserve in the basin. These second-order structures have one common phenomenon: oil is contained in all traps present in them. In other words, they are all belt-wide petroliferous reservoirs. Research has identified eight types of second-order structural belts under two categories in the eastern PRMB. Their petroliferous properties are subject to three typical constraints: petroliferous properties of subsags hosting these structural belts, locations of these belts in the petroleum system, and availability of traps prior to the hydrocarbon expulsion and migration. The formation and distribution of oil reservoirs in these belts are characterized by subsag–belt integration and "three-in-one". The former indicates that sags and the second-order structural belts within the supply range of the sags constitute the basic units of hydrocarbon accumulations and are therefore inseparable. The latter indicates that a belt-wide petroliferous second-order structural belt always contains three important elements: hydrocarbon richness, effective pathway and pre-existing traps.

Detrital K-feldspar^(40)Ar/^(39)Ar Ages:Source Constraints of the Lower Miocene Sandstones in the Pearl River Mouth Basin,South China Sea

The South China Sea began to outspread in the Oligocene.A great quantity of terraneous detritus was deposited in the northern continental shelf of the sea,mostly in Pearl River Mouth Basin,which constituted the main paleo-Pearl River Delta.The delta developed for a long geological time and formed a superimposed area.Almost all the oil and gas fields of detrital rock reservoir distribute in this delta.Thirty-three oil sandstone core samples in the Zhujiang Formation,lower Miocene(23-16 Ma),were collected from nine wells.The illite samples with detrital K feldspar(Kfs) separated from these sandstone cores in four sub-structural belts were analysed by the high-precision 40Ar/39Ar laser stepwise heating technique.All 33 illite 40Ar/39Ar data consistently yielded gradually rising age spectra at the low-temperature steps until reaching age plateaus at mid-high temperature steps.The youngest ages corresponding to the beginning steps were interpreted as the hydrocarbon accumulation ages and the plateau ages in mid-high temperature steps as the contributions of the detrital feldspar representing the ages of the granitic parent rocks in the provenances.The ages of the detrital feldspar from the Zhujiang Formation in the four sub-structural belts were different:(1) the late Cretaceous ages in the Lufeng 13 fault structural belt;(2) the late Cretaceous and early Cretaceous-Jurassic ages in the Huizhou 21 buried hill-fault belt;(3) the Jurassic and Triassic ages in the Xijiang 24 buried hill-fault belt;and(4) the early Cretaceous-late Jurassic ages in the Panyu 4 oil area.These detrital feldspar 40Ar/39Ar ages become younger and younger from west to east,corresponding to the age distribution of the granites in the adjacent Guangdong Province,Southern China.

Deep-water Depositional Features of Miocene Zhujiang Formation in Baiyun Sag,Pearl River Mouth Basin

十一张岩相和五个岩相协会根据详细核心分析是在中新世的 Zhujiang 形成的 indentified。三 depositional 打发达、也就是海底的扇子，盆和深海的拖拉，这能被决定电流六 microfacies 进一步在扇子以内被认出，包括在内部扇子，的主要隧道在中间的扇子，的支流隧道内部隧道，早朝和外部扇子。更低的 Zhujiang 形成，主要沙岩协会，是内部扇子和斜坡扇子。中间的部分，主要 mudstone 协会，是外部扇子存款。与违反，海底的扇子被 basinal 最后代替被 mudstone 协会， siltstone 协会，和深海的石灰石协会统治的远洋的存款。在弱严肃流动活动，更低的隧道，中间上面的外部扇子和盆期间，存款被深海的拖拉水流强烈修改。在中新世的 Zhujiang 形成的深海的拖拉免职的鉴定将是很重要的。自从迟了的渐新世(23.8 妈) ，深海的拖拉水流在 Baiyun 在架裂缝形成以后一直在存在，这能被推断下垂，影响并且控制沉积作文，分发，和 depositional 处理。它将在北华南海提供大启发给 paleo 海洋的当前的发行量。

Astronomical Dating of the Middle Miocene Hanjiang Formation in the Pearl River Mouth Basin,South China Sea

Langhian 年龄的 Hanjiang 形成(中间中新世) 在珍珠河嘴盆(PRMB ) ，华南海由三角形的 siliciclastic 和浅海的架组成焦化有韵律的引申，它形成盆的潜在的水库之一。在盆为勘探的烃和探索改进 stratigraphic 决定，现在的学习承担光谱高分辨率的自然 gamma 光线(NGR ) 的分析决定主导的频率部件并且测试 Milankovitch 轨道的信号是否在有韵律的继任被记录的记载得好的记录。分析结果显示领前(19 ka 和 23 ka ) 的轨道的周期，斜(41 ka ) ，和怪癖(100 ka 和 405 ka ) ，它在有韵律的引申继任为天文驾驶的气候变化提供充分证据。在 biochronological 限制以内，高分辨率的天体的 timescale 通过到地球的轨道的最近的天文计算的变化的 NGR 记录的天体的调节被构造。天文调节的 timescale 能被使用为在整个时期认出的地质的事件和 bioevents 计算天体的年龄。分别地，有孔虫目动物 Globorotalia peripheroronda 和 Globigerinoides sicanus 的第一 downhole 出现在 14.546 妈和 14.919 妈是过时的它与在华南海的更早的估计稍微不同。什么时候与全球海平面的变化相比制图，为基于微化石分布认出的序列的天体的估计有一样的结束时间但是不同开始时间。这可能由于在推迟了全球海平面的升起的效果的海面升降的上升附加的本地或地区性的构造活动。天体的 timescale 也为 Langhian 阶段解决 depositional 进化历史(中间中新世) 与一个变化，那强烈类似于从 13.65 妈预言到 15.97 妈的地球的轨道的怪癖的。我们推断在 Hanjiang 形成控制沉积率的可变性的主要因素与 405-ka-period 怪癖有关。

Characteristics of the turbidite fan in the Wenchang Formation of the Enping Sag, Pearl River Mouth Basin, China and its hydrocarbon significance

A turbidite fan in the Eocene upper Wenchang Formation in the Enping Sag, Pearl River Mouth Basin (PRMB) has been studied using seismic, logging and borehole data. The fan is characterized by parallel progradation on the dip seismic profile and is mound-shaped or lenticular-shaped on the strike seismic profile. The study of the core and logging data from well EP17-3-1, which is located in the front side of the turbidite fan, shows that this fan is a set of normal grading sand beds, interbedded within thick dark grey mudstones of semi-deep to deep lake deposits in the Wenchang Formation. The fan is interpreted as a sand/mud-rich turbidite fan that has an area of over 140 km2 and a maximum thickness of over 340 m. Combined with a study of the regional geological background and previous provenance analysis of the Eocene Wenchang Formation, the main potential provenances for the turbidite fan are considered to be the Panyu low-uplift and northern fault terrace zone. The Enping Sag is considered to be a half graben-like basin whose north side is faulted and whose south side is overlapped. Basement subsidence in the Eocene was mainly controlled by boundary faults which dip relatively steeply on the north side, causing the subsidence center of the Enping Sag in this stage to be close to the north boundary faults. Sustained faults developed in the Enping Sag during the Eocene caused an increase of the relative height difference between the north and the south uplift zone in the Enping Sag. Affected by the second episode of the Zhuqiong movement (39-36 Ma) in late Eocene, sediments which had accumulated on the Panyu low-uplift zone were triggered and moved toward the subsidence center of the Enping Sag and formed the turbidite fan. The second episode of the Zhuqiong movement is the most important triggering factor for the formation of the turbidite fan in the Wenchang Formation. Seismic attribute characterization shows that the low frequency energy is enhanced and high frequency energy is weakened when seismic waves propagate through the oil-bearing zone in this fan. Amplitude versus offset (AVO) anomalies are observed in the seismic data and abnormally high pressure is encountered. The turbidite fan in the Wenchang Formation has provided important information for sedimentary evolution in deep layers of the Enping Sag and pointed to a new direction for the hydrocarbon exploration in the study area.

The distribution of dimethylcarbazoles in oils from the Pearl River Mouth Basin, South China Sea

C2-carbazole isomers have been investigated in crude oils from the Hui-Liu Structure Ridge (HLSR) in the Pearl River Mouth Basin (PRMB), South China Sea. The NH shielded isomer, as well as the NH partially shielded isomers, was detected in high abundance and the NH exposed isomers in lower abundance. A small-enrichment trend of 1,8-dimethylcarbazole (DMC) was observed in crude oils along the western part of HLSR (WPHLSR), which may indicate little effect of migration on the C2-carbazole distributions. Two strikingly different distribution patterns of NH partially shielded isomers were observed in the reservoirs along the WPHLSR: one with a preference of 1,3- and 1,6-DMCs and the other with a preference of 1,4- and 1,5-DMCs. All of the oils occurring in the Upper reservoirs have a preference of 1,3- and 1,6-DMCs, whereas those trapped in the Lower reservoirs show a preference of 1,4- and 1,5-DMCs, which may indicate there are two petroleum migration systems in the WPHLSR.

The controls on the composition of biodegraded oils in the Liuhua11-1 Oilfield, Pearl River Mouth Basin, South China Sea

(1) The geochemical characteristics of extracted oils in three oil columns from the Liuhua11-1 Oilfield in the Pearl River Mouth Basin of South China Sea were analyzed in detail. The results show that the oil of Liuhua 11-1 Oilfield is generated from a single source rock, and belongs to maturate oil and their maturities have little difference.(2) The characteristics of saturated hydrocarbon gas chromatography,bulk composition, concentrations of biomarkers and gas chromatography–mass spectrometry of saturated hydrocarbon indicate that all samples studied were biodegraded, and the maximal level of biodegradation is less than PM level 6.(3) Bulk composition and the degree of biodegradation presented excellent gradient variations in the oil columns, and the highest degradation rates occurred at or near the oil–water contact(OWC). The key biomarkers of steranes and terpanes and maturity parameters of saturated hydrocarbon were not affected by degradation, which means that they are stable in slight to moderately biodegraded oils. Across the oilfield, the degree of biodegradation of LH11-1-3 was higher than that of LH11-1-1A and LH 11-1-4 in general.(4) We infer that the strong hydrodynamic conditions(tectonic control) and high reservoir temperatures(50–65 °C) are the primary controllers of the degree of oil biodegradation in the Liuhua 11-1 Oilfield and the late hydrocarbon accumulation may also have an important effect. The bulk composition and degree of biodegradation with excellent gradient variations in the oil columns were obviously controlled by the vertical distance from its in situ place to OWC. The highest supply of nutrient at the OWC results in high abundance of microorganisms and the highest degradation rate. And the lateral variation in level of biodegradation across the oil reservoir may be mainly control by the salinity of bottom water, the supply of nutrient and the energy of the bottom water.

Novel C15 sesquiterpanes and their origin in different crude oils from the Pearl River Mouth Basin of China

In the saturate fractions of crude oils from the Zhu1 and Zhu2 depressions of the Pearl River Mouth Basin,two novel C_(15) sesquiterpanes have been detected except for drimanes and rearranged drimanes.By comparison with spectra and retention times in previous studies,they have been identified as 2,2,4α,7,8-pentamethyl-trans-decalin and 2,2,4α,7,8-pentamethyl-cis-decalin,respectively,which are related to the D/Eringsof18α(H)-and18β(H)-oleanane.ResultsshowthatthenovelC15sesquiterpanesarerelativelyabundantincondensateoilsthathavehighPr/PhratiosandarerichinterrigenoustriterpanessuchasoleananeandbicadinanesfromtheZhu2depression,butevidentlylowinlacustrineoilsthathavelowPr/Phratiosandarerichin4-methylsteranesfromtheZhu1depression.ThissuggeststhatthenovelC15sesquiterpanescouldberelatedtoterrigenousorganicmatterinoriginandcanhelpdistinguishtheoriginandsourceofcrudeoilsinthestudyarea.Inaddition,thereexistcompletelydifferentcorrelationsbetweentherelativeabundancesofdifferentC15sesquiterpanesthathavedifferentsourcesandthePr/Phratio,whichindicatesredoxconditionsinthecrudeoils.Forexample,thenovelC15sesquiterpanesandcadinanethatindicateterrigenousinputarepositivelycorrelatedwiththePr/Phratio,butdrimanesandrearrangeddrimaneshavingmicrobialoriginarenegativelycorrelatedwiththePr/Phratio.ThisimpliesthattheformationofthenovelC15sesquiterpanesrequiresrelativelyoxicconditions.ThenovelC15sesquiterpanesaremuchmoreabundantincarbonaceousmudstone(Pr/Ph=7.39)thanindarkmudstone(Pr/Ph=2.19)fromtheEnpingformation,althoughtheirorganicmatterishumic.However,theyarenotcommoninsapropeliclacustrinesourcerocks(Pr/Ph=2.29)fromtheWenchangFormation.Thissuggeststhatterrigenousorganicmatterisonlyasufficientcondition,butanoxicenvironmentduringdepositionanddiagenesismaybeaprerequisitefortheformationofnovelC15sesquiterpanes.

Basin-filling processes and hydrocarbon source rock prediction of low-exploration degree areas in rift lacustrine basins:a case from the Wenchang Formation in low-exploration degree areas,northern Zhu I Depression,Pearl River Mouth Basin,E China

Hydrocarbon source rocks, as a main geologic factor of petroliferous systems in a sedimentary basin, play a key role in the accumulation of oil and gas and the formation of hydrocarbon accumulations. This study, which focuses on difficulties in prediction of hydrocarbon source rocks in basins or sags with low exploration degree and insufficient hydrocarbon source rock indicators, taking the Wenchang Formation of northern Zhu I Depression, Pearl River Mouth Basin as an example, proposed a hypothesis of “finding lakes and hydrocarbon source rocks”. Detailed steps include, first, determination of the lacustrine basin boundary according to analysis of seismic foreset facies, determination of the depositional area based on the compilation of strata residual thickness maps, determination of the lacustrine basin shape according to deciphering slope break belt system, determination of the fluctuation of paleo-water depth according to biogeochemical indicators of mature exploration areas, determination of the lacustrine basin scale based on analyses of tectonics intensity and accommodation space, which prove the existence of the lacustrine basin and identify the range of semi deep-deep lake;second, further analyses of tectonopalaeogeomorphology, paleo-provenance,palaeoclimate and paleo-water depth to reconstruct the geologic background of the original basin and semideep-deep lacustrine facies, to determine the distribution of semi-deep/deep lacustrine sediments in combination with studies of logging facies, core facies, seismic facies and sedimentary facies, and to rank the sags’ potential of developing hydrocarbon source rocks from controlling factors of source-to-sink system development;third, on the basis of regional sedimentary facies analysis, through identification and assessment of seismic facies types of semi-deep/deep lacustrine basins in mature areas, establishing “hydrocarbon source rock facies” in mature areas to instruct the identification and depicting of hydrocarbon source rocks in semideep/deep lacustrine basins with low exploration degree;fourth, through systematical summary of hydrocarbon-rich geological factors and lower limit index of hydrocarbon formation of the sags already revealed by drilling wells(e.g., sag area, tectonic subsidence amount, accommodation space, provenance characteristic, mudstone thickness, water body environment, sedimentary facies types of hydrocarbon source rocks), in correlation with corresponding indexes of sags with low exploration degree, then the evaluation and sorting of high-quality source rocks in areas with sparsely distributed or no drilling wells can be conducted with multi-factors and multiple dimensions. It is concluded that LF22 sag, HZ10 sag and HZ8 sag are II-order hydrocarbon rich sags;whereas HZS, HZ11 and HZ24 are the III-order hydrocarbon-generating sags.

Numerical modeling of the anomalous post-rift subsidence in the Baiyun Sag,Pearl River Mouth Basin

The Baiyun Sag is the deepest sag in the Pearl River Mouth Basin in northern continental margin of South China Sea,with the maximum sediment thickness over 12.5 km above the basement including >6.5 km sediments above the 30 Ma breakup unconformity.According to the theoretical models for the rifted basins,the post-rift subsidence is driven solely by the thermal contraction and can be calculated as the function of the lithospheric stretching factor.A method combining the forward modeling and reverse backstripping was designed to estimate lithospheric stretching factor.Using the 2D forward modeling based on the flexural cantilever model,we simulated the multi-rifting process of the Baiyun Sag with constrain of the backstripped profiles.By doing this the lithospheric stretching factor was obtained,and then the theoretical post-rift subsidence was calculated.The calculated theoretical subsidence was much smaller than the observed subsidence given by backstripping.Along the 1530 line in the Baiyun Sag,the anomalous post-rift subsidence is over 2 km in the sag center,and varies slightly to the north and south edges of the sag.This suggests that the anomalous post-rift subsidence continues beyond the sag both in the continental shelf to the north and in the continental slope to the south.The sensitivity tests in the forward modeling process indicate that only the use of low-angle faults (≤13°) can we simulate the shape of the backstripped profile.

The Cenozoic activities of Yangjiang-Yitongdong Fault: insights from analysis of the tectonic characteristics and evolution processes in western Zhujiang(Pearl) River Mouth Basin

The Yangjiang-Yitongdong Fault (YJF) is an important NW-trending regional fault, which divides the Zhujiang (Pearl) River Mouth Basin (ZRMB) into western and eastern segments. In Cenozoic, the northern continental margin of the South China Sea (SCS) underwent continental rifting, breakup, seafloor spreading and thermal subsidence processes, and the Cenozoic activities of YJF is one part of this series of complex processes. Two long NW-trending multichannel seismic profiles located on both sides of the YJF extending from the continental shelf to Continent-Ocean Boundary (COB) were used to study the tectonic and sedimentary characteristics of western ZRMB. Using the 2D-Move software and back-stripping method, we constructed the balance cross-section model and calculated the fault activity rate. Through the comprehensive consideration of tectonic position, tectonic evolution history, featured structure, and stress analysis, we deduced the activity history of the YJF in Cenozoic. The results showed that the YJF can be divided into two segments by the central uplift belt. From 65 Ma to 32 Ma, the YJF was in sinistral motion as a whole, inherited the preexisting sinistral motion of Mesozoic YJF, in which, the southern part of YJF was mainly in extension activity, controlling the formation and evolution of Yunkai Low Uplift, coupled with slight sinistral motion. From 32 Ma to 23.8 Ma, the sinistral motion in northern part of YJF continued, while the sinistral motion in southern part began to stop or shifted to a slightly dextral motion. After 23.8 Ma, the dextral motion in southern part of YJF continued, while the sinistral motion in northern part of YJF gradually stopped, or shifted to the slightly dextral motion. The shift of the YJF strike-slip direction may be related to the magmatic underplating in continent-ocean transition, southeastern ZRMB. According to the analysis of tectonic activity intensity and rift sedimentary structure, the activities of YJF in Cenozoic played a regulating role in the rift extension process of ZRMB.

Tectonic Evolution and Dynamics of Deepwater Area of Pearl River Mouth Basin, Northern South China Sea

Quantitative studies on the evolution and dynamics of the deepwater area of Pearl River Mouth basin (PRMB) were carried out based on the latest geological and seismic data. The study area is generally in an extensional state during the Cenozoic. The major extension happened in the earlier syn-rift stages before 23 Ma and the extension after 23 Ma is negligible. Two rapid subsidence periods, 32-23 Ma and 5.3-2.6 Ma, are identified, which are related to the abrupt heat decay during margin breakup and the collision between the Philippine Sea plate and the Eurasian plate, respectively. The strongest crustal thinning in the Baiyun (白云) sag may trigger the syn-rift volcanism along the weak faulted belt around the sag. The Cenozoic tectonic evolution of the study area could be divided into five stages: rifting (～50-40 Ma), rift-drift transition (～40-32 Ma), early post-breakup (～32-23 Ma), thermalsubsidence (～23-5.3 Ma) and neotectonic move-ment (～5.3-0 Ma).