Megafossils of Betulaceae from the Oligocene of Qaidam Basin and their paleoenvironmental and phytogeographic implications

Understanding the paleoenvironment and phytogeographical history of the Tibetan Plateau,China relies on discovering new plant fossils.The Qaidam Basin has long been regarded as an ideal‘field laboratory’to investigate the paleoclimate and paleobiological evolution of the northern Tibetan Plateau.However,fossil angiosperms from the Qaidam Basin are rare,and our knowledge of its paleovegetation is poor.Here,we report fossil leaves and fruits of Betulaceae found from the Oligocene Shangganchaigou Formation of northwestern Qaidam Basin(Huatugou area).Comparative morphological analysis led us to assign the fruits to the Betula subgenus Betula and the leaves to Carpinus grandis.These findings,together with other reported fossil plants from the same locality,reveal a close floristic linkage between the Qaidam Basin and Europe during the Oligocene.The northern pathway of this floristic exchange may have crossed through the Qaidam Basin during the late Paleogene.This floristic linkage may have been facilitated by the continuous narrowing of the Turgai Strait and stronger westerlies,which transported moisture and provided favorable climatic conditions.Indeed,fossil plants collected from the Qaidam Basin suggest that during the Oligocene this region had warm and humid deciduous broad-leaf forest,which differs from the region’s modern vegetation and indicates that the Qaidam Basin may have been a suitable region for these plants to flourish and spread during the Oligocene.

Diagenetic evolution and reservoir quality of the Oligocene sandstones in the Baiyun Sag, Pearl River Mouth Basin, South China Sea

The Oligocene Zhuhai sandstones are significant reservoirs for hydrocarbons in the Baiyun Sag, South China Sea.For effective appraisal, exploration and exploitation of such a deep-water hydrocarbon sandstone, samples of five wells from depths of 850 m to 3 000 m were studied. A series of comprehensive petrographic and geochemical analyses were performed to unravel the diagenetic features and their impact on the reservoir quality.Petrographically, the sandstones are dominated by feldspathic litharenites and lithic arenites with fine to medium grain sizes and moderate to good sorting. The reservoir quality varies greatly with a range of porosity from 0.2% to 36.1% and permeability from 0.016 ×10~(–3) μm~2 to 4 301 ×10~(–3) μm~2, which is attributed to complex diagenetic evolution related to sedimentary facies;these include compaction, cementation of calcite, dolomite, siderite and framboidal pyrite in eogenetic stage;further compaction, feldspar dissolution, precipitation of ferrocalcite and ankerite, quartz cements, formation of kaolinite and its illitization, precipitation of albite and nodular pyrite, as well as hydrocarbon charge in mesogenetic stage. The dissolution of feldspar and illitization of kaolinite provide internal sources for the precipitation of quartz cement, while carbonate cements are derived from external sources related to interbedded mudstones and deep fluid. Compaction is the predominant factor in reducing the total porosity, followed by carbonate cementation that leads to strong heterogeneity. Feldspar dissolution and concomitant quartz and clay cementation barely changes the porosity but significantly reduces the permeability.The high-quality reservoirs can be concluded as medium-grained sandstones lying in the central parts of thick underwater distributary channel sandbodies(>2 m) with a high content of detrital quartz but low cement.

Discovery and petroleum geological significance of delta in the third member of Oligocene Lingshui Formation in southern Baodao Sag,Qiongdongnan Basin,South China Sea

Based on the 3D seismic data and the analysis and test data of lithology,electricity,thin sections and chronology obtained from drilling of the Qiongdongnan Basin,the characteristics and the quantitative analysis of the source-sink system are studied of the third member of the Upper Oligocene Lingshui Formation(Ling 3 Member)in the southern fault step zone of the Baodao Sag.First,the YL10 denudation area of the Ling 3 Member mainly developed two fluvial systems in the east and west,resulting in the formation of two dominant sand transport channels and two delta lobes in southern Baodao Sag,which are generally large in the west and small in the east.The evolution of the delta has experienced four stages:initiation,prosperity,intermittence and rejuvenation.Second,the source-sink coupled quantitative calculation is performed depending on the parameters of the delta sand bodies,including development phases,distribution area,flattening thickness,area of different parent rocks,and sand-forming coefficient,showing that the study area has the material basis for the formation of large-scale reservoir.Third,the drilling reveals that the delta of the Ling 3 Member is dominated by fine sandstone,with total sandstone thickness of 109-138 m,maximum single-layer sandstone thickness of 15.5-30.0 m,and sand-to-strata ratio of 43.7%-73.0%,but the physical properties are different among the fault steps.Fourth,the large delta development model of the small source area in the step fault zone with multi-stage uplift is established.It suggests that the episodic uplift provides sufficient sediments,the fluvial system and watershed area control the scale of the sand body,the multi-step active fault steps dominate the sand body transport channel,and local fault troughs decide the lateral propulsion direction of the sand body.The delta of the Ling 3 Member is coupled with fault blocks to form diverse traps,which are critical exploration targets in southern Baodao Sag.

Genetic mechanism and main controlling factors of high-quality clastic rock reservoirs in deep and ultradeep layers:A case study of Oligocene Linhe Formation in Linhe Depression,Hetao Basin,NW China

Based on new data from cores,drilling and logging,combined with extensive rock and mineral testing analysis,a systematic analysis is conducted on the characteristics,diagenesis types,genesis and controlling factors of deep to ultra-deep abnormally high porosity clastic rock reservoirs in the Oligocene Linhe Formation in the Hetao Basin.The reservoir space of the deep to ultra-deep clastic rock reservoirs in the Linhe Formation is mainly primary pores,and the coupling of three favorable diagenetic elements,namely the rock fabric with strong compaction resistance,weak thermal compaction diagenetic dynamic field,and diagenetic environment with weak fluid compaction-weak cementation,is conducive to the preservation of primary pores.The Linhe Formation clastic rocks have a superior preexisting material composition,with an average total content of 90%for quartz,feldspar,and rigid rock fragments,and strong resistance to compaction.The geothermal gradient in Linhe Depression in the range of(2.0–2.6)°C/100 m is low,and together with the burial history of long-term shallow burial and late rapid deep burial,it forms a weak thermal compaction diagenetic dynamic field environment.The diagenetic environment of the saline lake basin is characterized by weak fluid compaction.At the same time,the paleosalinity has zoning characteristics,and weak cementation in low salinity areas is conducive to the preservation of primary pores.The hydrodynamic conditions of sedimentation,salinity differentiation of ancient water in saline lake basins,and sand body thickness jointly control the distribution of high-quality reservoirs in the Linhe Formation.

Sedimentary systems of the Oligocene Huagang Formation in the central anticline zone of the Xihu Depression,East China Sea Shelf Basin

The Xihu Depression is the largest hydrocarbon-bearing depression of the East China Sea Shelf Basin(also referred to as the ECSSB).However,the depositional systems and reservoir distribution of the Oligocene Huagang Formation in the Xihu Depression are still controversial.Under the guidance of sedimentology and stratigraphy,this study documented a marine-terrestrial transitional environment in the restricted bay setting of the Oligocene Huagang Formation through core description,well logging,and seismic data analysis.This study also revealed that the Oligocene Huagang Formation is dominated by tidal delta,estuary,and gravity flow deposits in the central anticline zone of the Xihu Depression.The new understanding of the sedimentary systems and the discovery of the transgressive gap in the eastern Diaoyu Islands uplift explain the origin of fine-grained sediments and the EW-trending sand bodies in the central depression and the sand bodies parallel to shoreline in the west slope belt,which cannot be explained by previous study results,such as southern transgression or fluvial deltas and even lacustrine deposition.Moreover,the tidal channels,tidal sand flats,and gravity flow sand bodies formed by the transgressive tides are high-quality reservoirs.The study will provide a basis for well placement and serve as guidance for the selection of favorable hydrocarbon exploration areas in the Xihu Depression.

Palynostratigraphy and Paleoenvironment of Bukit Tigapuluh Area in the Jambi Subbasin from Oligocene to Miocene

The Oligocene-Miocene trajectory exposed at the Lubuk Lawas and Lubuk Bernai Stratigraphic Tracks in Bukit Tigapuluh, Jambi Subbasin, Indonesia, archives remnants of equatorial vegetation during extreme global warming and near the beginning of the East Java-Eurasia microcontinent collision, and was carried out using mapping surface analysis, petrological analysis, sedimentology, stratigraphy and palinology. The rock units were deposited during one sedimentation phase, that is the continental deposition phase, which consists of conglomerates, gravel sandstones and sandstones that fill the basin followed by transgressive deposits associated with the deepening of the basin environment. Three palynozones Meyeripollis naharkotensis (Oligocene), Florschuetzia trilobata (Early Miocene) and Florschuetzia meridionalis (Middle Miocene) were identified stratigraphically on the basis of these pollen. The rock layers are deposits from the Early Oligocene to Middle Miocene from bottom to top. The depositional environment changed over time, passing from a narrow, steep-sided tectonic basin, during the Early to Late Oligocene, followed by a lacustrine basin to a palustine with oceanic influence, as a result of distensive E-W movement between the Jambi Fault and the Sunda Fault in the Late Oligocene to the Middle Miocene. Occurrence of taxonomically highly diverse angiosperm pollen in all three palynozones attests to an extremely rich inland and nearshore tropical flora under a strong seasonal rainfall regime. The climate remained warm and became increasingly humid towards the end of the Miocene. The nature of the environment is related to the dynamics of the opening of the basin opening.

Petrogenesis of the Late Eocene to Early Oligocene Yao'an Shoshonitic Complex,Southeastern Tibet:Partial Melting of an Ancient Continental Lithospheric Mantle beneath the Yangtze Block

Cenozoic potassic-ultrapotassic igneous rocks are widespread in the southeastern Tibetan Plateau.Their petrogenesis and magmatic processes remain subject to debate in spite of numerous publications.Almost all of the Cenozoic extrusive and intrusive rocks in the Yao’an area,western Yunnan Province,SW China,are geochemically shoshonitic,collectively termed here the Yao’an Shoshonitic Complex(YSC).The YSC is located in the(south)easternmost part of the ENE-WSW-trending,~550 km-long and~250 km-wide Cenozoic magmatic zone;the latter separates the orthogonal and oblique collision belts of the India-Eurasia collision orogen.Previously published geochronological and thermochronological data revealed that the rocks of the YSC were emplaced over a short timespan of 34-32 Ma.This and our new data suggest that the primary magma of the YSC likely was formed by partial melting of ancient continental lithospheric mantle beneath the Yangtze Block.This part of the continental lithospheric mantle had likely not been modified by any oceanic subduction.Fractionation crystallization of an Mg-and Ca-bearing mineral and TiFe oxides during the magmatic evolution probably account for the variable lithologies of the YSC.

Late Oligocene fossil acorns and nuts of Quercus section Cyclobalanopsis from the Nanning Basin,Guangxi,South China

Quercus is the largest genus within the Fagaceae and has a rich fossil record.Most of the fossil material is attributed to the subgenus Quercus based on leaves,pollen or rarely acorns and nuts.Fossil records of Q.section Cyclobalanopsis characterized by ring-cupped acorns are relatively few and especially those described based on nuts are scant.In this study,we described four new species of Quercus section Cyclobalanopsis based on mummified acorns and nuts:Q.paleodisciformis X.Y.Liu et J.H.Jin sp.nov.,Q.paleohui X.Y.Liu et J.H.Jin sp.nov.,Q.nanningensis X.Y.Liu et J.H.Jin sp.nov.and Q.yongningensis X.Y.Liu et J.H.Jin sp.nov.These species closely resemble the extant species Q.disciformis,Q.hui,Q.kerrii,and Q.dinghuensis.The occurrence of Q.section Cyclobalanopsis in the Oligocene stratum of Guangxi,South China,suggests that the section has diversified within its extant distribution center since the Oligocene.By combining records from other areas,we propose that the section first appeared in the middle Eocene of East Asia(Sino-Japan),has diversified in situ with a few elements scattering into West Asia and southern Europe since the Oligocene and Pliocene,respectively,and finally became restricted in East Asia since the Pleistocene.This indicates that the section originated and diversified in East Asia,before spreading into West Asia no later than the Oligocene and into southern Europe by the Pliocene.Subsequently it disappeared from South Europe and West Asia due to the appearance of the(summer dry)Mediterranean climate and widespread cooling during the Pleistocene.

Petrogenesis of Oligocene volcanic rocks of the Lake Tana area,Ethiopian large Igneous Province

The Lake Tana area is located within a complex volcano-tectonic basin on the northwestern Ethiopian plateau.The basin is underlain by a thick succession of Oligocene transitional basalts and sub-alkaline rhyolites overlain in places,particularly south of the lake,by Quaternary alkaline to mildly transitional basalts,and dotted with Oligo-Miocene trachyte domes and plugs.This paper presents the results of integrated field,petrographic,and major and trace element geochemical studies of the Lake Tana area volcanic rocks,with particular emphasis on the Oligocene basalts and rhyolites.The studies reveal a clear petrogenetic link between the Oligocene basalts and rhyolites.The Oligocene basalts are:(1)plagioclase,olivine,and/or pyroxene phyric;(2)show an overall decreasing trend in MgO,Fe_(2)O_(3),and CaO with silica;(3)have relatively low Mg#,Ni and Cr contents and high Nb/La and Nb/Yb ratios;and(4)show LREE enriched and generally flat HREE patterns.All these imply the origin of the Oligocene basalts by shallow-level fractional crystallization of an enriched magma sourced at the asthenospheric mantle.The Oligocene rhyolites:(1)are enriched in incompatible while depleted in compatible trace elements,P and Ti;(2)show a strong negative Eu anomaly;(3)contain appreciable amounts of plagioclase,apatite,and Fe-Ti oxides;and(4)show clear geochemical similarity with well-constrained rhyolites from the Large Igneous Province(LIP)of the northwestern Ethiopian plateau.Low-pressure fractional crystallization of mantle-derived basaltic magma in crustal magma chambers explains the origin of these rhyolites.Our study further shows that the Oligocene basalts and rhyolites are co-genetic and the felsic rocks of the Lake Tana area are related differentiates of the flood basalt volcanism in the northwestern Ethiopian plateau.

柴达木盆地南八仙地区渐新世浅水三角洲物源及沉积演化

以古近系渐新统下干柴沟组上段(E23)为研究对象,在精细岩芯观察和钻(测)井资料综合分析的基础上,结合重矿物组合、ZTR指数、砂岩碎屑组分、粒度累积概率图和砂地比等参数综合分析结果,对柴达木盆地南八仙地区E23湖盆扩张期弱退积型浅水三角洲物源方向、砂体展布规律和沉积微相发育演化进行了深入研究。结果表明:①南八仙地区E23沉积时期物源主要来自南祁连山中酸性岩浆岩和中高级变质岩发育区,物源方向为近北东—南西向,稳定重矿物(磁铁矿、石榴子石、白钛矿和锆石)含量高(质量分数介于81.0%~99.9%)、不稳定重矿物(绿帘石和角闪石)含量低。②南八仙地区E23沉积时期为浅水三角洲—滨浅湖沉积环境,主要发育水下分流河道砂体,岩性主要为岩屑长石砂岩,其次为长石岩屑砂岩,砂体的成分成熟度一般中等—较差、分选磨圆度中等—较差,粒度概率图以反映河流作用的一跳跃一悬浮式为主;沉积构造类型多样,主要发育块状层理、正粒序层理、平行层理和槽状交错层理。③E23沉积时期,南八仙地区继承性发育3条北东—南西向呈条带状展布的砂体富集带(砂地比值介于35%~65%之间),早期到中期砂体发育程度增强,呈进积式;中期到晚期砂体发育程度有所减弱,呈弱退积式。④E23沉积时期,南八仙地区处于浅水三角洲前缘亚相沉积环境,持续发育3个呈北东—南西向展布的沉积朵叶体,主要发育水下分流河道微相,其次为水下分流间湾微相,早期到中期浅水三角洲前缘发育规模增大,中期到晚期发育规模减小。

喜马拉雅造山带聂拉木地区渐新世深熔作用的厘定

喜马拉雅新生代淡色花岗岩记录了地壳深熔作用和花岗岩侵位的地球化学和构造物理效应。与形成于增厚地壳条件下和伸展背景下的始新世和中新世花岗岩相比,渐新世花岗岩分布比较局限,且源区和形成机制还存在着较大的争议。位于喜马拉雅造山带中部的聂拉木地区,可见32.1Ma的含电气石黑云母的花岗岩、29.8Ma的含黑云母的花岗岩、26.6Ma的含电气石黑云母的伟晶花岗岩侵入到高喜马拉雅岩系上部。这三组渐新世花岗岩具有以下特征:(1)较高的SiO_(2)、Al_(2)O_(3)、K_(2)O和Na_(2)O,A/CNK>1.0;(2)Ba、Nb、Ta、Sr和Ti的负异常;(3)略微富集轻稀土,亏损中稀土和重稀土,高度变化的Eu异常和微弱的负Nd异常;(4)均一的初始Sr-Nd同位素比值,^(87)Sr/^(86)Sr(t)=0.7463~0.7471,ε_(Nd)(t)=-15.0~-14.6。此外,含电气石黑云母的花岗岩和含电气石黑云母的伟晶花岗岩具有较高的Sr和Ba,较低的Rb/Sr比值,其Rb/Sr比值与Ba和Sr含量都无相关性,表明其为富B流体参与高喜马拉雅变沉积岩含水部分熔融作用的产物。而含黑云母的花岗岩具有较低的Sr和Ba,较高的Rb/Sr比值,且其Rb/Sr比值与Ba和Sr含量显示明显的负相关关系,表明其为变沉积岩发生脱水熔融作用的产物。综合喜马拉雅造山带已有的研究成果,得出以下认识:渐新世花岗岩的源区从下地壳基性物质转变为中地壳变泥质岩,展示了源区向上迁移的过程,并经历了一期重要的岩浆热液交代作用;渐新世深熔作用记录了造山带从缩短增厚向伸展垮塌转换阶段深部地壳的响应,从而促使了高喜马拉雅岩系的折返。

长江三峡贯通过程的动态古地貌重建

长江是亚洲第一大河,其形成和演化是新生代以来中国东西部构造—气候—地貌综合演变的结果。长江三峡的贯通是现代长江形成的标志性事件,但受限于传统沉积学、地球化学等单一方法的制约,对“长江三峡何时贯通”这一关键科学问题一直存在争议。综合考虑了构造运动、古气候及海平面变化等影响河流发展的关键因素,并将这些因素输入Badlands古地貌模拟软件,动态重建了长江“第一弯”以东地区晚白垩世(80 Ma)以来的长江流域地貌及水系演化过程;并利用四川盆地和江汉盆地的地震剖面资料验证了模拟结果的可靠性。模拟结果表明,青藏东部及上扬子西南缘晚始新世—渐新世的阶段性隆升迫使四川盆地原有南流水系下切受阻,沉积物在盆内堆积形成冲积河道并促使四川盆地地貌由“东北高西南低”反转为“西南高东北低”;新生代早期,江汉盆地长期受控于中国东部的裂陷环境,持续处于较低基准面。四川盆地的水系反转和江汉盆地的持续低基准面,最终导致位于二者之间的长江三峡在晚渐新世发生贯通。由此,本研究提出一种上扬子地区水系反转并被下游捕获的三峡贯通机制。

西湖凹陷花港组浅水三角洲平原分流河道沉积构型

渐新统花港组上段是东海陆架盆地西湖凹陷中南部地区的主要产油气层位,但对其沉积体系的认识一直存在争议,砂体构型特征及控制因素的研究也相对匮乏。综合运用岩心、测井及地震等资料,研究渐新统花港组上段沉积环境、沉积微相类型、砂体沉积构型特征及其控制因素。结果表明:西湖凹陷中南部花港组上段以浅水三角洲平原沉积为主,分流河道是最主要的砂体类型。根据弯曲度差异,可将分流河道划分为辫流型(弯曲度1~1.05)、曲流型(弯曲度大于1.15)和辫-曲转换型(弯曲度1.05~1.15)3种类型,其中辫流型分流河道砂体“宽而厚”,平面组合呈交织宽条带状,心滩呈斑点状排列,砂体垂向叠置样式以切叠型为主;曲流型分流河道砂体“窄而薄”,平面样式为单一条带型或分叉-合并型,厚砂以点坝的形式分布在河道边缘,砂体垂向叠置样式以“孤立型”为主;辫-曲转换型分流河道是辫流型和曲流型的过渡类型,河道砂体规模中等,河道内心滩和点坝同时发育。3类分流河道的厚度与宽度呈对数正相关,宽厚比具有辫流型>辫-曲转换型>曲流型的特征。推测古地貌和基准面变化是影响浅水三角洲分流河道构型特征的主要因素,其中古地貌控制了分流河道的河型变化,较陡的地形有利于低弯度辫流型分流河道的形成,而长期基准面上升造成了分流河道规模逐渐下降,中期基准面的上升造成了分流河道砂体叠置样式发生切叠型→叠加型→孤立型的演变。研究成果完善了浅水三角洲的沉积构型知识库,对地下浅水三角洲分流河道储集层的预测及开发具有重要的指导意义。

渐新世-中新世东南亚干湿气候格局演变及驱动机制

新生代印尼海道的启闭对印度-太平洋暖池演化和大气环流模式变迁有重大影响。然而,受限于构造和古环境重建资料的缺乏,这三者之间的逻辑关系和驱动机制还缺乏清晰的图景。本文梳理了孢粉记录、煤层沉积、浅海碳酸盐沉积和生物地理演化等方面的证据,提出东南亚水文气候在渐新世与中新世之交发生重大调整的认识,即从渐新世的相对干旱条件转型为贯穿整个中新世的持续湿润状态。结合最近的模拟研究,认为东南亚水文气候演变同时受到全球因素和区域构造要素的影响。渐新世与中新世之交和中中新世晚期至晚中新世早期,印尼海道的持续关闭可以通过限制太平洋-印度洋次表层水的交换,进而扩大太平洋一侧的温跃层深度以及经纬向的海表温度梯度,进一步增强沃克环流,最终可能促使东南亚在渐新世与中新世之交发生了干湿格局的转换,并抵消了中中新世晚期至晚中新世全球降温对区域水文气候的影响。目前的研究仍存在不确定性,未来亟需更多的地质记录和模拟研究来准确厘定海道关闭-暖池演化-大气环流之间的联系。

冈底斯岩浆弧东段米林地区晚渐新世花岗岩成因

青藏高原南部的冈底斯岩浆弧广泛发育新生代的花岗岩,然而这些花岗岩的成因仍存在争议。本文对冈底斯东段米林地区的晚渐新世二云母花岗岩进行了年代学和地球化学研究。全岩化学分析结果显示,这些花岗岩为中钾—高钾钙碱性系列,偏铝质,富集轻稀土元素和Rb、Th、U等大离子亲石元素,亏损重稀土元素和Nb、Ta、Ti等高场强元素,高Sr、低Y,具有较高的Sr/Y比值(37~85),显示出埃达克质岩石的地球化学特征。锆石U-Pb定年结果显示,所研究的二云母花岗岩结晶年龄为~26.6 Ma。这些花岗岩中的锆石具有较低的正εHf(t)值(+0.4~+7.3)。综合现有的研究结果,我们认为冈底斯弧东段的晚渐新世花岗岩起源于加厚下地壳始新世弧岩浆岩的部分熔融,但具有古老地壳物质的贡献。印度与亚洲大陆的持续汇聚导致冈底斯弧地壳加厚,加厚下地壳发生高级变质和部分熔融形成了晚渐新世的埃达克质花岗岩。

深层—超深层优质碎屑岩储层成因及主控因素——以河套盆地临河坳陷古近系渐新统临河组为例

基于岩心、钻井和测井等新资料,结合大量岩矿测试分析,对河套盆地临河坳陷古近系渐新统临河组深层—超深层碎屑岩储层特征、成岩作用类型、异常高孔储层成因及发育控制因素开展系统分析。研究表明:(1)临河组深层—超深层碎屑岩异常高孔储层储集空间以原生孔为主,抗压实能力强的物质组构基础、弱压实成岩动力场和流体弱压实-弱胶结成岩环境等3大有利成岩要素耦合利于原生孔大量保存;(2)临河组碎屑岩先存物质组构优越,石英、长石和刚性岩屑总含量平均值可达90%,抗压实能力强;(3)临河坳陷地温梯度低,仅为(2.0~2.6)℃/100 m,与早期长期浅埋—晚期快速深埋的埋藏方式共同形成弱压实成岩动力场环境;(4)咸化湖盆沉积地层的成岩环境中,成岩流体产生的压实作用弱,同时古盐度具有分异性,低盐度区胶结作用弱,利于原生孔保存;(5)沉积水动力条件、咸化湖盆古水体盐度分异和砂体厚度等3大因素共同控制临河组优质储层分布。

莺歌海盆地莺北区渐新统资源潜力与勘探方向

莺歌海盆地莺北区中新统烃源岩资源潜力有限,制约了油气勘探。利用地质、地球物理和地球化学等资料,开展了莺北区渐新统三角洲识别与含煤性分析、资源潜力评价和不同构造单元成藏条件比较等工作,明确了莺北区渐新统资源潜力和有利勘探方向。研究表明:①莺北区渐新世具备发育大型(扇)三角洲的物质基础和构造条件,崖城组—陵水组继承性发育大型煤系三角洲,形成崖城组三角洲平原煤系和陵水组陆源海相泥岩2类烃源岩;②莺北区渐新统煤系三角洲是莺北区主力烃源岩,主要处于高成熟—过成熟度大规模生气阶段,天然气资源量达1.13万亿方;③莺北区凹陷带、东部斜坡带中组合是有利勘探方向,构造脊、鼻状构造等具有良好汇聚背景上的大型岩性、地层圈闭是发现大中型气田的重要领域。本项研究成果指导了莺北区的勘探突破。

琼东南盆地宝岛凹陷南部渐新统陵水组三段三角洲的发现及石油地质意义

基于琼东南盆地三维地震资料及钻井获取的岩性、电性、薄片及测年等分析测试数据,开展宝岛凹陷南部断阶带渐新统陵水组三段(简称陵三段)源-汇体系特征及源-汇耦合定量分析等研究。研究表明:①陵三段沉积期YL10剥蚀区发育东、西2大物源水系,在宝岛凹陷南部形成了2大优势输砂通道及2个三角洲朵叶体,整体具有西大东小的特征,可划分为初始期、昌盛期、间歇期和回春期4个演变阶段;②综合三角洲砂体发育期次、展布面积、碾平厚度及不同类型母岩区面积、成砂系数等参数进行源-汇耦合定量计算,结果显示研究区具备形成规模储集体的物质基础;③钻井揭示陵三段三角洲沉积以细砂岩为主,砂岩总厚度为109~138 m,最大单层厚度为15.5~30.0 m,砂地比为43.7%~73.0%,不同断阶间储层物性差异明显;④断阶带多期隆升背景下小物源区规模三角洲发育模式揭示幕式隆升提供充足的物源供给,物源水系及流域面积控制砂体的规模,多级活动断阶控制砂体的输送通道,局部断槽控制砂体的侧向推进方向。陵三段三角洲砂体与断块耦合形成多类型圈闭,成为凹陷南部重要的勘探突破口和新领域,有望为盆地内其他构造背景相似区域油气勘探及储层研究提供借鉴。

Paleoclimate Changes during the Early Oligocene in the Hoh Xil Region, Northern Tibetan Plateau

Sedimentological, cyclic-stratigraphic, paleomagnetic, and clay-mineralogical studies on the early Oligocene Yaxicuo Group in the Hoh Xil Basin, the largest Cenozoic sedimentary basin in the hinterland of the Tibetan Plateau, provide abundant information of paleoclimate changes. A 350-m thick section in the middle-lower Yaxicuo Group was analyzed to reveal the climatic history that occurred in the Hoh Xil region during the early Oligocene interval 31.30-30.35 Ma, dated with the paleomagnetic chronostratigraphy. The results indicate that arid and cold climate dominated the Hoh Xil region during the early Oligocene in general, being related to the global cooling and drying events that occurred in the earliest Oligocene. Within this period, relatively warm and wet climate accompanied by strong tectonic activity occurred in the 31.05-30.75 Ma interval; while arid and cold climate and relatively inactive tectonics occurred in the 31.30-31.05 and 30.75-30.35 Ma intervals. Furthermore, spectral analyses of high-temporal resolution paleoclimatic records show orbital periods including eccentricity, obliquity, and precession. It is concluded that paleoclimate changes during the early Oligocene in the Hoh Xil region were forced by both tectonic activity and orbital periods.

The main controlling factors and developmental models of Oligocene source rocks in the Qiongdongnan Basin,northern South China Sea

Coals developed in the Oligocene Yacheng and Lingshui formations in the Qiongdongnan Basin have high organic matter abundance, and the dark mudstones in the two formations have reached a good source rock standard but with strong heterogeneity. Through the analysis of trace elements, organic macerals and biomarkers, it is indicated that plankton has made little contribution to Oligocene source rocks compared with the terrestrial higher plants. The organic matter preservation depends on hydrodynamics and the redox environment, and the former is the major factor in the study area. During the sedimentary period of the Yacheng Formation, tidal flats were developed in the central uplift zone, where the hydrodynamic conditions were weak and the input of terrestrial organic matter was abundant. So the Yacheng Salient of the central uplift zone is the most favorable area for the development of source rocks, followed by the central depression zone. During the sedimentary period of the Lingshui Formation, the organic matter input was sufficient in the central depression zone due to multiple sources of sediments. The semi-enclosed environment was favorable for organic matter accumulation, so high quality source rocks could be easily formed in this area, followed by the Yacheng salient of central uplift zone. Source rocks were less developed in the northern depression zone owing to poor preservation conditions,