Autocyclic behaviour of fan deltas: an analogue experimental study

Fan deltas are excellent recorders of fan‐building processes because of their high sedimentation rate, particularly in tectonically active settings. Although previous research focuses mainly on allogenic controls, there is clear evidence for autogenically produced storage and release of sediment by flume and numerical modelling that demands further definition of characteristics and significance of autogenically forced facies and stratigraphy. Analogue experiments were performed on fan deltas with constant extrinsic variables (discharge, sediment supply, sea‐level and basin relief) to demonstrate that fan‐delta evolution consists of prominent cyclic alternations of channellized flow and sheet flow. The channellized flow is initiated by slope‐induced scouring and subsequent headward erosion to form a channel that connected with the valley, while the removed sediment is deposited in a rapidly prograding delta lobe. The resulting decrease in channel gradient causes a reduction in flow strength, mouth‐bar formation, flow bifurcation and progressive backfilling of the channel. In the final stage of channel filling, sheet flow coexists for a while with channellized flow (semi‐confined flow), although in cycle 1 this phase of semi‐confined flow was absent. Subsequent autocyclic incisions are very similar in morphology and gradient. However, they erode deeper into the delta plain and, as a result, take more time to backfill. The duration of the semi‐confined flow increases with each subsequent cycle. During the period of sheet flow, the delta plain aggrades up to the ‘critical’ gradient required for the initiation of autocyclic incision. This critical gradient is dependent on the sediment transport capacity, defined by the input conditions. These autogenic cycles of erosion and aggradation confirm earlier findings that storage and release of sediment and associated slope variation play an important role in fan‐delta evolution. The erosional surfaces produced by the autocyclic incisions are well‐preserved by the backfilling process in the deposits of the fan deltas. These erosional surfaces can easily be misinterpreted as climate, sea‐level or tectonically produced bounding surfaces.     

基于支持向量机的浊积扇低渗透储层流动单元研究

由于浊积扇储层成因机制的复杂性,本文通过因子分析从反映储层岩石物理特征、微观孔隙结构特征、储层沉积特征及储层非均质特征的13个储层参数中提取了5个主因子,并以此作为预测模型的输入,采用支持向量机的算法来建立流动单元预测模型。应用该模型对大芦湖油田樊29块10口关键井的预测样本进行检验,正判率达90.38%。运用建立的预测模型对研究区4⁴¹单砂层流动单元进行划分,将预测结果与定性分析结果对比,具有较高的吻合度。平面上,流动单元划分结果与沉积微相展布具有较高的对应关系：Ⅰ类、Ⅱ类流动单元储层质量最好,仅在浊积水道及朵叶体微相发育;Ⅲ类流动单元分布广泛,在浊积水道、水道侧缘及中扇侧缘微相均有分布;Ⅳ类流动单元主要分布在浊积扇外缘的扇缘亚相。结果表明,建立的流动单元模型能够较为系统地反映多种地质因素与流动性能之间的复杂映射规律。     

Quaternary alluvial fans in the Gobi of southern Mongolia: evidence for neotectonics and climate change

Alluvial fans in southern Monglia occur along a group of narrow discontinuous mountain ranges which formed as transpressional uplifts along a series of strike-slip faults. They provide information on the nature of neotectonic activity in the eastern Gobi Altai range and on palaeoclimate change. Alluvial fan formation was dominated by various geomorphological processes largely controlled by climatic changes related to an increase in aridity throughout late Quaternary times. Their sedimentology shows that initially they experienced humid conditions, when the sedimentary environments were dominated by perennial streams, followed by a period of increasing aridity, during which coarse fanglomerates were deposited in alluvial fans by ephemerial streams and active-layer structures were produced by permafrost within the alluvial fan sediments. With climatic amelioration during early Holocene times, the permafrost degraded and fan incision and entrenchment dominated. Sedimentation was then confined to the upper reaches of the fans, adjacent to steep mountain slopes, and within the entrenched channels. The alluvial fans have been neotectonically deformed, faulted and their surface warped by small thrust faults that propagate from the mountain fronts into their forelands. Localised uplift rates are in the order of 0.1 to 1 m Ka⁻¹. © 1997 John Wiley & Sons, Ltd.     

巴西砂矿金刚石产地特征及比较研究

金刚石产地来源的确定是国际性难题,其中由于砂矿金刚石可能存在搬运过程的混合,其产地来源的确定更加具有争议性。巴西金刚石绝大部分来源于砂矿,从成矿地质背景、形貌和颜色、内部结构、矿物包裹体、微量元素、C同位素组成分布等六个要素总结分析了巴西金刚石研究的相关成果,为国际砂矿来源金刚石产地辨识提供参考。研究结果显示,巴西大部分矿区金刚石的特征与世界范围其它矿区的相似,以橄榄岩型为主,难以相互区分。但其中Juina地区和Machado河产地的金刚石具有独特的氮含量（无氮的Ⅱ型为主）和聚集状态,内部结构、矿物包裹体组合及温压环境特征均显示出深部地幔来源的特点,和巴西其它产地具有一定的区分度。与世界范围不同产地砂矿来源金刚石的基本特征对比显示,不同国家具体地区砂矿金刚石在上述不同要素的组合上存在一定的差异性,但总体上要借助现有资料进行产地来源的准确判断还非常困难,需要进行更深入细致的工作。     

山西左云地区下白垩统左云组沉积相及其古气候意义

最近,山西左云地区下白垩统左云组中发现了保存完好的剑龙化石,笔者对左云组的沉积相、剑龙的埋藏环境及当时的古气候和进行了分析。系统的沉积学研究表明,左云组为一套典型的洪积扇沉积,是由泥石流和暂时性水流作用交替发育而形成的产物。根据岩性特征和剖面结构,可将左云组划分为扇根、扇中、扇缘及扇间四个亚相,剖面上可进一步识别出泥石流沉积、主槽—河道、漫洪—漫流带、辫流线四个微相。结合孢粉及古生物数据分析,早白垩世晚期,左云地区总体为亚热带沙漠边缘干旱环境,但仍体现出干旱和潮湿的无规律变化,突发的季节性暴雨或特大暴雨(千年一次)的出现,形成特大洪峰,短时间内形成巨厚的洪积扇砾岩—泥岩。剑龙埋藏环境为扇根—扇中部位漫洪—漫流带,是特大洪水期溢出主槽而于山前形成的"片汜沉积",剑龙陷于厚层泥质中而被埋藏保存下来。     

砾质辫状河型冲积扇沉积微相及沉积模式:以准噶尔盆地西北缘现代白杨河冲积扇为例*

冲积扇砂砾岩储集层具有相变快、连续性差等特点,显示冲积扇内部沉积环境的复杂性。本次研究以准噶尔盆地西北缘现代白杨河冲积扇为例,在丰富的野外露头资料基础上,结合区域水文资料以及冲积扇文献资料,对白杨河冲积扇沉积演化特征及沉积模式进行探讨。按其沉积机制,白杨河冲积扇属于辫状河型冲积扇,具有规模大(扇体总面积约327.6km²)、坡度平缓(约4‰~7‰)、沉积粒度粗和漫洪细粒沉积物不发育的典型特征。在白杨河冲积扇内共可识别出16种岩相类型,并根据岩相形成的流体动力差异划归为5类成因,即重力流成因、高流态牵引流成因、低流态牵引流成因、静水沉积成因以及风成沉积成因。白杨河冲积扇扇体建造过程可划分为洪水期和间洪期2个时期,洪水期以沉积作用为主,从扇根至扇缘依次发育扇根补给水道沉积、扇根片流沉积、扇中片流沉积、扇中辫状水道沉积、扇缘径流水道沉积和扇缘湿地沉积6种沉积微相类型;间洪期以改造作用为主,从扇根至扇缘依次发育扇根主槽沉积、扇中辫状沟槽沉积和扇缘湿地沉积3种沉积微相类型。最后,依据白杨河冲积扇建立了砾质辫状河型冲积扇洪水期和间洪期的沉积模式。     

Evolution and architecture of a West Mediterranean Upper Pleistocene to Holocene coastal apron‐fan system

The Quaternary deposits of tectonically stable areas are a powerful tool to investigate high‐frequency climate variations (<10 ka) and to distinguish allogenic and autogenic factors controlling deposition. Therefore, an Upper Pleistocene–Holocene coastal apron‐fan system in north–western Sardinia (Porto Palmas, Italy) was studied to investigate the relations between climate changes, sea‐level fluctuations and sediment source‐supply that controlled its development. The sedimentary sequence records the strong influence of local (wet/dry) and worldwide (sea‐level) environmental variations in the sedimentation and preservation of the deposits. A multi‐disciplinary approach allowed subdivision of the succession into four major, unconformity‐bounded stratigraphic units: U1 U2, U3 and U4. Unit U1, tentatively dated to the warm and humid Marine Isotopic Stage (MIS) 5, consists of sandy, gravelly coastal/beach deposits developed during high sea‐level in low‐lying areas. Unit U2 consists of debris‐flow dominated fan‐deposits (ca 74 ka; MIS 4), preserved as partial fills of small valleys and coves. Unit U2 is mainly composed of reddish silty conglomerate to pebbly siltstones sourced from the Palaeozoic metamorphic inland hills (bedrock), superficially disintegrated during the preceding warm, vegetation‐rich MIS 5. The cold and semi‐arid climate strongly reduced vegetation cover along the valley flanks. Therefore, sediment gravity‐flow processes, possibly activated by rainstorms, led to deposition of debris‐flow dominated fans. Unit U3 consists of water‐flow dominated alluvial‐fan deposits (ca 47 to 23 ka; MIS 3), developed on a slightly inclined coastal plain. Unit U3 is composed of sandstone and sandy conglomerate fed from two main sediment sources: metamorphic inland bedrock and Quaternary bioclastic‐rich shelf‐derived sands. During this cold phase, sea‐level dropped sufficiently to expose bioclastic sands accumulated on the shelf. Frequent climate fluctuations favoured inland aeolian transport of sand during dry phases, followed by reworking of the aeolian bodies by flash floods during wet phases. Bedrock‐derived fragments mixed with water‐reworked, wind‐blown sands led to the development of water‐flow dominated fans. The Dansgaard–Oeschger events possibly associated with sand landward deflation and main fan formations are Dansgaard–Oeschger 13 (ca 47 ka), Dansgaard–Oeschger 8 (ca 39 ka) and Dansgaard–Oeschger 2 (ca 23 ka). No record of sedimentation during MIS 2 was observed. Finally, bioclastic‐rich aeolianites (Unit U4, ca 10 to 5 ka; MIS 1), preserved on a coastal slope, were developed during the Holocene transgression (ca 10 to 5 ka; MIS 1). The studied sequence shows strong similarities with those of other Mediterranean sites; it is, however, one of the few where the main MIS 4 and MIS 3 climatic fluctuations are registered in the sedimentary record.     

川西北九龙山地区下侏罗统自流井组珍珠冲段沉积相研究

根据单因素分析多因素综合作图法,对川西北九龙山地区珍珠冲段岩石学特征、沉积构造、植物化石特征等进行详细研究,并结合测井资料,分析九龙山地区珍珠冲段沉积相,深入研究其平面展布特征进行,探讨其演化规律,建立研究区沉积相发育模式。研究认为珍珠冲段沉积期九龙山地区主要发育湿地型扇三角洲相,并且发育一套正旋回的层序,为该地区油气勘探工作提供指导。      

Sedimentary Patterns and Stratigraphic Trap Models of Deeply Buried Intervals in the Baxian Depression, North China

The Baxian depression is a typical half-graben located in the Jizhong sub-basin, north China. Commercial petroleum traps have been discovered in the Jizhong sub-basin. However, the 3rd and 4th members of the Shahejie Formation in this sub-basin have been poorly explored. These two members, belonging to the Lower Paleogene age, are buried deeply in the depression. Favorable petroleum reservoir conditions exist in such deep intervals of the half-graben due to the presence of different types and extent of deltas and turbidity fans in various areas. In fact, three types of turbidite fans are developed in the sag below the transitional belt on the eastern gentle slope. This work summarized three stratigraphic trap belts, i.e., the steep slope, gentle slope, and sag. On the steep slope, structural-stratigraphic traps with small-scale delta fronts and turbidite sandbodies are well developed. On the gentle slope, hydrocarbons generally accumulate in the large-scale delta front, onlapping beds and those sandbodies adjacent to unconformities. In the sag, petroleum trap models are typically characterized by pinched-out turbidite sandbodies. Stratigraphic traps were easily formed in turbidite fans below the eastern transitional belt. The petroleum traps that have already been discovered or predicted in the study area indicate that stratigraphic traps have favorable petroleum exploration potential in deeply buried areas (depth >5000 m) in a half-graben basin or depression.     

歧口凹陷歧北斜坡远岸重力流湖底扇沉积的 地震相特征

以古地貌认识为指导,克服钻井少的特点,应用landmark地震地质解释平台,从平面和剖面两个角度开展了地震相分析,以进一步明确该区远岸湖底扇沉积的地震相特征、扇体的延伸距离及波及范围,为今后该区的井位部署工作提供有力的技术支撑。研究表明,在剖面上,横切物源方向能识别出强振幅、中频、低连续性的杂乱反射结构,代表了水动力不稳定的高能环境,是湖底扇、重力滑动或构造变动的标志之一,而在顺物源方向能识别出规模较小的叠瓦型前积反射构造,前积层倾角十分平缓,代表了该区发育小型沟道;在平面上,提取了均方根振幅、总能量及瞬时频率等地震属性,反映了该区砂体平面上的展布特征及岩性的变化规律,与上述地震剖面特征综合判断,该区沙一下段在张北断层及歧东断层下降盘处发育4个湖底扇体,主要发育中扇辫状沟道及外扇席状砂微相,向西北方向展布,与北部沧县物源砂体交互。