塔中I号坡折带上奥陶统台缘礁滩复合体建造模式

塔中Ⅰ号坡折带的台缘高能区发育上奥陶统良里塔格组礁滩体,主要包括生物礁丘、灰泥丘、粒屑滩、滩间海等沉积类型,纵向上表现为粒屑滩、灰泥丘、礁丘的多旋回组合。单个礁滩复合体下部发育粒屑滩,上部为灰泥丘和(或)礁丘,之上为下一旋回的粒屑滩亚相所覆盖。横穿宽3～10km的区域,特别是台地边缘向陡坡转折的高地为礁生长相对集中区,而向内侧则更多出现滩相。不同沉积期礁滩单元分布范围不同,从良三段到良一段沉积期间内呈现逐步向塔中Ⅰ号坡折带迁移的缩减趋势。经后期多次成岩作用和构造破裂的改造,形成沿台缘分布且具有非均质性变化的礁滩型储层。     

塔里木盆地东部古城地区奥陶系碳酸盐岩微相特征和沉积环境

对塔里木盆地东部的勘探程度较低,是塔里木盆地未来油气勘探的新远景区,对其沉积微相类型及沉积演化认识较薄弱.采用碳酸盐岩微相分析的方法,以沉积学理论为指导,充分利用岩心、录井、测井、地震及岩石薄片等资料,分析了塔里木盆地东部古城地区中-下奥陶统碳酸盐岩微相类型、沉积特征及分布规律.研究表明,塔里木盆地东部古城地区中—下奥陶统碳酸盐岩发育8种微相类型,根据微相类型组合特征,尝试建立了研究区碳酸盐岩微相的空间展布模式,同时将研究区划分出局限台地、开阔台地、台地边缘和台缘斜坡等4个沉积相带;探讨了古城地区早-中奥陶纪沉积环境的演化,认为广泛发育的台地边缘浅滩形成了奥陶系碳酸盐岩是最为有利的储集相带.     

Carbonate petrology of an Upper Ordovician—Silurian section at the Lone Mountain,Eureka County,Nevada

This preliminary petrographic study of a Paleozoic carbonate section (Upper Ordovician to Silurian) in the Lone Mountain district in east-central Nevada provides new information on its microfacies and dolomitization. The occurrence of diversified microfacies in rocks from this section reveals the complex nature of their depositional history, as well as the subsequent diagenesis. Most microfacies from these rocks fall into two general groups: (1) dolomite facies which predominates in the Hanson Creek Formation (Upper Ordovician to Lowest Silurian) and the Lone Mountain Unit 1; and (2) dolomitized limestone facies which is common in the Lone Mountain Formation (Upper Silurian).The dolomite in the mid-Paleozoic carbonate section of the Lone Mountain is overwhelmingly diagenetic in origin. The early diagenetic dolomite is characterized by its homogeneous texture, uniform medium-to-coarse grain size and poorly preserved fossil impressions. The fossil content in this type of dolomite indicates its original calcareous nature. On the other hand, the late-diagenetic dolomite is characterized by its heterogeneous texture, non-uniform grain-size, detrital-quartz content and the presence of partly or incompletely altered fossil structures.     

塔里木盆地奥陶系碳酸盐岩颗粒滩沉积组合及展布特征

本文综合露头、钻/测井、地震以及薄片等资料的研究,开展了塔里木盆地奥陶系碳酸盐岩颗粒滩沉积特征的研究。本次研究首次厘定了颗粒滩的划分标准,总结了塔里木盆地奥陶系发育藻粒滩、内碎屑滩、鲕粒滩和生物碎屑滩四类颗粒滩;明确了四类颗粒滩的微观和宏观沉积特征:藻粒滩由藻粒、球粒和藻团块构成,与结晶云岩互层沉积;内碎屑的含量和形态揭示了内碎屑滩的部位和沉积区的能量,内碎屑滩自鹰山组开始大规模发育;鲕粒滩主体区其他颗粒类型不发育,鲕粒滩主要分布在塔中和玉北地区的台缘带;生物碎屑滩由生物骨架碎屑和生物颗粒构成,与生物礁复合共生,集中在台缘带及台缘内侧分布,一间房组开始大规模发育。颗粒滩发育段对应低GR值,主体部位GR曲线呈箱型、漏斗型或钟型,侧缘部位呈指状或尖峰状,台缘带颗粒滩向台内迁移呈退积反射特征。在此基础上,建立了塔里木盆地碳酸盐岩颗粒滩的四种沉积组合样式:藻礁滩互层式、厚层滩加积式、薄层滩退积式和生物礁滩复合式。其中,藻礁滩互层式和礁滩复合式颗粒滩分布受相带影响,具有局限性,易于形成有效油气藏,在塔中地区奥陶系已得到证实。厚层滩加积式和薄层滩退积式颗粒滩在鹰山组分布广泛,结合颗粒滩的沉积展布规律及储层成因分析,得出鹰山组颗粒滩储层具有区域优势分布特征。     

四川盆地东北部长兴期沉积特征与沉积格局

四川盆地东北部长兴组（大隆组）为海洋环境的产物,根据沉积特点,可以分为碳酸盐台地沉积体系和盆地沉积体系。碳酸盐台地沉积体系又可进一步分为局限台地、开阔台地、台地边缘礁滩及缓坡等沉积相。在详细研究分析各沉积体系的沉积特点的基础上,探讨了该期沉积相带的空间分布,提出了不存在“开江一梁平”海槽的认识。笔者等认为在“开江梁平海槽”区域内,长兴组只是水体相对台地较深环境（台棚环境）的产物,为碳酸盐缓坡,不宜称为海槽。指出台地边缘浅滩及生物礁是储层最有利相带,礁白云岩及颗粒白云岩等是储层的有利微相。研究区的生物礁为碳酸盐台地边缘缓坡点礁群,沿着台地边缘断续分布。     

鄂尔多斯盆地临兴—神府地区奥陶系马五段沉积相发育控制因素及沉积模式

鄂尔多斯盆地奥陶系碳酸盐岩是重要的天然气赋存单元。通过对盆地东缘的临兴—神府地区奥陶系马家沟组五段发育的沉积相类型和沉积模式进行研究和总结,结合岩芯、薄片、测井和综合地质录井数据等资料,判定了研究区属陆表海潮坪相沉积。受矿物蒸发析出机理、白云岩生成机理、盆山耦合关系3个基础地质模型控制,结合海平面变化,建立了研究区陆表海碳酸盐岩-蒸发岩综合沉积模式:海退-蒸发背景下,自隆向坳主要发育云坪(泥云坪)—含膏云坪—膏坪等潮上—潮间相带微相;海侵背景下,自隆向坳主要发育灰云坪、云灰坪和灰坪等潮间—潮下带微相。该沉积模式较好地解释了研究区马家沟组五段的复杂岩相和沉积微相的分布规律,预测结果后被钻井证实。成果对提高后续勘探成效和指导类似地区的沉积储层研究具有积极意义。     

塔里木盆地良里塔格地区奥陶系生物礁的沉积特征和成礁模式*

塔里木盆地奥陶系生物礁储集层受到广泛关注。笔者优选塔里木盆地良里塔格地区奥陶系鹰山组顶部生物礁体(NYG-1)和一间房组底部生物礁体(NYG-2)为研究对象,通过详细的野外观察、实测和薄片鉴定,对生物礁的生物类型、岩性特征和沉积相单元进行分析。结果表明:塔里木盆地良里塔格地区鹰山组和一间房组的造礁生物主要由瓶筐石组成,与生物礁相关的岩性有障积礁灰岩、粘结礁灰岩、生屑粒泥灰岩、生屑泥粒灰岩、内碎屑泥粒灰岩、生屑颗粒灰岩、内碎屑颗粒灰岩和球粒颗粒灰岩。NYG-1礁体发育礁基、礁核、礁顶、礁翼和礁盖5个沉积相单元;NYG-2发育礁基、礁核与礁盖3个沉积相单元。从生物类型来看,鹰山组的礁为托盘—障积礁,为近似圆形的台内点礁,展布无明显的方向性;一间房组的礁为托盘障积—粘结礁,属台缘礁,常呈现出礁体迁移的特征。鹰山组和一间房组成礁模式的共同点是都经历了礁基的形成、礁体的发育和礁体的衰亡3个阶段,而差异主要表现在生物礁体的灭亡阶段: 对鹰山组生物礁而言,海平面的快速下降和高能水动力条件导致了瓶筐石的灭亡和内碎屑灰岩的产生;对一间房组生物礁而言,海平面的上升和浮游藻类生物大量发育造成造礁生物甁筐石的灭亡。以上研究对于建立符合塔里木盆地地区条件的礁体沉积模式和探索露头与井下生物礁对比研究有重要意义。     

西沙海域西琛1井生物礁性质及储层岩石学特征

西沙海域西琛1井生物礁主要是由红藻门壳状珊瑚藻、有节珊瑚藻和绿藻门仙掌藻等钙藻组成的植物礁,其次为珊瑚礁。礁相类型主要有礁核相和礁后泻湖相。岩石矿物成分单一,以碳酸盐矿物为主,包括低镁方解石和铁白云石;结构组分有生物骨架、粒屑、泥晶和亮晶;结构类型有生物格架结构、生物障积结构、生物节片结构、生物捆扎结构和生物粘结结构。岩石类型包括骨架石灰岩/白云岩、粘结石灰岩/白云岩、粒屑石灰岩/白云岩。储集空间类型有粒间孔、生物体腔孔和藻架孔等原生孔隙和铸模孔、裂缝、颗粒内溶蚀孔、藻类溶孔和扩大的粒间溶孔等次生孔隙。孔隙组合类型以粒间孔＋溶孔＋晶间孔最为发育,储集性能较好。     

Early Palaeozoic carbonate reservoirs from the Yingshan Formation of Well block ZG‐43 in Tazhong Low Rise,Central Uplift,Tarim Basin,NW China: geological features and controlling factors

Marine carbonate reservoirs, as a focus of petroleum exploration and development in China, are involved with high exploration risk and prediction difficulty owing to high heterogeneity and diversity of reservoir beds. In the Tarim Basin, NW China, carbonate reservoirs host about 38% of the whole basin's hydrocarbon resources in a large prospecting area mainly distributed in the Cambrian and Ordovician rocks in central (Tazhong) and northern (Tabei) Tarim. Recently, a better understanding has been made of the karsted weathering crust at the top of the Lower Ordovician Yingshan Formation in the northern slope area of the Tazhong Low Rise, Central Uplift, Tarim Basin. As a new frontier of exploration, oil/gas distribution and controlling factors of carbonate reservoirs in the Yingshan Formation are not clearly understood. In this work, we investigated the reservoir beds and oil/gas properties in 13 wells in Well block ZG‐43 on the No. 10 structural belt in the Tazhong Low Rise, and studied hydrocarbon accumulation characteristics with seismic and geochemical data. The Yingshan Formation in Well block ZG‐43 is mainly composed of calcarenite, dolomitic limestone, dolomite, cryptite, as low porosity and low permeability reservoir beds, with fracture‐void porosity constituting the main reservoir pore space. Oil/gas is quasi‐layer distributed beneath the unconformity between the Yingshan and Lianglitag formations to a depth of 140 m. The oil in Well block ZG‐43 is condensate with low density, low viscosity, low sulphur, low resin, low asphaltene, and high wax. The gas is 87.3% methane, generally containing H₂S. The oil/gas distribution pattern is oil in the east and gas in the west, and H₂S content in the west is lower than that in the east. The controlling factors for hydrocarbon are multi‐source supply and multi‐phase charging, interstratal karstification, hydrothermal activity, structural location and sealing condition. A structural–lithological trap is the main type of oil/gas accumulation. Oil/gas distribution was clearly affected by strike–slip faults. Oil/gas with multi‐source supply and multi‐phase charging was controlled by favourable local palaeo‐highs, and affected by later karsting and hydrothermal activity, as well as gas invasion in the Himalayan (Cenozoic) period. Under the caprock of compact limestone in the third to fifth members of the Lianglitag Formation, oil/gas migrated up along the strike–slip fault planes, and moved laterally to both sides in a ‘T’ shape, and formed large‐scale quasi‐layer condensate gas reservoirs controlled by reservoir bed quality. Copyright © 2013 John Wiley & Sons, Ltd.     

塔中Ⅰ号坡折带上奥陶统成礁背景分析

自塔中Ⅰ号坡折带上奥陶统良里塔格组发现生物礁滩作为重要储集层后,有关礁的形态和规模一直存在较大的争议。本研究解析该地区礁的古生物组合、古生态结构和沉积环境,以此为基础系统论证生物礁的沉积序列。礁的建造方式可归属于珊瑚骨架礁丘、层孔虫—海绵骨架礁丘、海绵—层孔虫骨架礁丘以及管孔藻—层孔虫骨架礁丘几种生态类型,这些骨架礁丘无论从造礁生物多样性、生态组合特征以及造礁规模,皆不同于前时代的前寒武纪叠层石礁、寒武纪古杯礁、早奥陶世的灰泥丘和托盘海绵礁,而呈现出单个礁体厚度大、发育旋回多之特点,多期礁体的垂向叠加、横向迁移造成礁体叠置连片, 形成了呈条带状展布的大型生物礁群。     

鄂尔多斯盆地南部中寒武统张夏组颗粒滩沉积特征及控制因素*

碳酸盐岩颗粒滩是一种十分重要的油气储集体。综合野外露头剖面、岩心、测井、录井及薄片资料,对鄂尔多斯盆地南部地区中寒武统张夏组颗粒滩沉积特征、分布规律及控制因素进行分析。结果表明:研究区张夏组碳酸盐岩颗粒滩沉积广泛发育,以鲕粒滩为典型代表的颗粒滩沉积占绝对优势;颗粒滩岩性主要为颗粒石灰岩、颗粒白云岩和晶粒白云岩3种,颗粒类型主要有鲕粒、生屑、砾屑和砂屑等;颗粒滩纵向上通常表现为下细上粗的反旋回序列,多期旋回垂向叠置,形成相当厚度规模的颗粒滩沉积。结合鄂尔多斯盆地寒武纪岩相古地理特征,认为研究区颗粒滩发育特征及平面展布规律受海平面相对变化、基底古地貌及构造活动综合控制,尤其是鄂尔多斯盆地南部寒武纪近北东向裂陷槽的发育,对张夏组台地边缘带鲕粒滩发育及分布的控制作用更为显著。     

中国海相碳酸盐岩油气储层基本特征

中国在碳酸盐岩油气勘探中相继发现了陕中气田、轮南—塔河油田、塔中油气田、川东北大气区等大型油气田。这些发现揭示了非均质性强的碳酸盐岩储层,主要有4种类型:台缘生物礁(滩)、岩溶风化壳、白云岩和台内颗粒滩四大类储层。中国的台缘礁滩储层主要有两类,一类是面积小的孤立状的塔礁(川东和川东北),另一类是连片分布似层状的台缘礁滩复合体。岩溶风化壳储层依岩溶成熟度和构造型式有三种,即克拉通内岩溶老年期的低幅度地貌层状白云岩储层,岩溶壮年期的高幅度地貌非均质裂缝-洞穴单元储层,以及断块掀斜翘倾的岩溶青年期独立岩溶储层。有油气前景的白云岩储层主要为三类:与蒸发台地相关的准同生白云岩,埋藏成岩作用白云岩和生物成因白云岩。台内颗粒滩主要有两类:即白云石化颗粒滩和裂缝性颗粒滩储层。中国的碳酸盐岩分布广,达300km2,四大类储层发育良好,除已发现油气的四大盆地外,相信不久的将来在中国南方和羌塘地区古特提斯域碳酸盐岩沉积区将取得大的突破。     

峡东宜昌黄花场下奥陶统生物礁的研究

首次发现峡东下奥陶统生物礁。造礁生物为攀苔藓虫、托盘藻、古钵海绵、蓝细菌和Pulchrilamina等。按造礁生物形成的格架类型有:粘结粒泥岩,藻粘结岩,攀苔藓虫障积岩,粘结障积岩等4种礁岩微相。这些微相在礁核中有、无和分布差异形成了灰泥岩隆礁,藻岩隆礁,攀苔藓虫障积岩隆礁和藻粘结-古钵海绵障积岩隆礁。各类礁都经历了奠基、发育、衰亡3个阶段。这一演化过程反映了沉积环境和水能量强度对礁发育的制约作用。     

塔北奥陶系碳酸盐岩沉积演化特征及台地发育模式

通过对塔北奥陶系碳酸盐岩沉积相类型、相带区域组合及分布特征的研究,根据岩石特征、沉积结构和相序特征的分析,建立了塔北奥陶系碳酸盐岩的沉积演化模式。研究认为塔北下奥陶统蓬莱坝组、中下奥陶统鹰山组、中奥陶统一间房组和上奥陶统吐木休克组沉积相横向分布较稳定。蓬莱坝组、鹰山组鹰四段、鹰三段为局限台地相;鹰二段为半局限台地相;鹰一段和一间房组属于开阔台地相,但向上水体不断变浅,一间房组沉积时期开始出现点礁;上奥陶统吐木休克组沉积时期区域整体沉降,为沉没台地-斜坡-盆地沉积。上奥陶统良里塔格组沉积时期盆地整体抬升,区域沉积厚度和沉积相出现较大变化,早中期发育碳酸盐开阔台地相,后期为典型的台地边缘相。     

塔北西部一间房组碳酸盐岩礁滩体同生期暴露溶蚀作用模式

在详细的钻井资料和岩心的宏观及微观特征研究的基础上,通过对塔北隆起西部奥陶系中统一间房组礁滩体沉积相特征及展布规律的分析,结合礁滩体储层同生期溶蚀作用特征研究,按照古沉积地貌单元结合岩相的方法建立了一间房组礁滩体同生期溶蚀作用模式。研究认为一间房组开阔台地相包括台内滩、滩间海和台内点礁3个亚相,沉积相平面分布稳定;一间房组的同生期暴露溶蚀作用明显,普遍反映出滩体的间歇性暴露的特征:生物钻孔和溶蚀孔洞内充填礁滩体暴露形成的古土壤泥;发育潮间带的化石碎片堆积和软体生物活动遗迹;岩心观察和成像测井可以识别出渗流-潜流带溶蚀作用旋回。按照礁滩相沉积地貌结合岩相的模式,将一间房组开阔台地的浅滩和点礁沉积划分出潮上长期暴露滩、潮间间歇暴露滩、潮下高能滩、瓶筐石障积礁、潮汐水道等地貌单元。     

塔里木盆地轮南地区奥陶系沉积储层研究新进展

近期轮南奥陶系沉积储层研究取得了几个方面的重要进展:发现了两期三套风化壳岩溶储集系统,其中海西期一套,存在于奥陶系顶部,加里东期两套,存在于奥陶系内部;确定了轮古东奥陶系储集层为风化壳岩溶缝洞性储层;发现了桑塔木组礁滩相储集层。桑塔木组沉积早期轮南地区具有成礁环境,局部发育有小型生物礁;修正了以往对奥陶系沉积相的划分,将奥陶系碳酸盐台地边缘的位置东移30km;提出了奥陶系内部风化壳储层形成的热沉降机制,认为晚奥陶世早期塔里木盆地存在一次热升降运动,该运动使轮南地区发生不均衡抬升,奥陶系良里塔格组出露剥蚀,形成轮南地区广泛分布的风化壳岩溶储集系统,从而揭示了轮南地区奥陶系更为良好的勘探前景。     

Sedimentary facies and model of shallow water carbonates platform of the Lower Cambrian Longwangmiao Formation in Sichuan Basin

Building the sedimentary model accurately and delineating the spatial-temporal distribution of grain bank are the keys to increase the rate of success in natural gas exploration. Thus,the types and characteristics of sedimentary facies of the Longwangmiao Formation in the Sichuan Basin were studied based on field outcrops,drilling,logging and seismic data. Taking a fourth-order sequence systems tract as a unit,the spatial-temporal distribution characteristics of sedimentary facies within the chronostratigraphic framework were precisely described,and the depositional model of the Longwangmiao Formation was established to reveal the development mechanisms of grain bank and point out the future natural gas exploration. The results showed that: (1)The typical shallow carbonate platform depositional system was developed in the Longwangmiao Formation of the Sichuan Basin and its surrounding areas. Mixed tidal flat,restricted platform(microfacies types including grain bank,dolomitic tidal flat,lagoon,and gypsum lagoon),open platform(microfacies types including grain bank,lime tidal flat),platform margin,and slope-basin were developed from the edge to the interior of the basin;(2)A sedimentary model was established,which was characterized by “one depression,two uplifts,and two grain banks in the form of belts”. The development of grain banks was mainly controlled by palaeo-topography and relative sea-level fluctuation. It was mainly developed in the highstand systems tract of a fourth-order sequence vertically,and distributed as a belt along the platform margin and around the lagoon area horizontally. Our paper pointed out the grain shoal deposits are the material basis for the development of high-quality reservoirs,the results thus provide a sedimentary geological basis for future natural gas exploration of the Cambrian Longwangmiao Formation in the Sichuan Basin.     

四川盆地下寒武统龙王庙组浅水碳酸盐岩台地沉积相特征及模式*

准确建立四川盆地龙王庙组沉积模式和精细刻画颗粒滩时空分布规律,是提高天然气勘探成功率的关键。依据野外露头、钻井、测井和地震等资料,综合分析四川盆地及周缘下寒武统龙王庙组沉积相类型及特征;以四级层序为单元,精细刻画等时地层格架内沉积相时空展布特征,建立沉积模式,揭示颗粒滩发育规律,指出下一步天然气勘探方向。结果表明: (1)四川盆地及周缘龙王庙组发育典型浅水碳酸盐岩镶边台地沉积体系,自盆地西缘向东依次发育混积潮坪、局限台地(颗粒滩、云坪和咸化潟湖等微相类型)、开阔台地(颗粒滩、灰坪等微相类型)、台地边缘和斜坡—盆地等沉积相类型;(2)建立了“一凹两隆,双滩成带”的沉积模式,颗粒滩的发育主要受控于古地貌和相对海平面升降,具体表现为垂向上主要发育于四级层序海退时期,平面上沿台地边缘带和围绕咸化潟湖的相对隆起区呈带状分布。由于颗粒滩是优质储集层发育的有利相带,因此该结果可为四川盆地寒武系龙王庙组下一步天然气勘探提供沉积地质基础。     

Distribution of Chancelloriids in a Middle Cambrian Carbonate Platform Deposit, Taebaek Group, Korea

The onset of the Cambrian witnessed the diversification of “small shelly fossils (SSF)”, which affected carbonate depositional system. One of the problematic SSF, chancelloriids, are common components of the early to middle Cambrian carbonate and shale, and their contributions toward Cambrian carbonates are not yet fully understood. This study assesses distribution patterns of chancelloriid sclerites in the Cambrian Series 3 Daegi Formation based on microfacies analysis and discusses their sedimentologic implications. In the lower part of the formation, partially articulated chancelloriid sclerites occur mainly in bioclastic packstone and grainstone facies, with isolated sclerite rays in nodular packstone to grainstone facies. In the middle part of the formation, chancelloriid fragments occur only sporadically in bioclastic wackestone to packstone, bioclastic grainstone and oolitic packstone to grainstone facies, whereas boundstone facies are nearly devoid of their fragments. There are no chancelloriid fragments in the upper part of the formation, which consists of oolitic packstone to grainstone facies. Chancelloriids are interpreted to have primarily occupied platform margin shoal environments, shedding their sclerites to surrounding areas, and thus contributed as sediment producers. The distribution of Daegi chancelloriid sclerites is similar to other Cambrian examples, with the exception of common chancelloriids in Cambrian Series 2 reefs and their apparent near absence in the Daegi and other Cambrian Series 3 reefs. This disparity resulted from changes in the ecologic niche of chancelloriids after the end-Cambrian Series 2 reef crisis, coupled with an overall decline of chancelloriids in the middle Cambrian.     

塔中地区中上奥陶统台地镶边体系分析

塔里木盆地塔中地区中上奥陶统碳酸盐岩台地边缘以发育良好的多种粒屑滩和生物礁组成的镶边沉积体系为特征。通过精细的单井相分析,识别出砂屑滩、砾屑滩、生屑滩以及鲕粒滩等,并且区分出3种不同类型的生物礁：①隐藻灰泥丘;②主要由枝状苔藓虫、海绵或（和）珊瑚建造的障积礁;③由石质海绵、托盘类、层孔虫、珊瑚、管孔藻等建造的骨架礁。在面向深水盆地的台缘外带,以发育中～高能粒屑滩和骨架礁组合为特征。在背靠开阔海台地的台缘内带,主要表现为中低能粒屑滩、隐藻灰泥丘以及障积礁的组合。这个台缘镶边沉积体系总体上沿塔中1号断层西侧呈北西-南东向长带状展布,长度100多公里,是本区重要的油气聚集带之一。储层质量台缘外带总体上优于台缘内带,其中骨架礁礁核和粒屑滩灰岩最好,礁间海和滩间海沉积物较差。