鄂尔多斯盆地延长组致密砂岩孔喉结构与油藏物性表征

基于高压压汞和核磁共振测试方法,结合分形理论对鄂尔多斯盆地延长组致密砂岩孔喉结构与油藏物性进行了表征。采用毛管束模型和润湿相模型计算了高压压汞孔喉分形维数,利用核磁共振测试T₂谱分别计算了大孔、中孔、小孔以及总孔隙的分形维数;对各分形维数与油藏物性之间的关系进行了对比分析。研究表明：基于高压压汞曲线计算岩心分形维数时,相比于润湿相模型,毛管束模型计算得到的分形维数与油藏物性之间具有更好的相关性,随着分形维数增加,平均半径减小,孔喉结构非均质性增强,油藏物性变差。核磁总孔隙分形维数与油藏物性相关性较差,大孔和中孔的分形维数与油藏物性具有较好的相关性,其中随着大孔和中孔分形维数增加,岩心渗透率降低,油藏物性变差;与小孔相比,大孔和中孔的分形维数与油藏物性的相关性更强,表明致密砂岩储层物性主要受大孔和中孔控制,分形维数可以有效表征致密砂岩小孔、中孔和大孔对油藏物性的影响。     

准噶尔盆地中侏罗统西山窑组与头屯河组间不整合面特征及其油气勘探意义

准噶尔盆地侏罗系顶、底及其内部不整合广泛发育,其中中侏罗统下部西山窑组和上部头屯河组之间的角度不整合尤为清楚。它表现为区域性不整合,沉积间断时限达4～10 Ma,剥蚀厚度达100～340 m。该不整合“面”的结构由不整合面之上的砂岩、不整合面之下的风化粘土层和半风化岩石组成,以砂岩-泥岩-砂岩的组合为主,平均厚度达70～90 m。依靠风化粘土层,不整合面之上头屯河组的砂岩形成了地层超覆圈闭,不整合面之下西山窑组的砂岩形成了地层削截不整合遮挡圈闭。油气勘探实践表明：该不整合面上、下是油气聚集的重要场所;该不整合面的后期掀斜演变对油气聚集和调整再分配产生了重要影响。     

常规砂岩与砂砾岩分形特征对比及成因分析:以乌石凹陷X构造流沙港组为例

基于常规物性、铸体薄片、高压压汞等实验分析资料,运用分形理论研究乌石凹陷X油田始新统流沙港组常规砂岩与砂砾岩储层分形特征,分析分形曲线特征,分形维数及其与储层物性、孔隙结构参数之间的关系,评价分形维数对不同类型砂岩储渗能力表征的有效性.结果表明:相比常规砂岩,砂砾岩高压压汞分形曲线呈现分段特征,指示发育大孔、小孔、微孔...     

Distinguishing eustatic and epeirogenic controls on Lower–Middle Cambrian boundary successions in West Gondwana (Morocco and Iberia)

Major eustatic fall has been invoked to explain Lower–Middle Cambrian boundary sandstones and faunal replacements on a number of Cambrian palaeocontinents. This proposal has been tested on the Moroccan and Spanish margins of West Gondwana and found to be inadequate to explain stratigraphical developments. In these regions, sandstone intervals long presumed to be regressive and late Early Cambrian in age are now shown to be early Middle Cambrian, and composed of a lower regressive and an overlying transgressive sandstone separated by a regional unconformity. Only the lower tidalites (i.e. Tazlaft Formation in Morocco and lower Daroca sandstones in Spain) record the Hawke Bay eustatic regression in West Gondwana. The Tazlaft is overlain by a newly recognized, unconformably overlying sandstone (Talelt Formation) that onlapped southern Morocco with reactivation of a pull‐apart or transcurrent regime. Up to 150 m of erosion on uplifted blocks in the High Atlas range and foundering of the Souss Basin to the south preceded onlap and deposition of the volcanic‐rich Tatelt, the correlative and depositional analogue of the upper Daroca and lower Valdemides Formations in northern Spain. With folding and erosion, a type 1 depositional sequence boundary also caps the Tatelt at its contact with an overlying, lower Middle Cambrian mudstone‐dominated succession. This unconformity probably occurs in Spain within the Valdemiedes Formation and corresponds to a faunal discontinuity called the ‘Valdemiedes geoevent’. The Iberian ‘Daroca regression’ and Moroccan ‘Asrir regression’ are misnomers, as the sandstones on which they are based are composite units with a lower regressive interval that records eustatic fall and an upper transgressive unit that records epeirogenically driven onlap.     

Depositional history and stratigraphical evolution of the Sakamena group (Middle Karoo Supergroup) in the southern Morondava Basin, Madagascar

The Karoo Supergroup in Madagascar is subdivided into three lithostratigraphical units: the Late Carboniferous-Early Permian Sakoa Group; the Late Permian-Middle Triassic Sakamena Group; and the Triassic-Early Jurassic Isalo Group. The Sakamena Group is fairly well exposed in the southern Morondava Basin, where it is approximately 4000 m thick. The Sakamena Group is separated from the Sakoa Group by an angular unconformity. The Lower Sakamena Formation is characterised by two major facies associations: (1) interbedded muddy conglomerates and coarse sandstones; and (2) interbedded sandstones and mudstones, which were deposited in a rejuvenated rift setting by coarse-grained fluvial systems and debris flows on the rift margins. In the Vatambe area, facies represent fandelta deposition in a saline lake or tongue of the ocean. The Middle Sakamena Formation comprises three major facies: (1) laminated mudstones and sandstones; (2) sandstones; and (3) mudstones. The Middle Sakamena facies were deposited by low gradient meandering streams and in shallow lakes. The Upper Sakamena Formation was deposited in similar environments, except that it is comprised predominantly of red beds. The Isalo Group consists predominantly of coarse-grained sandstones (up to 6000 m thick). These sandstones were deposited by braided streams with the coarse detritus derived from a structural uplift in the east.     

藏北羌塘奥陶纪平行不整合面的厘定及其构造意义

西藏羌塘块体有无变质基底、其前新生代构造属性与演化过程是长期争论的议题。本文报道南羌塘块体北部,中、上奥陶统塔石山组底砾岩平行不整合于浅变质中厚层石英砂岩夹薄层泥灰岩之上。近600粒碎屑锆石测年结果表明浅变质石英砂岩的最大沉积年龄为527±7Ma,300余粒碎屑锆石测年结果表明塔石山组底部石英砂岩的最大沉积年龄为471±6Ma。不整合面上、下石英砂岩最大沉积年龄之差达56Myr,表明这两套石英砂岩之间存在明显的沉积间断,证实了该平行不整合面的时代为奥陶纪早期。另一独立的证据是在邻区发现了早奥陶世花岗岩类岩石(471~477Ma)侵位于该浅变质石英岩,因此将不整合面之下的浅变质石英岩暂命名为荣玛组,归入寒武系地层。阴极发光与年代学研究进一步表明不整合面之上的碎屑锆石主要来源于在"泛非"运动晚期形成的结晶岩,为近源锆石,表明"泛非运动"晚期所形成的结晶岩在奥陶纪早期就已隆升,遭受剥蚀,为区内中上奥陶统沉积岩的形成提供物质来源。该奥陶纪平行不整合面的发现,表明南羌塘块体与喜马拉雅、拉萨等块体相似,同属冈瓦纳大陆体系。南、北羌塘早古生代地层系统之间的显著差异表明在寒武-奥陶纪之交,南、北羌塘块体就已被古大洋盆分隔开,开始各自独立演化。     

鄂尔多斯盆地东缘C-P煤系致密砂岩储层敏感性分析

鄂尔多斯盆地临兴地区上古生界致密砂岩储集层中矿物组成多样、孔隙结构复杂且黏土矿物含量高,直接影响储层改造和开发效果。文章基于X 衍射、铸体薄片、气测孔渗、压汞和敏感性实验,系统研究了储层敏感性及其影响因素。结果表明研究区砂岩中石英和岩屑含量高,长石含量较低,以岩屑砂岩、岩屑石英砂岩、长石岩屑砂岩和石英砂岩为主。黏土矿物主要为伊利石、高岭石、绿泥石以及伊/蒙混层;储层普遍低孔低渗,孔隙结构较差。速敏以太原组最强,山西组最弱,与伊利石+绿泥石含量正相关,高岭石含量负相关。水敏下石盒子组最强,太原组最弱,与伊/蒙混层含量正相关。盐敏与水敏有类似特点,与伊/蒙混层含量表现出正相关。酸敏山西组最强,下石盒子组最弱,与绿泥石和铁白云石矿物含量正相关。碱敏性山西组最强,太原组最弱,受长石、石英和高岭石含量影响。相关认识有助于指导研究区钻井、压裂等施工工艺选择和排采控制。     

Geology of Neogene basins of Buldan-Sarıcaova region and their importance in Western Anatolia neotectonics

The Büyük Menderes and Gediz (Ala?ehir) grabens are two significant segments of the Western Anatolian extensional province. They merge around Buldan-Sar?caova in the east. Outcropping Neogene sedimentary units in this area display a rather complex structure. This paper summarizes the importance and meaning of the data obtained during a detailed investigation of the Neogene units and aims to improve our understanding of the neotectonic evolution of Western Anatolia. The Buldan-Sar?caova Neogene sequence is composed of three different sedimentary units: (1) the Lower Unit, (2) the Middle Unit, and (3) the Upper Unit. The Lower Unit crops out on the Buldan horst which is located between the Büyük Menderes and Ala?ehir grabens. The sequence starts as a coarse conglomerate and sandstone (?salar Formation) and continues as lagoonal-lacustrine mudstone, interbedded with coal seams and shales (Bostanyeri Formation) and also with lacustrine limestones. The age of this succession is Lower-Middle Miocene. The development of the basin is structurally controlled by NNW-trending normal faults. The Middle Unit begins with a conglomerate–mudstone sequence (K?z?lburun Formation), followed by a sandstone–mudstone–marl sequence (Sarayköy Formation). A lacustrine limestone–marl unit occurs at the top (Aktepe Formation). Some thin gypsum lenses and layers are observed in the Sarayköy Formation. The unit contains some brackish-water fossils. The rocks of the Middle Unit crop out mostly at the low altitudes of the Buldan horst, i.e. the southeast piedmont, around the junction of the Büyük Menderes and the Gediz grabens. The Middle Unit was deposited in fluvial and lacustrine environments during the Late Miocene–Pliocene period. These rocks were formed in response to the uplift of the Buldan horst. The Upper Unit, which is composed of conglomerates, was deposited within the Büyük Menderes Graben–Gediz Graben depressions as alluvial fill.     

致密气储层孔喉分形特征及其与渗流的关系--以鄂尔多斯盆地下石盒子组盒8段为例

选取鄂尔多斯盆地盒8段16块致密砂岩样品进行恒速压汞测试,结合同位样品核磁共振实验,分析了致密气储层孔喉分布特征;在此基础上,运用分形几何原理和方法,开展了致密气储层孔喉分形研究,并表征了分形与储层渗流特征和孔隙结构参数的关系。结果表明:致密气储层有效孔隙被亚微米-微米级孔喉所控制,其中孔隙主要为大孔和中孔,喉道由微喉道、微细喉道和细喉道所组成;致密气储层孔隙分布不具分形特征,而孔喉整体和喉道则符合分形结构,且分别对应分形维数D₁和D₂;基于储层孔喉分形结构与其渗流特征,将盒8段致密气储层孔喉分形结构划分为2种类型:Ⅰ型表现为阶段式分形特征,以进汞压力1 MPa为界,大于1 MPa孔喉具有分形特征,且储层阶段进汞饱和度主要由喉道贡献,反之,孔喉不符合分形特征,其进汞饱和度增量由孔隙贡献;Ⅱ型为整体式分形,进汞饱和度几乎全由喉道贡献。储层孔喉分形维数与渗透率、平均喉道半径和主流喉道半径存在较好的负相关性,与微观非均质系数呈现较明显的正相关性,而与孔隙度、平均孔隙半径和平均孔喉半径比之间没有明显的相关性。     

碳封存砂岩储层的孔隙迂曲度分形特征研究

目前“双碳”减排的背景下,砂岩咸水层是最具潜力的二氧化碳封存介质,其孔隙结构特征决定了流体的运移及封存效率,对其进行研究具有重要的科学意义。分形维数常用于定量表征砂岩孔隙结构在三维空间中的分布规律,但以往对砂岩孔隙迂曲度分形特征进行研究时,多依赖于孔隙率、平均孔隙半径、平均迂曲度等特征值计算分形维数,并不能很好的反映岩石孔隙排列分布及孔隙连接的非均质性。文章将砂岩孔隙结构概化为具有迂曲度的毛细管,首先通过核磁共振成像（MRI）技术获取岩石片层图像,以盒维数法获得孔径分布分形维数;随后以孔径分布分形维数为目标值,结合不同迂曲度下的毛细管数量与孔径的分形标量关系,迭代计算迂曲度分形维数。与传统的分形维数计算方法相比,该研究确定的迂曲度分形维数更能体现岩石内部不同孔隙排列方式引起的迂曲度差异。     

Pore Structure Heterogeneity of the Xiamaling Formation Shale Gas Reservoir in the Yanshan Area of China: Evaluation of Geological Controlling Factors

Micro-heterogeneity is an integral parameter of the pore structure of shale gas reservoir and it forms an essential basis for setting and adjusting development parameters. In this study, scanning electron microscopy, high-pressure mercury intrusion and low-temperature nitrogen adsorption experiments were used to qualitatively and quantitatively characterize the pore structure of black shale from the third member of the Xiamaling Formation in the Yanshan area. The pore heterogeneity was studied using fractal theory, and the controlling factors of pore development and heterogeneity were evaluated in combination with geochemical parameters, mineral composition, and geological evolution history. The results show that the pore structure of the reservoir was intricate and complicated. Moreover, various types of micro-nano scale pores such as dissolution pores, intergranular pores, interlayer pores, and micro-cracks are well developed in member 3 of the Xiamaling Formation. The average porosity was found to be 6.30%, and the mean value of the average pore size was 4.78 nm. Micropores and transition pores provided most of the storage space. Pore development was significantly affected by the region and was mainly related to the total organic carbon content, vitrinite reflectance and mineral composition. The fractal dimension, which characterizes the heterogeneity, is 2.66 on average, indicating that the pore structure is highly heterogeneous. Fractal dimension is positively correlated with maturity and clay mineral content, while it is negatively correlated with brittle mineral content and average pore size. These results indicate that pore heterogeneity is closely related to thermal history and material composition. Combined with the geological background of this area, it was found that the pore heterogeneity was mainly controlled by the Jurassic magmatism. The more intense the magma intrusion, the stronger the pore heterogeneity. The pore structure and its heterogeneity characteristics present today are a general reflection of the superimposed geological processes of sedimentary-diagenetic-late transformation. The influence of magmatic intrusion on the reservoir is the main geological factor that should be considered for detailed evaluation of the Xiamaling Formation shale gas reservoir in the Yanshan area.     

The role of the late Cimmerian unconformity for the distribution of kaolinite in the Gullfaks Field, northern North Sea

A study of the Middle Jurassic Rannoch Formation in the Gullfaks Field shows that there is no relationship between the content and distribution of kaolinite and location relative to the late Cimmerian unconformity. From petrographic data most of the kaolinite is interpreted to be detrital, and only traces of authigenic kaolinite are observed. Mass-balance calculations are used to provide guidelines as to the likelihood of kaolinite being formed and preserved in sandstones under the unconformity. The result of the calculations shows that the propagation rate of the dissolution front in the sandstones was probably on average slower than the erosion rates during the formation of the late Cimmerian unconformity. Hence, transformation of significant amounts of feldspar and mica to kaolinite probably did not take place within the sandstones which at present underlie the unconformity in the Gullfaks Field. Periods of exposure during formation of unconformities may thus be less effective causes of kaolinitization in sandstones than has been assumed.     

东昆仑造山带东段下中三叠统研究进展

东昆仑造山带东段下中三叠统包括下三叠统洪水川群和中三叠统闹仓坚沟组.根据实体化石和遗迹化石的研究, 结合区域上的岩性对比, 将洪水川群划分为3个岩性单元, 即底部砂砾岩段、中部灰岩夹砂岩段和上部砂板岩段, 属于下三叠统印度阶和奥伦尼克阶.闹仓坚沟组包括闹一段砂砾岩和闹二段砂板岩, 从中三叠统早安尼锡期延续到中安尼锡期.砂岩的常量元素分析结果表明, 大部分砂岩为杂砂岩, 属于复理石盆地的浊积岩范畴; 砂岩稀土元素的特征值接近于活动大陆边缘和大陆岛弧元素的参数值.      

Siliciclastic deposit of the Nahr Umr Formation, sedimentological and depositional environment studies, central and southern Iraq

The depositional facies and environments were unraveling by studying 21 subsurface sections from ten oilfields in the central and southern Iraq and a large number of thin sections of the Nahr Umr (siliciclastic deposit) Formation (Albian). This formation is mainly composed of sandstone interlaminated with minor siltstone and shale, with occurrence of thin limestone beds. Nahr Umr Formation is subdivided into three lithostratigraphic units of variable thicknesses on the basis of lithological variations and log characters. Mineralogically and texturally, mature quartz arenite and sandstones are the common type of the Nahr Umr Formation. The sandstones are cemented by silica and calcite material and have had a complex digenetic history. Compaction, dissolution, and replacements are the main diagenetic processes. Prodelta, distal bar, distributary mouth bar, distributary channel, over bank, and tidal channel are the main depositional environments recognized for the Nahr Umr Formation, within the studied wells. This formation was deposited in shallow marine and fluvial–deltaic environments and exhibit progradational succession of facies. Eight sedimentary facies that have been identified in the Nahr Umr Formation include claystone lithofacies, claystone siltstone lithofacies, lenticular-bedded sandstone–mudstone lithofacies, wavy-bedded sandstone–mudstone lithofacies, flaser-bedded sandstone–mudstone lithofacies, parallel and cross lamination sandstone lithofacies, trough cross-bedded sandstone lithofacies, and planar cross-bedded sandstone lithofacies. The depositional model of the Nahr Umr Formation environment was built based on the lithofacies association concepts.     

湖南邵阳地区茅口期晚期重力流沉积的发现及意义*

湖南邵阳市邵阳大道两侧出露的龙潭组下段(中二叠世茅口期晚期)发育一套地质特征典型、沉积序列明显的重力流沉积。重力流沉积以块状砂岩、粉砂岩及泥岩互层为主,砂岩底面发育有重荷模、槽模、工具痕及冲刷充填构造,砂岩内部有块状层理、平行层理、包卷层理及粒序层理,互层的泥岩中见类似于古网迹的痕迹化石。可识别出砂质碎屑流、浊流与滑塌沉积。剖面下部含泥砾块状砂岩发育,剖面中上部以发育薄到中厚层砂岩、粉砂岩与泥岩互层为特点。根据重力流沉积物特征及其垂向序列特征,建立了重力流沉积模式,将海底扇划分出内扇、中扇与外扇。通过与附近的短陂桥矿区的龙潭组下段沉积特征进行对比研究,结合华南地区岩相古地理特征,认为位于华南盆地范围的邵阳地区,在中二叠世茅口期晚期(龙潭组下段)发育的重力流沉积,可能意味着华南盆地在扬子陆块和华夏陆块之间的深大断裂在中二叠世末期曾经发生拉张,形成裂谷盆地。     

渗流-应力耦合作用下砂岩声发射及分形损伤特征研究

为探究渗流-应力耦合作用下岩石内部微元体的损伤演化特征,对取自某矿井的砂岩进行了三轴渗透及声发射试验。研究结果表明:砂岩的渗透率变化历经降低、动态平衡、快速增加以及略微回落四个阶段;声发射现象呈阶段性变化特征,随着围压的升高,声发射最大值越滞后;基于柱状分形理论得到的分形特征表明,分形维数逐渐降低,表明砂岩内部损伤经历了一个从无序到有序的变化趋势,分形维数突变点的出现,可作为砂岩即将失稳破坏的前兆;渗流-应力耦合作用下砂岩的损伤值呈指数型函数增加,渗透损伤Ds主要集中于屈服阶段之后,渗透损伤与总损伤的比值随围压的升高呈线性减小;应力加载是损伤产生的主要因素,渗透作用为次要因素;分形维数df与损伤D、渗透损伤Ds之间呈良好的负指数型函数关系。     

准噶尔盆地卡姆斯特地区侏罗系铀成矿模式研究

本文对卡姆斯特地区中侏罗统头屯河组、西山窑组层间氧化带及铀矿化特征进行了归纳与总结,运用层间氧化带砂岩型铀矿成矿理论,重建了该区铀成矿模式,初步认为铀矿化、异常符合层间氧化带控矿理论,受层间氧化带及不整合面控制,总体上遵循层间氧化带砂岩型铀成矿规律。指出区内铀矿找矿目的层主要为中侏罗统头屯河组,次为西山窑组,主要找矿类型为层间氧化带砂岩型,次为不整合面砂岩型、煤岩型,明确了今后该地区找矿方向,可为准噶尔盆地今后铀矿资源勘查工作提供参考。     

ASD field hyperspectral measurements for discrimination of the ferruginous rocks and the iron ore types at El Gedida-Ghorabi area,Bahariya Oasis,Western Desert,Egypt

The ironstone succession at El Gedida-Ghorabi-Naser area of El Bahariya depression is subdivided into lagoonal manganiferous mud and fossiliferous ironstone consisting mainly of hematite and goethite-hydrogoethite. The application of the ASD field spectroradiometer measurements (spectral range) in the ASTER data led to the interpretation of the presence of ferruginous units as quartzitic sandstone, gluconitic sandy clay, and pink marly limestone. The existing iron ore minerals in the iron ore localities were also classified into high Mn hematite, low Mn hematite, goethite, hydrogoethite as well as low- and high-grade Hematite and Barite. Quartz, feldspars, rutile, and clay minerals (e.g., kaolinite and illite) are mainly associated with the iron ore. Accessory minerals of manganese, e.g., psilomelane and pyrolusite, were also present. The Barite mineral is recorded as a common mineral association with the iron ore deposits at El Gedida and Ghorabi localities. The stratigraphical units investigated in the study area include the oldest gravely clayey sandstones of the Bahariya Formation overlain by the fossiliferous and oolitic limestones of the El-Hamra, Qazzun, and Naqb Formations. Quartztic sandstones and clayey sandstones of the Radwan Formation and youngest Quaternary sediments of sandy-clayey materials were often found as intermittent cover and overburden in unconformity surfaces over the iron ore bands.     

Facies model for a sandy ephemeral stream and clay playa complex; the Middle Devonian Trentishoe Formation of North Devon, U.K.

Sediments in the Trentishoe Formation of the Middle Devonian Hangman Sandstone Group (North Devon, U.K.) provide the basis of a model for sandy ephemeral stream and clay playa deposition. Three types of sequence are found, representing proximal, medial and distal areas on an extensive alluvial plain. The Proximal sequence consists of cross-cutting channel-fill sandstones which represent the deposits of a network of low sinuosity sand bed streams. The Medial sequence comprises upwards coarsening cyclothems which start with relatively distal, thinly bedded sandstone and siltstone flood sheets cut by complexes of silt draped channel-fill sandstones and single channel fill sandstones. The flood sheets coarsen and thicken upwards to more proximal multistorey sheet sandstones. The Distal sequences consist of laminated mudstone and sandstone, cut by desiccation and water escape features, alternating with wave rippled sandstones, and represent playa lakes occasionally incised by high sinuosity channels with laterally accreting sandstones. The three sequence types represent the downslope progression from a low sinuosity channel network which passed into an ephemeral flood deposit complex which in turn drained into clay playas.     

Controls on the accumulation of coal and on the development of anastomosed fluvial systems in the Cretaceous Dakota Formation of southern Utah

Alluvial strata of the Cretaceous Dakota Formation of southern Utah are part of a transgressive systems tract associated with a foreland basin developed adjacent to the Sevier orogenic belt. These strata contain valley fill deposits, anastomosed channel systems and widespread coals. The coals constitute a relatively minor part of the Dakota Formation in terms of sediment volume, but may represent a substantial amount of the time represented by the formation. The coals are separated by clastic units up to 20 m thick. The stratigraphically lowest clastic unit of the Dakota Formation lies above an unconformity cut into Jurassic rocks. Incised valleys associated with the unconformity are up to 12 m deep. Two discrete episodes of valley fill sedimentation are recognized, including a lower sandstone unit with conglomerate layers, and an upper, discontinuous, coal-bearing unit. After the valleys filled, the area became one of low relief where extensive mires formed. Peat accumulation was interrupted at least three times by deposition of clastic sediment derived from the west. The clastic units consist of sandstone, mudstone or heterolithic ribbon bodies, stacked tabular sandstones, and laminated mudstones, and contain minor coal beds less than 0·35 m thick. Ribbon bodies are 1–9 m thick and 15–160 m wide, have pronounced basal scours, and are filled with both lateral and vertical accretion deposits. An anastomosed channel complex is suggested by the large number of coeval channels of varying dimensions, the variation in the structure and grain size of channel fills, and the presence of abundant tabular sandstones interpreted as crevasse splays. Although some sandstone bodies have well developed lateral accretion surfaces, the overall ribbon geometry indicates that mature meandering streams were not well developed. This is in contrast to modern anastomosed systems, which are commonly thought to be a transitional morphology caused by avulsion of a meander belt to a new position on its floodplain. Rather than being a transitional channel pattern related to river avulsion, the anastomosed channels of the Dakota Formation may have formed part of a large inland delta that episodically invaded widespread mires. The mires developed during periods when clastic influx was reduced either by high rates of subsidence close to the thrust belt or by deflection of rivers by emergent thrusts.