Pseudo-unconformities in seismic models of large outcrops

Traditionally, seismic modeling has concentrated on one-dimensional borehole modeling and two-dimensional forward modeling of basic structural-stratigraphic schemes, which are directly compared with real seismic data. Two-dimensional seismic models based on outcrop observations may aid in bridging the gap between the detail of the outcrop and the low resolution of seismic lines. Examples include the Dolomites (North Italy), the Vercors (SE France), and the High Atlas (Morocco). The seismic models are generally constructed using the following procedure: (a) construction of a detailed lithologic model based on direct outcrop observations; (b) division of the lithologic model into lithostratigraphic units; (c) assignment of petrophysical properties to these lithostratigraphic units; (d) ray tracing to compute time- or depth sections of reflectivity; (e) convolution of the reflectivity sections with source wavelets of different frequencies. The lithologic detail modeled in the case studies led to some striking results, particularly the discovery of pseudo-unconformities. Pseudo-unconformities are unconformities in seismics, but correspond to rapid changes of dip and facies in outcrop. None of the outcrop geometries studied were correctly portrayed seismically at 25-Hz peak frequency. However, in some instances the true relationship would gradually emerge at peak frequencies of 50–100 Hz. The examples given in this study demonstrate that detailed, outcrop-derived, seismic models can reveal what stratigraphic relationships and features are likely to be resolved under ideal or less-ideal conditions, and what pitfalls may befall the interpreter of real seismic data.     

Facies mosaic and diagenetic patterns of the early Devonian (Late Pragian–Early Emsian) microbialite‐dominated carbonate sequences,Qasr Member,Jauf Formation,Saudi Arabia

The Al‐Jawf area of northern Saudi Arabia provides spectacular outcrops of Early Devonian carbonate bioherms in the Wadi Murayr and Dumat Al‐Jandal areas. These carbonate bioherms belong to the Qasr Member of the Late Pragian–Early Emsian Jauf Formation (~405 Ma) and are surrounded by a bioclastic carbonate succession. The Qasr Member is the first major carbonate unit of the Palaeozoic succession in Saudi Arabia that mainly consists of microbialite carbonates and metazoan reefs exhibiting distinct mound features. These bioherm complexes and their associated carbonate facies are pervasively dolomitized. Stratigraphic, petrographic and geochemical analyses were conducted to determine the facies distribution and interpret their depositional and diagenetic processes. A total of 11 facies are identified from a range of depositional environments within a carbonate platform system, ranging from tidal flats, lagoon, shoal, patch reefs to reef front. The main diagenetic processes are carbonate cementation and dolomitization. Dolomitization occurred as both fabric preserved (mostly in grain‐dominated facies) and fabric destructive (mud‐dominated facies). The microbialites and coralline sponges facies show poor reservoir with visual porosity less than 5%, but this succession may have a potential to serve as a good source for the underlying and overlying facies. Ooid and peloidal grainstone facies show fair to good visual porosity that locally exceeds 10% with intergranular porosity as the dominant type. However, in the most studied samples, vuggy and intraparticle porosities are observed as the dominant type. Copyright © 2015 John Wiley & Sons, Ltd.     

Integration of sedimentology and ground‐penetrating radar for high‐resolution imaging of a carbonate platform

Ground‐penetrating radar has not been applied widely to the recognition of ancient carbonate platform geometries. This article reports the results of an integrated study performed on an Upper Jurassic outcrop from the south‐east Paris basin, where coral bioherms laterally change into prograding depositional sequences. Ground‐penetrating radar profiles illustrate the different bedding planes and major erosional unconformities visible at outcrop. A ground‐penetrating radar profile conducted at the base of the cliff displays a palaeotopographic surface on which the outcropping bioherms settled. The excellent penetration depths of the ground‐penetrating radar (20 m with a monostatic 200 MHz antenna) images the carbonate platform geometries, ranging between outcrop workscale (a few metres) and seismic scale (several hundreds of metres). This study supports recent evidence of icehouse conditions and induced sea‐level fluctuations controlling the Upper Jurassic carbonate production.     

Three-dimensional modelling of an isolated carbonate buildup (Triassic, Dolomites, Italy)

The Middle Triassic carbonate buildups of the Dolomites show facies similarities with mud mounds but display apparent architectural elements of flat‐topped carbonate platforms. In order to test whether the facies similarities to mounds are also reflected in the internal buildup architecture, a three‐dimensional modelling study of the Middle Triassic Monte Cernera buildup has been carried out. The Cernera buildup exhibits apparent geometries suggesting a mounded platform in the lower and uppermost part of the buildup, separated by an interval with apparent platform geometry and a retrogradational platform interior, which is difficult to explain with a flat top platform model. For this purpose, a number of three‐dimensional models were constructed using the three‐dimensional modelling programme petrel ^TM. Key geological horizons were constructed based on outcrop measurements, intermediate horizons were calculated in the modelling program, and the intersections of the modelled layers with a digital topography surface were displayed and compared with outcrop photographs. The models were refined stepwise until a best fit with the actual bedding architecture was achieved. The best fit model shows that the mounded geometries in the lower and uppermost part of the buildup are real architectural elements. The intermediate platform stage, about 1·5 km across, had probably retained a mounded top with a relief of up to 50 m, which is difficult to distinguish from an absolutely flat top, but necessary to explain the retrogradational platform interior. The present study shows that Monte Cernera was dominated by mounded geometries at all stages of platform development. The mounded geometry plus facies data suggest that the platform is a deep‐water accumulation, below the zone of intense wave energy, but within the photic zone. The Cernera represents a tropical buildup type, which did not have the capacity to grow into continuously wave‐swept environments because of the small size and the absence of a wave‐resistant energy barrier. Such buildup types are probably common after major crises in earth history, when reef organisms were virtually absent.     

Subtropical to temperate facies from a transition zone, mixed carbonate–siliciclastic system, Palaeogene, North Carolina, USA

Palaeogene passive margin sediments on the US mid‐Atlantic coastal plain provide valuable insight into facies interaction and distribution on mixed carbonate–siliciclastic shelves. This study utilizes well cuttings, outcrop, core, and seismic data to document temporal and spatial variations in admixed bryozoan‐rich skeletal carbonates and sandy siliciclastic units that were deposited on a humid passive margin located in the vicinity of a major marine transition zone. This zone was situated between north‐flowing, warm waters of the ancestral Gulf Stream (carbonate dominated settings) and south‐flowing, cold waters of the ancestral Labrador Current (siliciclastic dominated settings). Some degree of mixing of carbonates and siliciclastics occurs in all facies; however, siliciclastic‐prone sediments predominate in nearshore settings, while carbonate‐prone sediments are more common in more open marine settings of the inner shelf break and deep shelf. A distinctive dual‐break shelf depositional profile originated following a major Late Cretaceous to Palaeocene transgression that drowned the earlier shallow platform. This profile was characterized by prominent mid‐shelf break dividing the shallow shelf from the deep shelf and a major continental shelf/slope break. Incomplete filling of available accommodation space during successive buildup of the shallow shelf preserved the topographic break on this passive margin. Storm wave base also contributed to the preservation of the dual‐break shelf geometry by beveling shallow shelf sediments and transporting them onto and seaward of the mid‐shelf break. Sediment fines in deep shelf facies were produced in place, transported downdip from the shallow shelf by storm ebb currents and boundary currents, and reworked from adjacent areas of the deep shelf by strike‐parallel boundary currents. Regional climate and boundary currents controlled whether carbonate or siliciclastic material was deposited on the shelf, with warmer waters and more humid climates favouring carbonate deposition and cooler, more arid conditions favouring glaucony and siliciclastic dominated deposition. Continuous wave and current sweeping of the shallow shelf favoured deposition of mud‐lean facies across much of the shallow shelf. Skeletal components in much of the carbonate‐rich strata formed in warm, nutrient‐rich subtropical waters, as indicated by widespread occurrences of larger benthic foraminifera and molluscan assemblages. These indicators of warm water deposition within the bryozoan‐mollusk‐rich carbonate assemblage on this shelf provide an example of a warm water bryomol assemblage; such facies generally are associated with cooler water depositional settings.     

Tertiary syntectonic carbonate platform development in Indonesia

Cenozoic tropical carbonate sedimentation was strongly influenced by local and regional tectonics in SE Asia. This paper outlines the evolution of the syntectonic Eocene to middle Miocene Tonasa Formation of South Sulawesi, evaluating controls on sedimentation, facies distribution and sequence development. Development of a facies model for this Cenozoic tropical carbonate platform provides a meaningful analogue for similar, less well‐studied SE Asian carbonates, which commonly comprise targets for hydrocarbon exploration. This study also has considerable implications for the study of syntectonic carbonates, controls on carbonate sedimentation, carbonate platform development in backarc areas and SE Asian tectonics. Detailed facies mapping, logging, petrographic and biostratigraphic analyses indicate that the Tonasa Formation was deposited initially as part of a transgressive sequence in a backarc setting. By late Eocene times, shallow‐water carbonates were being deposited over much of South Sulawesi forming a widespread (100‐km long) platform area. Shallow‐water sedimentation continued unabated in some areas of the platform until the middle Miocene. Elsewhere, active normal faulting resulted in fault‐block platforms, with local subaerial exposure of footwall blocks and the formation of basinal graben in adjacent hangingwall areas. Platform‐top facies were aggradational and dominated by larger benthic foraminifera. Low‐angle slopes, particularly hangingwall dip slopes, were characterized by the development of ramps. Faults, controlled in part by pre‐existing structures, were periodically active and formed steep escarpment margins. Variable regional subsidence strongly influenced the development of the Tonasa Carbonate Platform, whereas platform‐wide effects caused by regional eustacy have not been identified. Computer modelling of the Tonasa Platform confirms that the accommodation space and sedimentary geometries observed can be produced by block faulting and regional subsidence alone. Modelling also reveals that regional subsidence and extension, oblique to the main stretching direction, were low on the margins of the backarc basin. Shallow‐water accumulation rates for this foraminifera‐dominated tropical carbonate platform were an order of magnitude lower than those for modern warm‐water platforms dominated by corals or ooids.     

Research on Diageneses of Cambrian Shoal Facies Carbonate Rocks in the Xiadong Area, Hubei Province

With continuous outcrops, developed shoal fades rocks, complete types of diagenesis and changeable diagenetic environments, Cambrian strata are well developed in the Xiadong area, Yichang, Hubei Province. Under the combined influence of numerous diageneses, secondary pores can be formed, which result in better reservoir properties of the rock strata.The Cambrian rocks in this area consist of mainly carbonate rocks and secondarily detrital rocks. The carbonate rocks are dominated by grainstones including wormkalk, calcirudite-calcarenite, oolitic limestone and oncolitic limestone. Graded bedding and cross bedding are well developed in the strata, which indicates that the formation environment is of a high-energy shoal facies.In this area, there has developed a sequence of stable Cambrian platform carbonate deposition. The evolution trend is as follows: open sea shelf facies→intertidal low-energy restricted sea facies→Hntertidal high-energy shoal facies→coastal shoal facies→evaporite tidal-flat facies     

Porosity distribution in cyclic dolomites of the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin,China

Increasing interests in hydrocarbon resources at depths have drawn greater attentions to the deeply-buried carbonate reservoirs in the Tarim Basin in China. In this study, the cyclic dolomite rocks of Upper Cambrian Lower Qiulitag Group from four outcrop sections in northwestern Tarim Basin were selected to investigate and evaluate the petrophysical properties in relation to depositional facies and cyclicity. The Lower Qiulitag Group includes ten lithofacies, which were deposited in intermediate to shallow subtidal, restricted shallow subtidal, intertidal, and supratidal environments on a carbonate ramp system. These lithofacies are vertically stacked into repeated shallowing-upward, meter-scale cycles which are further grouped into six third-order depositional sequences (Sq1 to Sq6). There are variable types of pore spaces in the Lower Qiulitag Group dolomite rocks, including interparticle, intraparticle, and fenestral pores of primary origin, inter crystal, and vuggy pores of late diagenetic modification. The porosity in the dolomites is generally facies-selective as that the microbially-originated thrombolites and stromatolites generally yield a relatively high porosity. In contrast, the high-energy ooidal grainstones generally have very low porosity. In this case, the microbialite-based peritidal cycles and peritidal cycle-dominated highstand (or regressive) successions have relatively high volumes of pore spaces, although highly fluctuating (or vertical inhomogeneous). Accordingly, the grainstone-based subtidal cycles and subtidal cycle-dominated transgressive successions generally yield extremely low porosity. This scenario indicates that porosity development and preservation in the thick dolomite successions are primarily controlled by depositional facies which were influenced by sea-level fluctuations of different orders and later diagenetic overprinting.     

Impacts of Depositional Facies and Diagenesis on Reservoir Quality: A Case Study from the Rudist-bearing Sarvak Formation, Abadan Plain, SW Iran

An integrated geological-petrophysical analysis of the rudist-bearing sequence of the Cretaceous Sarvak Formation is given one giant oilfield, and provides an improved understanding of this main reservoir in the Abadan Plain, in the Zagros Basin, SW Iran. The main objective of this study is to evaluate reservoir potential of the Sarvak Formation, and then to utilize the calibrated well log signature to correlate reservoir potential in un-cored wells. Eight main facies are recognized and categori...     

碳酸盐工厂与浅水碳酸盐岩台地: 研究进展与展望*

碳酸盐岩具有广泛的时空分布,主要为生物成因,而该生物成因属性注定了其与海洋生物、海洋化学条件等密切相关。海相碳酸盐岩沉积相模式对于描述、表征和解释碳酸盐岩地层结构成因具有重要的指导意义。本文回顾了近70年来碳酸盐岩沉积相模式从二维水体能量模型到三维地形模型的发展变化。如何深入分析不同形态特征和内部构成的碳酸盐岩台地的成因机制,解读其相应的地球生物-环境演化信息,是当前和下一步需要努力的方向。为此本文介绍了碳酸盐工厂和碳酸盐岩台地成因方面的研究进展,强调了碳酸盐岩地层的时代特殊性,并以滇黔桂地区为例,从碳酸盐工厂分析的角度剖析了该区二叠纪吴家坪期碳酸盐岩沉积学研究现状,展望了下一步的研究方向,旨在引起同行重新认识生物和海洋环境条件在碳酸盐岩台地演化中的作用,重视全球空间尺度范围内同时代碳酸盐岩的相似性与差异,并从古生态角度入手开展碳酸盐工厂分析,发掘碳酸盐岩台地的生物学和海洋学意义。     

土库曼斯坦阿姆河盆地卡洛夫—牛津阶沉积相特征

以岩心描述、薄片和古生物鉴定为依据,结合测井相和地震相特征和区域构造-沉积背景,对土库曼斯坦阿姆河盆地卡洛夫—牛津阶沉积相特征进行研究,确定该地层单元属于碳酸盐台地沉积体系,识别出蒸发台地、局限台地、开阔台地、台地边缘礁滩、前缘缓斜坡、盆地等6种沉积相类型。以连井沉积相剖面对比、沉积相平面分布特征和纵向演化分析为基础,进一步揭示出该盆地卡洛夫—牛津阶经历了台地前缘缓斜坡、台地边缘礁滩、开阔台地、局限台地、蒸发台地等交替发育构成多期次海进—海退旋回演化和"开放性宽缓型镶边缓斜坡台地"沉积模式。     

Three-dimensional sequence analysis of a subsurface carbonate ramp, Mississippian Appalachian foreland basin, West Virginia, USA

Well‐cuttings, wireline logs and limited core and outcrop data were used to generate a regional, three‐dimensional sequence framework for Upper Mississippian (Chesterian), Greenbrier Group carbonates in the Appalachian foreland basin, West Virginia, USA. The resulting maps were used to document the stratigraphic response of the basin to tectonics and to glacio‐eustasy during the transition into ice‐house conditions. The ramp facies include inner ramp red beds and aeolianites, lagoonal muddy carbonates, mid‐ramp ooid and skeletal grainstone shoal complexes, and outer ramp wackestone–mudstone, that grades downslope into laminated silty lime mudstone. The facies make up fourth‐order sequences, a few metres to over 90 m (300 ft) thick. The sequences are bounded along the ramp margin by lowstand sandstones and calcareous siltstones. On the ramp, sequence boundaries are overlain by thin transgressive siliciclastics and aeolianites, and only a few are calichified. Maximum flooding surfaces on the outer ramp lie beneath deeper water facies that overlie lowstand to transgressive siliciclastic or carbonate units. On the shallow ramp, maximum flooding surfaces overlie siliciclastic‐prone transgressive systems tracts, that are overlain by highstand carbonates with significant grainstone units interlayered with lagoonal lime mudstones. The fourth‐order sequences are the major mappable subsurface units; they are bundled into weak composite sequences which are bounded by red beds. In spite of differential subsidence rates across the foreland basin (1 to 3 cm/k.y. up to 25 cm/k.y.), eustatic sea‐level changes controlled regional sequence development. Thrust‐load induced differential subsidence of fault‐blocks, coupled with in‐plane stress, controlled the rapid basinward thickening of the depositional wedge, whose thickness and facies were influenced by subtle structures such as arches trending at high angles as well as parallel to the margin.     

Early and Middle Permian paleoclimates of the Baoshan Block, western Yunnan, China: insight from carbonates

In Permian times the Baoshan Block of western Yunnan, southwest China formed the eastern part of the Cimmerian Continent. Most biogeographical and sedimentological data indicate that the Early Permian Dingjiazhai Formation formed on the block under conditions strongly influenced by the Permo-Carboniferous glaciation. After Early Permian rifting, with post-glaciation climatic amelioration, and as the Baoshan Block drifted northwards to approach South China and Indochina, faunal elements characteristic of Gondwana affinity decreased, while those of Cathaysian affinity increased. Finally, Late Permian faunas are characterized by exclusively Cathaysian elements. This shift of marine provinciality becomes an important indicator in understanding the Permian paleoclimatic evolution of the region. This research investigated the composition of carbonate grain associations and the early diagenetic features of limestones from the upper part of the Dingjiazhai Formation, and from the overlying Yongde and Shazipo formations. A sharp distinction in petrological and diagenetic features is recognized between the Dingjiazhai Formation and the two overlying formations. The Dingjiazhai carbonates are characterized by the bryonoderm (bryozoan-echinoderm)-extended facies of the heterozoan association, with no non-skeletal grains. Because early diagenetic cement was rarely formed, the Dingjiazhai carbonates experienced strong diagenetic compaction. In contrast, the Yongde and Shazipo carbonates show a chloroforam facies of photozoan association, with the common occurrence of non-skeletal grains. These carbonates were well cemented during early diagenetic processes. From comparison with Permian cool-water carbonates from northern Pangea and Tasmania, Australia, the Dingjiazhai carbonates are interpreted as deposits of warm-temperate conditions, while the overlying carbonates are considered to be deposits of subtropical or tropical conditions. This climatic interpretation, based on the petrographic features of the Permian carbonates, agrees well with existing biogeographical data from the region.     

Depositional systems and sequence stratigraphy analysis of the Upper Callovian to Tithonian sediments in the Central and Western Kopet‐Dagh Basin,Northeast Iran

Upper Callovian to Tithonian (late Jurassic) sediments represent an important hydrocarbon reservoir in the Kopet‐Dagh Basin, NE Iran. These deposits consist mainly of limestone, dolostone, and calcareous mudstone with subordinate siliciclastic interbeds. Detailed field surveys, lithofacies and facies analyses at three outcrop sections were used to investigate the depositional environments and sequence stratigraphy of the Middle to Upper Jurassic interval in the central and western areas of the basin. Vertical and lateral facies changes, sedimentary fabrics and structures, and geometry of carbonate bodies resulted in recognition of various carbonate facies related to tidal flats, back‐barrier lagoon, shelf‐margin/shelf‐margin reef, slope and deep‐marine facies belts. These facies were accompanied by interbedded beach and deep marine siliciclastic petrofacies. Field surveys, facies analysis, parasequences stacking patterns, discontinuity surfaces, and geometries coupled with relative depth variation, led to the recognition of six third‐order depositional sequences. The depositional history of the study areas can be divided into two main phases. These indicate platform evolution from a rimmed‐shelf to a carbonate ramp during the late Callovian–Oxfordian and Kimmeridgian–Tithonian intervals, respectively. Significant lateral and vertical facies and thickness changes, and results obtained from regional correlation of the depositional sequences, can be attributed to the combined effect of antecedent topography and differential subsidence related to local tectonics. Moreover, sea‐level changes must be regarded as a major factor during the late Callovian–Tithonian interval. Copyright © 2015 John Wiley & Sons, Ltd.     

吉林新立油田下白垩统泉头组成岩储集相及储集空间演化

吉林新立油田位于吉林省松原市郭县以北新庙乡境内。综合应用25口取心井薄片、镜质体反射率、铸体、电镜、X衍射等岩心分析资料，研究了泉头组的成岩作用、成岩演化序列、成岩阶段划分及储集空间类型。根据成岩作用及特有的储集空间组合，划分了4种成岩储集相类型，即A相(不稳定组分强溶解次生孔隙成岩储集相)、B相(中等压实弱-中胶结混合孔隙成岩储集相)、C相(强压实中等胶结残余粒间孔成岩储集相)、D相(碳酸盐强胶结微孔隙成岩储集相)；并阐述了各类成岩储集相的特征，其中A相储集物性最好，B相次之，C相较差，D相最差。最后探讨了泉头组储集空间演化，在中成岩阶段A期形成了大量次生溶蚀孔隙，为首次在该区泉头组特低渗储层中预测相对有利的成岩储集相带提供了研究思路和科学依据，也为改善特低渗油田的开发效果奠定了坚实的地质基础。     

Controls and predictability of carbonate facies architecture in a Lower Jurassic three-dimensional barrier-shoal complex (Djebel Bou Dahar, High Atlas, Morocco)

Spatial information on lithofacies from outcrops is paramount for understanding the internal dynamics, external controls and degree of predictability of the facies architecture of shallow‐water carbonate‐platform tops. To quantify the spatial distribution and vertical stacking of lithofacies within an outer‐platform shoal‐barrier complex, integrated facies analysis and digital field technologies have been applied to a high‐relief carbonate platform exposed in the Djebel Bou Dahar (Lower Jurassic, High Atlas, Morocco). The outer platform is characterized by subtidal, cross‐bedded, coarse grainstone to rudstone grading into supratidal, pisoidal packstone‐rudstone with tepees that together formed a 350 to 420 m wide shoal‐barrier belt parallel to the margin. This belt acted as a topographic high separating a restricted lagoon from the subtidal, open marine region. Low‐energy tidal flats developed on the protected flank of the barrier facing the lagoon. Lithofacies patterns were captured quantitatively from outcrop and integrated into a digital outcrop model. The outcrop model enabled rapid visualization of field data and efficient extraction of quantitative data such as widths of facies belts. In addition, the spatial heterogeneity was captured in multiple time slices, i.e. during different phases of cyclic base‐level fluctuations. In general, the lateral continuity of lithofacies is highest when relative water depth increased during flooding of the platform top, establishing low‐energy subtidal conditions across the whole platform, and when the accommodation space was filled with tidal flat facies. Heterogeneity increased during deposition of the relief‐building bar facies that promoted spatial diversification of depositional environments during the initial phases of accommodation space creation. Cycles commonly are composed of a thin transgressive tidal flat unit, followed by coated‐grain rudstone bar facies. Lateral to the bar facies, pisoidal‐grainstone beach deposits accumulated. These bar and beach deposits were overlain by subtidal lagoonal facies or would grow through the maximum flooding and highstand. There the bars either graded into supratidal pisoidal facies with tepees (when accommodation space was filled) or were capped by subaerial exposure (due to a sea‐level fall). Modified embedded Markov analysis was used to test the presence of common ordering in vertical lithofacies stacking in a stationary interval (constant depositional mode). Analysis of individual sections did not reveal any ordering, which may be related to the limited thickness of these sections. Composite sections, however, rejected the null hypothesis of randomness. The addition of stratigraphically significant information to the Markov analysis, such as exposure surfaces and lateral dimensions of facies bodies, strengthens the verdict of unambiguous preferential ordering. Through careful quantitative reconstruction of stratal geometry and facies relationships in fully integrated digital outcrop models, accurate depositional models could be established that enhanced the predictability of carbonate sediment accumulation.     

Tectonic, eustatic and environmental controls on mid-Cretaceous carbonate platform deposition, south-central Pyrenees, Spain

Cenomanian–Turonian strata of the south‐central Pyrenees in northern Spain contain three prograding carbonate sequences that record interactions among tectonics, sea level, environment and sediment fabric in controlling sequence development. Sequence UK‐1 (Lower to Upper Cenomanian) contains distinct lagoonal, back‐margin, margin, slope and basin facies, and was deposited on a broad, flat shelf adjacent to a deep basin. The lack of reef‐constructing organisms resulted in a gently dipping ramp morphology for the margin and slope. Sequence UK‐2 (Upper Cenomanian) contains similar shallow‐water facies belts, but syndepositional tectonic modification of the margin resulted in a steep slope and deposition of carbonate megabreccias. Sequence UK‐3 (Lower to Middle Turonian) records a shift from benthic to pelagic deposition, as the shallow platform was drowned in response to a eustatic sea‐level rise, coupled with increased organic productivity. Sequences UK‐1 to UK‐3 are subdivided into lowstand, transgressive and highstand systems tracts based on stratal geometries and facies distribution patterns. The same lithologies (e.g. megabreccias) commonly occur in more than one systems tract, indicating that: (1) the depositional system responded to more than just sea‐level fluctuations; and (2) similar processes occurred during different times throughout sequence development. These sequences illustrate the complexity of carbonate platform dynamics that influence sequence architecture. Rift tectonics and flexural subsidence played a major role in controlling the location of the platform margin, maintaining a steep slope gradient through syndepositional faulting, enhancing slope instability and erosion, and influencing depositional processes, stratal relationships and lithofacies distribution on the slope. Sea‐level variations (eustatic and relative) strongly influenced the timing of sequence and parasequence boundary formation, controlled changes in accommodation and promoted platform drowning (in conjunction with other factors). Physico‐chemical and climatic conditions were responsible for reducing carbonate production rates and inducing platform drowning. Finally, a mud‐rich sediment fabric affected platform morphology, growth geometries (aggradation vs. progradation) and facies distribution patterns.     

北京下苇甸地区寒武系张夏组鲕粒滩成岩演化与勘探前景

成岩作用及成岩演化序列控制着鲕粒滩储集空间的发育。下苇甸地区寒武系张夏组主要为鲕粒滩微相。通过野外实测、薄片观察、X衍射等方法,阐明了下苇甸地区寒武系张夏组鲕粒滩的主要成岩作用类型和成岩序列,并对张夏组鲕粒滩的勘探前景进行了展望,得出如下结论:(1)对下苇甸地区张夏组鲕粒滩储集性能有明显影响的成岩作用主要包括压实-压溶、胶结作用、白云石化作用等。除此之外,硅化作用主要以围绕鲕粒的硅化套形式出现;(2)鲕粒白云石化主要发生在同生成岩阶段,白云石晶间孔是鲕粒滩主要的储集空间。早-晚成岩阶段主要以鲕粒滩的压实-压溶作用为主,造成了粒间孔隙的急剧减少;(3)鲕粒滩压溶产生的缝合线及其伴生的溶蚀孔隙与白云石晶间孔构成的疏导-储集复合体系是鲕粒滩可能的油气储集空间,在证实中新元古界烃源岩供源的前提下,华北寒武系张夏组是一套潜在的勘探新层系,值得研究和探索。     

Correlation between rock fabrics and physical properties of carbonate reservoir rocks

 Three carbonate core samples from an oil and gas reservoir of the NW German basin were chosen to study the correlation between rock fabrics and physical properties of reservoir rocks. Detailed fabric analyses and texture investigations were carried out as well as laboratory measurements of different physical properties, e.g. density, porosity, permeability, electrical conductivity, seismic compressional and shear wave velocities. Although the three core samples come from a similar depositional facies, they show great differences in the occurrence and three-dimensional distribution of the rock fabric elements. These heterogeneities are the result of various diagenetic and tectonic processes. For the correlation between the rock fabrics and the physical properties four main rock fabric types have to be considered: (a) major constituents, e.g. fossils, ooides, peloides and crystals; (b) pore space with different pore types; (c) fractures; and (d) stylolites. The results of the correlation clearly show that the values and anisotropies of the petrophysical properties are fairly related to the observed fabric elements, with their different arrangements, spatial distributions and preferred orientations. These results also provide a fundamental understanding of the petrophysical responses, such as seismics, to the different geological features (e.g. fractures) and their dynamic changes with pressure, which can be converted to different depths. The knowledge gained from such correlations may lead to an improved interpretation of geophysical data for hydrocarbon exploration and production and therefore to an advanced reservoir characterization. Received: 28 January 1999 / Accepted: 21 June 1999