Drowning history of a Miocene carbonate platform (Zhujiang Formation, South China Sea)

The identification and interpretation of drowning events in the geologic record can aid significantly to the reconstruction of the depositional, tectonic and eustatic history of a study area and often improve reservoir and seal prediction in carbonate rocks. The differentiation between drowned platforms showing a record of continuous deepening and those with a record of exposure followed by rapid deepening remains, however, problematic. The Zhujiang carbonate platform (Liuhua 11-1 field, South China Sea) study shown here provides an example of an integrated approach combining high-resolution geochemistry, microfacies analyses and foraminiferal biostratigraphy in order to improve the reconstruction of environmental conditions prior, during and after platform demise and drowning. The Zhujiang carbonate platform displays the following vertical succession of four facies types i) skeletal grain facies with a miogypsinid/lepidocyclinid-dominated fauna deposited in a moderately deep (< 50 m), oligotrophic back-reef setting; ii) in situ corals in patch-reef facies in an oligotrophic lagoon (< 10 m); iii) rhodoid facies with in situ red algal crusts, dominated by Heterostegina sp. and spiroclypeids, possibly capped by a subaerial exposure surface. Well-rounded rhodoids representing a mesotrophic lagoon dominate the upper portions of the rhodoid facies; iv) pelagic marine shales of the Hanjiang Formation burying the carbonate platform after drowning. This facies succession, in combination with geochemical evidence suggests a deepening-upward trend. This trend might have been interrupted by transient subaerial exposure but no evidence for meteoric diagenesis was found at the drowning unconformity topping the carbonate platform. Instead, microfacies analyses suggest that platform demise may be related to progressive changes in environmental conditions, including increasing nutrient-levels and/or decreasing temperature up-core towards the drowning unconformity. These findings are of significance for those concerned with Miocene carbonate factories and, more specifically, the demise of carbonate platforms in general.     

Crescentiella morronensis (Crescenti) (incertae sedis) dominated microencruster association in Lower Cretaceous (lower Aptian) limestones from the Rarău Massif (Eastern Carpathians,Romania)

Analysis of eight outcrops from Rarău Massif (Eastern Carpathians, Romania) revealed a rich assemblage of encrusting organisms mostly of problematic biological affiliation within platform-margin facies – Crescentiella morronensis, Radiomura cautica, Koskinobullina socialis, Pseudorothpletzella schmidi, Lithocodium aggregatum and bacinellid structures, encrusting calcified sponges (Calcistella jachenhausenensis, Neuropora lusitanica) and foraminifera (Coscinophragma cribrosa). Orbitolinids, calcareous green algae and rudists assign an early Aptian age (Bedoulian) for the studied limestones. In terms of species variety, abundance and structural microfabrics the studied microencruster association show similarity with the Upper Jurassic communities from reef and peri-reefal sedimentary settings. Comparisons with such assemblages are emphasized here. The presence of the sporolithacean and peyssonneliacean red algae (Sporolithon rude and Polystrata alba) contributes also to the construction of superimposed crusts. Even if the known stratigraphic range of the microencrusters is not strictly limited to the late Jurassic, reports of comparable associations in Urgonian-type facies is poorly documented. Therefore, it exemplifies the resistance and flourishing events of many encrusters throughout the Mesozoic.     

Microfacies and geochemistry of the Ilam Formation in the Tang-E Rashid area, Izeh, S.W. Iran

The Ilam Formation (Santonian–Campanian in age), part of the Bangestan Group, is disconformably overlain by the Sarvak Formation and underlain by the Gurpi Formation in the Tang-E Rashid, Peyon area, Izeh (Zagros), southwest of Iran.Facies analyses indicate that the Ilam carbonates formed in four microfacies belts: tidal flat, lagoon, shoal and open marine, in a platform ramp environment. Major and minor elements and carbon and oxygen isotope values were used to determine the original carbonate mineralogy of the Ilam Formation. Petrographic evidence and elemental and oxygen and carbon isotope values indicate that aragonite was the original carbonate mineralogy in the Ilam Formation. The elemental and isotopic compositions of the Ilam carbonates also illustrate that they have stabilized in the marine phreatic environment. Variations of Sr/Ca and δ¹⁸O values versus Mn suggest that diagenetic alteration occurred in a closed system. Temperature calculation based on the oxygen isotope value of the least-altered sample indicates that the very early shallow burial fluid temperature was around 28 °C.Recognition of the exact boundary between the Ilam and Sarvak Formations is difficult, due to similar lithologies and the absence of the Surgah Formation in the study area. However, elemental and oxygen and carbon isotope analysis were used to determine the boundary between these formations. The δ¹⁸O and δ¹³C values, along with elemental results, clearly indicate a subaerial exposure surface, below which meteoric diagenesis affected the sediments.     

The significant role of sediment bio-retexturing within a contemporary carbonate platform system: Implications for carbonate microfacies development

Assessments of carbonate platform reef–lagoon sediments and benthic habitats around Rodrigues Island (south-west Indian Ocean) have been undertaken in order to examine carbonate sediment textural properties and the controls on texturally-defined sediment fabrics. Reef–lagoon sediments, sampled from across the expansive (~ 8 km wide) carbonate-dominated windward platform, principally comprise poorly sorted medium- to coarse-grained bioclastic sands, composed of a low diversity of grain constituents — predominantly non-geniculate coralline algal bioclasts. Despite a marked homogeneity in sediment compositional and grain size properties, eight distinct sediment textural groups can be identified that form a heterogeneous mosaic across the contemporary reef–lagoon system. Only along the narrow outer platform margins (reef crest, sand apron and outermost lagoon environments) do we observe consistent (predictable) transitions in sediment textural groups, where physical processes are the primary drivers of selective sediment transport and sorting. In contrast, across the main expanse of the lagoon, the sediment substrates are characterised by an irregular mosaic of texturally-defined sediment groups — formed primarily as a function of sediment bio-retexturing. The burrowing activities of alpheid and callianassid shrimps are particularly important in this respect and impart a distinctly unique textural fabric to the upper sediment horizons in the environments in which the respective organisms occur. The consequence of this is that, at the platform system scale, individual, texturally-defined sediment groups are relatively poor indicators of prevailing hydrodynamic regimes or of local sediment production, reflecting more the biological action of organisms inhabiting the substrate. This has important implications for understanding the development of carbonate sediment fabrics in the context of palaeoenvironmental reconstructions and for interpreting a key diagnostic criteria of carbonate microfacies.     

Microfacies and depositional environment of Tertiary Tanjung Enim low rank coal, South Sumatra Basin, Indonesia

The South Sumatra basin is among the most important coal producing basins in Indonesia. Results of an organic petrography study on coals from Tanjung Enim, South Sumatra Basin are reported. The studied low rank coals have a mean random huminite reflectance between 0.35% and 0.46% and are dominated by huminite (34.6–94.6 vol.%). Less abundant are liptinite (4.0–61.4 vol.%) and inertinite (0.2–43.9 vol.%). Minerals are found only in small amounts (0–2 vol.%); mostly as iron sulfide.Based on maceral assemblages, the coals can be grouped into five classes: (1) humotelinite-rich group, (2) humodetrinite-rich group, (3) humocollinite-rich group, (4) inertinite-rich group and (5) humodetrinite–liptinite-rich group. Comparing the distribution of maceral assemblages to the maceral or pre-maceral assemblages in modern tropical domed peat in Indonesia reveals many similarities. The basal section of the studied coal seams is represented typically by the humodetrinite–liptinite-rich group. This section might be derived from sapric or fine hemic peat often occurring at the base of modern peats. The middle section of the seams is characterized by humotelinite-rich and humocollinite-rich groups. The precursors of these groups were hemic and fine hemic peats. The top section of the coal seams is typically represented by the humodetrinite-rich or inertinite-rich group. These groups are the counterparts of fibric peat at the top of the modern peats. The sequence of maceral assemblages thus represents the change of topogenous to ombrogenous peat and the development of a raised peat bog.A comparison between the result of detailed maceral assemblage analysis and the paleodepositional environment as established from coal maceral ratio calculation indicates that the use of coal maceral ratio diagrams developed for other coal deposits fails to deduce paleo-peat development for these young tropical coals. In particular, mineral distribution and composition should not be neglected in coal facies interpretations.     

克拉玛依油田八区克下组沉积特征研究

克拉玛依油田八区克下组油藏属山前陆相盆地边缘冲积扇成因砾岩油藏,油藏单一扇体具独立成藏体系。研究中需区分不同扇体,厘清扇体展布及演化特征,为油藏后期精细开发提供依据。在沉积微相特征分析基础上,充分运用砾岩组分和重矿物资料,落实八区克下组发育的3大冲积扇体,发现扇体具明显区带性,自下而上呈退积式演化特征。对油田生产状况分析认为,八区克下组油藏产能及剩余油分布受沉积微相控制,其中主槽和辫流线为最有利相带,具高产特征,是剩余油主要分布区。     

陕西定边张韩区块延长组长2储层流动单元研究

利用孔隙度和渗透率参数,将陕西定边张韩区块油藏储层分为4类流动单元.分析各流动单元的物性特征,阐明不同级别流动单元分布特点及其与沉积微相的关系,获得各类流动单元物性特征参数.     

柴达木盆地西北缘现代冲积扇沉积特征及石油地质意义

柴达木盆地处于古亚洲构造域和特提斯－喜马拉雅构造域的结合部，构造应力大而复杂，导致盆内地势起伏大，加上西南暖湿气流受喜马拉雅山系阻隔难以进入境内，盆内气候干旱，最终导致盆地内冲积扇极为发育。通过对大柴旦地区大头羊煤矿、鱼卡河、波门河和八里沟四个冲积扇的实地考察，共观测到3个亚相8个微相：扇根亚相沉积物最粗，分为古沟道、主水道和主水道间微相；扇中亚相沉积物偏细，成熟度增高，分为辫状水道、辫状水道间和纵坝微相；扇缘亚相沉积物最细，流体能量最低，分为水道径流和片流微相。不同沉积微相其沉积特征差异较大，认为古沟道、主水道和辫状水道微相具有较好的储集性能。勘探表明，冲积扇沉积与储层有着密切的关系，其内形成的油藏具有“自我保护”的能力；另外，冲积扇的形成很可能导致上覆地层形成扇背斜油藏，也可能导致下伏基岩形成基岩风化壳油藏。     

滇黔桂区中三叠世安尼期浊积岩微相划分与金矿关系

对滇黔桂区中三叠世安尼期浊积岩微相的划分,认为区内浊流有3处发源地,形成了3个浊积扇,每一扇中又由中心相、过渡相及末梢相组成,从各微相分布特征和含金性分析,认为浊积岩的过渡相与卡林型金矿密切,并提出了找矿方向.     

塔里木柯坪地区上奥陶统印干组碳酸盐岩微相和古地理

新疆塔里木西北缘柯坪地区上奥陶统印干组碳酸盐岩发育良好,不同地区岩性变化较大。通过对其不同剖面位置的微相类型进行研究,识别出10种微相类型,与威尔逊标准相模式对比,将其划分为5种沉积相,在印干组中首次识别出台缘浅滩相、台地边缘斜坡相。印干组沉积期古地理特征为南西水浅(主要为台缘浅滩-台缘斜坡相)、北东水深(主要为广海陆棚-盆地边缘相)。通过沉积相变化规律,识别出在印干组沉积中晚期,存在一次大规模海退事件。