At the northern end of the Cap Corse peninsula, several klippes of ultramafic rocks (peridotite and serpentinite), among which the Monte Maggiore klippe is the least serpentinized one, rest upon continental-crust derived rocks (Centuri gneisses) and basic or metasedimentary schists (Schistes Lustres). The Monte Maggiore ophiolitic klippe shares several characteristics with the Cima di Gratera klippe located 30 km further south. First, the two units are composed of a lherzolitic peridotite. Second, they record the same succession of metamorphic events. Third, in the Cap Corse tectonic pile, the two units occupy the highest structural position. Several differences are also observed. First, mafic rocks are significantly less abundant in the Monte Maggiore unit, where they are restricted to dykes cross-cutting the peridotite, than in the Cima di Gratera unit, where they constitute an entire sub-unit. Second, pyroxenite layers are more common at Monte Maggiore than at Cima di Gratera. Despite these differences, the Monte Maggiore and Cima di Gratera klippes can be considered as possible lateral equivalents of a single ophiolitic unit having covered the entire Cap Corse before subsequent erosion. Pseudotachylyte of seismic origin is newly discovered in the Monte Maggiore klippe. The host rock is a cataclastic serpentinized peridotite affected by a cataclastic foliation that is either flat-lying or steeply dipping. Pseudotachylyte fault veins are parallel to the host rock cataclastic foliation. The small lateral extension and the small thickness of fault veins along with frequent cross-cutting relationships suggest that the exposed pseudotachylyte most likely results from numerous small magnitude seismic events such as swarms or aftershocks rather than from large magnitude shocks. All these characteristics are also observed at the Cima di Gratera klippe where they are interpreted as the testimonies of a fossil intermediate-depth Wadati-Benioff zone at the time of subduction of the Ligurian Tethys oceanic lithosphere. Mineral assemblages that could constrain the depth of formation of the pseudotachylyte lack in the Monte Maggiore area. Despite this uncertainty, and given the similarities with the Cima di Gratera occurrences, the pseudotachylyte veins newly discovered at Monte Maggiore are tentatively related to the seismic activity linked with the subduction of the Piemonte-Ligurian oceanic lithosphere in Eocene times. This interpretation suggests that the fossil Wadati-Benioff zone could be traced further south in Alpine Corsica and further north in the Piemontese zone of the western Alps.
在科斯角半岛的北端，有几片超镁铁质岩石(橄榄岩和蛇纹岩)，其中马焦雷山是蛇纹岩化程度最低的一个，坐落在大陆地壳衍生的岩石(半人马座片麻岩)和基性或变质片岩(片岩)之上。Monte Maggiore蛇绿岩断裂带与位于其南部30公里处的Cima di Gratera断裂带有几个共同的特征。首先，这两个单元都是由一个橄榄岩组成的。其次，它们记录了相同的变质事件序列。③盖湖构造桩中，两个单元占据最高构造位置。还观察到一些不同之处。首先，在Monte Maggiore单元中，基性岩的数量明显少于Cima di Gratera单元，它们被限制在横切橄榄岩的岩脉中，而在Cima di Gratera单元中，它们构成了一个完整的亚单元。其次，辉石岩层在Monte Maggiore比Cima di Gratera更常见。尽管存在这些差异，Monte Maggiore和Cima di Gratera klippes可以被认为是在随后的侵蚀之前覆盖整个Cap Corse的单一蛇绿岩单元的可能的横向等量。在马焦雷山断裂带新发现了地震源伪岩。寄主岩为碎裂蛇纹岩化橄榄岩，受碎裂片理作用的影响，碎裂片理或平坦或陡倾。伪岩断层脉平行于寄主岩碎裂面理。断层脉横向延伸小，厚度小，且频繁的横切关系表明，暴露的伪岩很可能是由大量的小震级地震事件(如群震或余震)造成的，而不是由大震级地震造成的。所有这些特征在Cima di Gratera断裂带也被观察到，它们被解释为利古里亚-特提斯海洋岩石圈俯冲时期的化石中深Wadati-Benioff带的证据。在Monte Maggiore地区，矿物组合可能限制假木酸盐缺乏的形成深度。尽管存在这种不确定性，并考虑到与Cima di Gratera的相似之处，在Monte Maggiore新发现的伪水晶石脉初步与始新世皮埃蒙特-利古里亚海洋岩石圈俯冲有关的地震活动有关。这一解释表明，化石瓦达蒂-贝尼奥夫带可以在阿尔卑斯科西嘉山脉更南的地方找到，在西阿尔卑斯山脉的皮埃蒙特带更北的地方找到。
{"title":"Pseudotachylyte in the Monte Maggiore ophiolitic unit (Alpine Corsica): a possible lateral extension of the Cima di Gratera intermediate-depth Wadati-Benioff paleo-seismic zone","authors":"O. Fabbri, R. Magott, M. Fournier, Lucas Etienne","doi":"10.1051/BSGF/2018020","DOIUrl":"https://doi.org/10.1051/BSGF/2018020","url":null,"abstract":"At the northern end of the Cap Corse peninsula, several klippes of ultramafic rocks (peridotite and serpentinite), among which the Monte Maggiore klippe is the least serpentinized one, rest upon continental-crust derived rocks (Centuri gneisses) and basic or metasedimentary schists (Schistes Lustres). The Monte Maggiore ophiolitic klippe shares several characteristics with the Cima di Gratera klippe located 30 km further south. First, the two units are composed of a lherzolitic peridotite. Second, they record the same succession of metamorphic events. Third, in the Cap Corse tectonic pile, the two units occupy the highest structural position. Several differences are also observed. First, mafic rocks are significantly less abundant in the Monte Maggiore unit, where they are restricted to dykes cross-cutting the peridotite, than in the Cima di Gratera unit, where they constitute an entire sub-unit. Second, pyroxenite layers are more common at Monte Maggiore than at Cima di Gratera. Despite these differences, the Monte Maggiore and Cima di Gratera klippes can be considered as possible lateral equivalents of a single ophiolitic unit having covered the entire Cap Corse before subsequent erosion. Pseudotachylyte of seismic origin is newly discovered in the Monte Maggiore klippe. The host rock is a cataclastic serpentinized peridotite affected by a cataclastic foliation that is either flat-lying or steeply dipping. Pseudotachylyte fault veins are parallel to the host rock cataclastic foliation. The small lateral extension and the small thickness of fault veins along with frequent cross-cutting relationships suggest that the exposed pseudotachylyte most likely results from numerous small magnitude seismic events such as swarms or aftershocks rather than from large magnitude shocks. All these characteristics are also observed at the Cima di Gratera klippe where they are interpreted as the testimonies of a fossil intermediate-depth Wadati-Benioff zone at the time of subduction of the Ligurian Tethys oceanic lithosphere. Mineral assemblages that could constrain the depth of formation of the pseudotachylyte lack in the Monte Maggiore area. Despite this uncertainty, and given the similarities with the Cima di Gratera occurrences, the pseudotachylyte veins newly discovered at Monte Maggiore are tentatively related to the seismic activity linked with the subduction of the Piemonte-Ligurian oceanic lithosphere in Eocene times. This interpretation suggests that the fossil Wadati-Benioff zone could be traced further south in Alpine Corsica and further north in the Piemontese zone of the western Alps.","PeriodicalId":55978,"journal":{"name":"Bulletin de la Societe Geologique de France","volume":"5 1","pages":"18"},"PeriodicalIF":0.0,"publicationDate":"2018-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85425129","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
B. Cochelin, D. Chardon, Y. Denèle, C. Gumiaux, B. Bayon
A new structural map of the Paleozoic crust of the Pyrenees based on an extensive compilation and new kinematic data allows for the evaluation of the mechanical coupling between the upper and lower crust of the abnormally hot foreland of the Variscan orogen of SW Europe. We document partitioning between coeval lower crustal lateral flow and upper crustal thickening between 310 and 290 Ma under an overall dextral transpressive regime. Partitioning also involved syn-convergence transtensional gneiss domes emplacement during this period. Late orogen-normal shortening of the domes and strain localization in steep crustal-scale transpressive shear zones reflects increasing coupling between the lower crust and the upper crust. The combination of dextral transpression and eastward flow in the Pyrenees results from the shortening and lateral escape of a hot buoyant crust along the inner northern limb of the closing Cantabrian orocline at the core of the Iberian-Armorican arc between ca. 305 and 295 Ma. Delamination or thermal erosion of the lithosphere enhanced orocline closure and explains (1) the switch from crust-to mantle-derived magmatism in the Iberian-Armorican arc and (2) the abnormally hot and soft character of the Pyrenean crust that escaped the closing syntax.
{"title":"Vertical strain partitioning in hot Variscan crust: Syn-convergence escape of the Pyrenees in the Iberian-Armorican syntax","authors":"B. Cochelin, D. Chardon, Y. Denèle, C. Gumiaux, B. Bayon","doi":"10.1051/BSGF/2017206","DOIUrl":"https://doi.org/10.1051/BSGF/2017206","url":null,"abstract":"A new structural map of the Paleozoic crust of the Pyrenees based on an extensive compilation and new kinematic data allows for the evaluation of the mechanical coupling between the upper and lower crust of the abnormally hot foreland of the Variscan orogen of SW Europe. We document partitioning between coeval lower crustal lateral flow and upper crustal thickening between 310 and 290 Ma under an overall dextral transpressive regime. Partitioning also involved syn-convergence transtensional gneiss domes emplacement during this period. Late orogen-normal shortening of the domes and strain localization in steep crustal-scale transpressive shear zones reflects increasing coupling between the lower crust and the upper crust. The combination of dextral transpression and eastward flow in the Pyrenees results from the shortening and lateral escape of a hot buoyant crust along the inner northern limb of the closing Cantabrian orocline at the core of the Iberian-Armorican arc between ca. 305 and 295 Ma. Delamination or thermal erosion of the lithosphere enhanced orocline closure and explains (1) the switch from crust-to mantle-derived magmatism in the Iberian-Armorican arc and (2) the abnormally hot and soft character of the Pyrenean crust that escaped the closing syntax.","PeriodicalId":55978,"journal":{"name":"Bulletin de la Societe Geologique de France","volume":"26 1","pages":"39"},"PeriodicalIF":0.0,"publicationDate":"2017-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83686123","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
E. Hatem, N. Tribovillard, O. Averbuch, V. Bout‐Roumazeilles, A. Trentesaux, J. Deconinck, F. Baudin, T. Adatte
We studied a potential petroleum source rock deposited in a clastic-dominated ramp environment: the Argiles de Châtillon Formation (Kimmeridgian-Tithonian, Boulonnais area, northern France). The formation was deposited along a proximal-distal gradient on this ramp affected by synsedimentary fault movements. A sedimentological, mineralogical and geochemical study was conducted to decipher the distribution patterns of sedimentary parameters along such a depth increase over the ramp. It comes out that smectite distribution unexpectedly mimics the lateral depth evolution despite the good floatability of the mineral. It is also observed that the Argiles de Châtillon could accumulate noticeable amounts of organic matter in spite of paleoenvironmental conditions that were not specifically prone to organic matter preservation and burial (sedimentation rate, mineral particle grain size, productivity, oxygenation level…). Conversely, reactive iron, when being present in limited abundance, must have allowed sulfide ions to react with organic molecules instead of forming iron sulfides, which fostered organic matter preservation and accumulation. This protecting role of organic sulfide incorporation (additional to other favorable factors) cannot exist if reactive iron is relatively abundant. Lastly, our results (still fragmentary) suggest that smectite minerals could carry reactive iron, which would have occasionally hampered organic-matter preservation.
研究了沉积在碎屑岩为主的斜坡环境中的潜在烃源岩:Argiles de ch tillon组(法国北部Boulonnais地区Kimmeridgian-Tithonian)。受同沉积断层运动的影响，该斜坡上的地层沿近端-远端梯度沉积。通过沉积学、矿物学和地球化学研究，揭示了斜坡上深度增加的沉积参数分布规律。结果表明，尽管蒙脱石具有良好的可浮性，但其分布出人意料地模拟了侧向深度演化。研究还发现，尽管古环境条件(沉积速率、矿物颗粒粒度、生产力、氧合水平等)不利于有机质的保存和埋藏，但Argiles de chaltilon仍能积累大量的有机质。相反，活性铁，当含量有限时，一定会让硫化物离子与有机分子反应，而不是形成硫化铁，从而促进有机物的保存和积累。如果活性铁相对丰富，有机硫化物结合的这种保护作用(加上其他有利因素)就不存在。最后，我们的结果(仍然是碎片)表明蒙脱石矿物可能携带活性铁，这偶尔会阻碍有机物的保存。
{"title":"Small-scaled lateral variations of an organic-rich formation in a ramp-type depositional environment (the Late Jurassic of the Boulonnais, France): impact of the clastic supply","authors":"E. Hatem, N. Tribovillard, O. Averbuch, V. Bout‐Roumazeilles, A. Trentesaux, J. Deconinck, F. Baudin, T. Adatte","doi":"10.1051/BSGF/2017193","DOIUrl":"https://doi.org/10.1051/BSGF/2017193","url":null,"abstract":"We studied a potential petroleum source rock deposited in a clastic-dominated ramp environment: the Argiles de Châtillon Formation (Kimmeridgian-Tithonian, Boulonnais area, northern France). The formation was deposited along a proximal-distal gradient on this ramp affected by synsedimentary fault movements. A sedimentological, mineralogical and geochemical study was conducted to decipher the distribution patterns of sedimentary parameters along such a depth increase over the ramp. It comes out that smectite distribution unexpectedly mimics the lateral depth evolution despite the good floatability of the mineral. It is also observed that the Argiles de Châtillon could accumulate noticeable amounts of organic matter in spite of paleoenvironmental conditions that were not specifically prone to organic matter preservation and burial (sedimentation rate, mineral particle grain size, productivity, oxygenation level…). Conversely, reactive iron, when being present in limited abundance, must have allowed sulfide ions to react with organic molecules instead of forming iron sulfides, which fostered organic matter preservation and accumulation. This protecting role of organic sulfide incorporation (additional to other favorable factors) cannot exist if reactive iron is relatively abundant. Lastly, our results (still fragmentary) suggest that smectite minerals could carry reactive iron, which would have occasionally hampered organic-matter preservation.","PeriodicalId":55978,"journal":{"name":"Bulletin de la Societe Geologique de France","volume":"24 1","pages":"31"},"PeriodicalIF":0.0,"publicationDate":"2017-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85178096","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
N. Krim, C. Bonnel, N. Tribovillard, P. Imbert, C. Aubourg, A. Riboulleau, V. Bout‐Roumazeilles, G. Hoareau, B. Fasentieux
The Tithonian-Berriasian interval in the southern part of the Neuquen Basin is represented by the Vaca Muerta and the Picun Leufu Formations. Facies analysis and correlation of the Vaca Muerta Formation and the lower part of the Picun Leufu Formation in the Picun Leufu Anticline allow us to characterize the evolution of successive facies belts representing siliciclastic shelf and mixed ramp environments. Shoreface and offshore facies are developed on the siliciclastic shelf in the western and southern parts of the Picun Leufu Anticline. The offshore transition domain is characterized by storm beds and slump features, whereas the offshore domain corresponds to black to grey shales and turbidites. The mixed siliciclastic-carbonate ramp is characterized by the development of a lagoon and high-energy shoal in the proximal part of the inner ramp, whereas the distal part comprises a tidal complex. The mid-ramp zone is characterized by storm influence and the outer ramp by fine-grained deposits. Two major transgressive-regressive sequences and five high-frequency transgressive-regressive sequences are recognized. The high-frequency transgressive-regressive sequences make up three progradational sequences, an aggradational-progradational sequence and an aggradational sequence. The geochemical characteristics and clay mineralogy of the Tithonian-Berriasian interval in the southern Neuquen Basin indicate that (1) climate played a key role in the evolution of the sedimentary environment, (2) a “normal marine” depositional environment with oxic sea water and sediment pore waters, rapidly changing to suboxic conditions at shallow depth below the seabed and (3) the occurrence of episodically restricted water-mass circulation at the onset of deposition of the Vaca Muerta Formation.
{"title":"Paleoenvironmental evolution of the southern Neuquèn basin (Argentina) during the Tithonian-Berriasian (Vaca Muerta and Picún Leufú Formations): a multi-proxy approach","authors":"N. Krim, C. Bonnel, N. Tribovillard, P. Imbert, C. Aubourg, A. Riboulleau, V. Bout‐Roumazeilles, G. Hoareau, B. Fasentieux","doi":"10.1051/BSGF/2017196","DOIUrl":"https://doi.org/10.1051/BSGF/2017196","url":null,"abstract":"The Tithonian-Berriasian interval in the southern part of the Neuquen Basin is represented by the Vaca Muerta and the Picun Leufu Formations. Facies analysis and correlation of the Vaca Muerta Formation and the lower part of the Picun Leufu Formation in the Picun Leufu Anticline allow us to characterize the evolution of successive facies belts representing siliciclastic shelf and mixed ramp environments. Shoreface and offshore facies are developed on the siliciclastic shelf in the western and southern parts of the Picun Leufu Anticline. The offshore transition domain is characterized by storm beds and slump features, whereas the offshore domain corresponds to black to grey shales and turbidites. The mixed siliciclastic-carbonate ramp is characterized by the development of a lagoon and high-energy shoal in the proximal part of the inner ramp, whereas the distal part comprises a tidal complex. The mid-ramp zone is characterized by storm influence and the outer ramp by fine-grained deposits. Two major transgressive-regressive sequences and five high-frequency transgressive-regressive sequences are recognized. The high-frequency transgressive-regressive sequences make up three progradational sequences, an aggradational-progradational sequence and an aggradational sequence. The geochemical characteristics and clay mineralogy of the Tithonian-Berriasian interval in the southern Neuquen Basin indicate that (1) climate played a key role in the evolution of the sedimentary environment, (2) a “normal marine” depositional environment with oxic sea water and sediment pore waters, rapidly changing to suboxic conditions at shallow depth below the seabed and (3) the occurrence of episodically restricted water-mass circulation at the onset of deposition of the Vaca Muerta Formation.","PeriodicalId":55978,"journal":{"name":"Bulletin de la Societe Geologique de France","volume":"79 1","pages":"34"},"PeriodicalIF":0.0,"publicationDate":"2017-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81481593","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Autunian (Late Gzhelian-Sakmarian, ∼295 Ma) of the Autun Basin (Saone-et-Loire, France) is well known for its lacustrine oil-shales and boghead that were exploited during the 19th and 20th centuries. A study of the kerogen geochemistry, palynofacies, hydrocarbon (HC) distribution of pyrolysates and kinetics on these organic-rich sediments gave indications on the control of lacustrine primary fluid characteristics by palaeoenvironmental settings. Palynofacies results have shown a Botryococcus algae dominance in the boghead, whereas the oil-shales were dominated either by a mixing of terrestrial particles and lacustrine phytoplanktonic/bacterial organic matter (OM), or by a lacustrine OM that suffered biodegradation within the water column. During the Autunian, an increasing trend of degraded lacustrine OM concentration is observed in the successive oil-shales. Geochemical and kinetics analyses showed that, except for the boghead, hydrogen index values were n -alkanes with ( n -C 6 - n -C 14 ) / n -C 14+ n -alkanes ratio depending on the proportion of terrestrial OM. They were only encountered in the three oldest oil-shales. On the other hand, the HC profile of fluids originating from samples dominated by degraded lacustrine OM, which was obtained for the first time in this study, show low n -alkanes concentrations and a relatively prominent hump. These samples were only present in the three youngest oil-shale levels. Finally, the boghead showed a link between strong concentration of the Pila genus of Botryococcus algae and a primary fluid dominated by n -C 6 - n -C 14 n -alkanes.By comparing these data with published palaeoenvironmental studies, it appears that the kind of dominant OM, and thus the HC distribution of primary fluids, is controlled by the lake level: a lower depth preventing the settlement of anoxic conditions in the bottom waters, and the subsequent good preservation of the OM. During the Autunian, the lake level decreased, consequence of a reduced tectonic activity and of the beginning of a dryer period. Finally, these changes will have caused a strong decrease of the nutrient influx to the lake, favoring the bloom of Botryococcus algae that accumulated to form the boghead.
Autun盆地(Saone-et-Loire, France)的Autunian (Gzhelian-Sakmarian晚期，~ 295 Ma)以其在19世纪和20世纪开采的湖相油页岩和沼泽而闻名。通过对富有机质沉积物干酪根地球化学、孢粉相、热解产物烃分布及动力学的研究，揭示了古环境背景对湖泊原生流体特征的控制作用。孢粉相的结果显示，在沼泽中以葡萄球菌藻类为主，而油页岩则主要是陆相颗粒和湖泊浮游植物/细菌有机质(OM)的混合，或者是在水柱中遭受生物降解的湖泊有机质。在秋季，连续的油页岩中，湖相有机质浓度呈下降趋势。地球化学和动力学分析表明，除沼地外，氢指标值均为正构烷烃，根据陆相有机质的比例，氢指标值为(n - c6 - n - c14) / n - c14 + n -烷烃。它们只在三个最古老的油页岩中被发现。另一方面，本研究首次获得的以降解湖相有机质为主的样品流体HC剖面显示出较低的正构烷烃浓度和相对突出的驼峰。这些样本只存在于三个最年轻的油页岩层中。最后，boghead显示了高浓度的肉球菌属藻类与以n - c - 6 - n - c - 14 n -烷烃为主的初级流体之间的联系。通过将这些数据与已发表的古环境研究结果进行比较，可以看出，主要有机质的种类以及原生流体的HC分布都受到湖泊水位的控制:较低的深度防止了底部缺氧条件的沉降，并随后很好地保存了有机质。在秋期，由于构造活动减少和干旱期的开始，湖泊水位下降。最后，这些变化将导致流入湖泊的营养物质大幅减少，有利于botrococcus藻类的繁殖，这些藻类积累形成了botrococcus boghead。
{"title":"Palaeoenvironmental control on primary fluids characteristics of lacustrine source rocks in the Autun Permian Basin (France)","authors":"Sylvain Garel, F. Behar, J. Schnyder, F. Baudin","doi":"10.1051/BSGF/2017187","DOIUrl":"https://doi.org/10.1051/BSGF/2017187","url":null,"abstract":"The Autunian (Late Gzhelian-Sakmarian, ∼295 Ma) of the Autun Basin (Saone-et-Loire, France) is well known for its lacustrine oil-shales and boghead that were exploited during the 19th and 20th centuries. A study of the kerogen geochemistry, palynofacies, hydrocarbon (HC) distribution of pyrolysates and kinetics on these organic-rich sediments gave indications on the control of lacustrine primary fluid characteristics by palaeoenvironmental settings. Palynofacies results have shown a Botryococcus algae dominance in the boghead, whereas the oil-shales were dominated either by a mixing of terrestrial particles and lacustrine phytoplanktonic/bacterial organic matter (OM), or by a lacustrine OM that suffered biodegradation within the water column. During the Autunian, an increasing trend of degraded lacustrine OM concentration is observed in the successive oil-shales. Geochemical and kinetics analyses showed that, except for the boghead, hydrogen index values were n -alkanes with ( n -C 6 - n -C 14 ) / n -C 14+ n -alkanes ratio depending on the proportion of terrestrial OM. They were only encountered in the three oldest oil-shales. On the other hand, the HC profile of fluids originating from samples dominated by degraded lacustrine OM, which was obtained for the first time in this study, show low n -alkanes concentrations and a relatively prominent hump. These samples were only present in the three youngest oil-shale levels. Finally, the boghead showed a link between strong concentration of the Pila genus of Botryococcus algae and a primary fluid dominated by n -C 6 - n -C 14 n -alkanes.By comparing these data with published palaeoenvironmental studies, it appears that the kind of dominant OM, and thus the HC distribution of primary fluids, is controlled by the lake level: a lower depth preventing the settlement of anoxic conditions in the bottom waters, and the subsequent good preservation of the OM. During the Autunian, the lake level decreased, consequence of a reduced tectonic activity and of the beginning of a dryer period. Finally, these changes will have caused a strong decrease of the nutrient influx to the lake, favoring the bloom of Botryococcus algae that accumulated to form the boghead.","PeriodicalId":55978,"journal":{"name":"Bulletin de la Societe Geologique de France","volume":"59 1","pages":"29"},"PeriodicalIF":0.0,"publicationDate":"2017-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89009754","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Global inventory of shale-oil resources and reserves are far from being complete even in mature basins which have been intensively drilled and produced and in which the main parameters of the regional or local oil-prone source rocks are known. But even in these cases, difficulties still occur for deriving reserves from resources: reaching a plausible recovery factor is actually a complex task because of the lack of production history in many shale-oil ventures. This exercise is in progress in several institutions (EIA, USGS, AAPG) or private oil and gas companies on a basin-by-basin basis in order to estimate the global potential. This analytical method is very useful and accurate but also very time consuming. In the last EIA report in 2013 “only” 95 basins had been surveyed whereas for example, no Middle-East or Caspian basins have been taken into account. In order to accelerate the process and to reach an order of magnitude of worldwide shale-oil reserves, we propose hereafter a method based on the Petroleum System principle as defined by Demaison and Huizinga (Demaison G and Huizinga B. 1991. Genetic classification of Petroleum Systems. AAPG Bulletin 75 (10): 1626–1643) and more precisely on the Petroleum System Yield (PSY) defined as the ratio (at a source-rock drainage area scale) between the accumulated hydrocarbons in conventional traps (HCA) and hydrocarbons generated by the mature parts of the source-rock (HCG). By knowing the initial oil reserves worldwide we can first derive the global HCA and then the HCG. Using a proxy for amount of the migrated oil from the source-rocks to the trap, one can obtain the retained accumulations within the shales and then their reserves by using assumptions about a possible average recovery factor for shale-oil. As a definition of shale-oil or more precisely LTO (light tight oil), we will follow Jarvie (Jarvie D. 2012. Shale resource systems for oil & gas: part 2 – Shale Oil Resources Systems. In: Breyer J, ed. Shale Reservoirs . AAPG, Memoir 97, pp. 89–119) stating that “shale-oil is oil stored in organic rich intervals (the source rock itself) or migrated into juxtaposed organic lean intervals”. According to several institutes or companies, the worldwide initial recoverable oil reserves should reach around 3000 Gbo, taking into account the already produced oil (1000 Gbo) and the “Yet to Find” oil (500 Gbo). Following a review of more than 50 basins within different geodynamical contexts, the world average PSY value is around 5% except for very special Extra Heavy Oils (EHO) belts like the Orinoco or Alberta foreland basins where PSY can reach 50% (!) because large part of the migrated oils have been trapped and preserved and not destroyed by oxidation as it is so often the case. This 50% PSY figure is here considered as a good proxy for the global amount of expelled and migrated oil as compared to the HCG. Confirmation of such figures can also be achieved when studying the ratio of S1 (in-place hydroca
全球页岩油资源和储量的清查还远远不够完整，即使是在已经进行了密集钻探和生产的成熟盆地中，即使是在已知区域或局部易油烃源岩主要参数的盆地中。但即使在这些情况下，从资源中获取储量仍然存在困难:由于许多页岩油企业缺乏生产历史，达到一个合理的采收率实际上是一项复杂的任务。几个机构(EIA、USGS、AAPG)或私营石油和天然气公司正在逐个盆地地进行这项工作，以估计全球潜力。这种分析方法非常有用和准确，但也非常耗时。在2013年的上一份EIA报告中，“仅”调查了95个盆地，而没有考虑到中东或里海盆地。为了加快这一进程并达到世界范围内页岩油储量的一个数量级，我们在此提出了一种基于Demaison和Huizinga (Demaison G和Huizinga B. 1991)所定义的石油系统原理的方法。含油气系统成因分类。AAPG通报75(10):1626-1643)，更准确地说，石油系统产量(PSY)定义为常规圈闭(HCA)中聚集的烃与源岩成熟部分(HCG)产生的烃的比值(在源岩排水面积尺度上)。通过了解全球初始石油储量，我们可以首先得到全球HCA，然后是HCG。使用从源岩到圈闭的运移油量的代理，可以通过对页岩油可能的平均采收率的假设，获得页岩内的保留油藏，然后获得其储量。关于页岩油或更准确的LTO(轻质致密油)的定义，我们将遵循Jarvie (Jarvie D. 2012)。石油和天然气用页岩资源系统:第2部分-页岩油资源系统参见:Breyer J，主编。页岩储层。AAPG，回忆录97，第89-119页)指出，“页岩油是储存在有机富层(烃源岩本身)或运移到并列的有机贫层的油”。根据一些研究机构或公司的说法，考虑到已经开采的石油(1000亿吨)和尚未发现的石油(500亿吨)，全球初始可采石油储量应达到3000亿吨左右。在对50多个不同地球动力学背景下的盆地进行回顾后，世界平均PSY值约为5%，除了非常特殊的特稠油(EHO)带，如Orinoco或Alberta前陆盆地，PSY可以达到50%(!)，因为大部分运移的油被困住并保存下来，而不是像通常情况那样被氧化破坏。与HCG相比，这个50%的PSY值被认为是全球排出和迁移油量的一个很好的代表。通过研究岩石评价实验室测量中某些烃源岩的S1(原位烃)与S2(潜在可产烃)之比，也可以证实这些数据。以3000 Gbo作为世界石油储量，假设相当乐观的平均采收率为40%，相应的HCA接近7500 Gbo, HCG (= HCA/PSY)接近15万Gbo。假设排出(运移)因子为50%，我们得到全球烃源岩圈闭7.5万Gbo，与页岩油资源相对应。为了从这些资源中得出(可采的)储量，需要估计平均采收率(RF)。确定采收率的主要参数是孔隙度和饱和度的合理值，而在这些极细粒度、致密的非常规储层中，由于采样和实验室技术流程差异很大，很难获得合理的孔隙度和饱和度值。然而，新的测井技术(核磁共振)和扫描电镜图像显示，由于成熟度的提高，油页岩的主要有效孔隙度是在有机质内部产生的。因此，孔隙度随着总有机碳(TOC)的增加而增加，而与…此外，孔隙度从未含水，主要是油湿性的，因此含油饱和度非常高，测量和计算结果在75 - 90%之间。在水力压裂开始之前，Bakken LTO的第一口垂直生产井显示出非常低的含水率(在1%至4%之间)，累积产油量为300 Kbo，从而间接验证了这些高数据。因此，可以假设应使用几位研究人员建议的10%左右的最高射频值。据此，全球无风险页岩油储量应在7500亿吨左右。 然而，对于一些地下陷阱(主要是分散的III型干酪根烃源岩盆地或位于气窗的烃源岩)和许多人类活动造成的地面障碍(农业、住房、交通线路等)，应该考虑高风险因素，这些障碍可能会阻碍页岩油的开发。假设仅在(半)沙漠条件下(即中东、哈萨克斯坦、西西伯利亚、北非、中国西部、阿根廷西部、美国西部、加拿大、墨西哥和澳大利亚的部分地区)开发页岩油盆地，则可以使用20%的概率因子。因此，全球页岩油储量可能达到1500亿吨石油当量，这是初始常规储量的一半，因此可能是目前常规石油剩余储量的两倍。
{"title":"Worldwide shale-oil reserves: towards a global approach based on the principles of Petroleum System and the Petroleum System Yield","authors":"M. Blaizot","doi":"10.1051/BSGF/2017199","DOIUrl":"https://doi.org/10.1051/BSGF/2017199","url":null,"abstract":"Global inventory of shale-oil resources and reserves are far from being complete even in mature basins which have been intensively drilled and produced and in which the main parameters of the regional or local oil-prone source rocks are known. But even in these cases, difficulties still occur for deriving reserves from resources: reaching a plausible recovery factor is actually a complex task because of the lack of production history in many shale-oil ventures. This exercise is in progress in several institutions (EIA, USGS, AAPG) or private oil and gas companies on a basin-by-basin basis in order to estimate the global potential. This analytical method is very useful and accurate but also very time consuming. In the last EIA report in 2013 “only” 95 basins had been surveyed whereas for example, no Middle-East or Caspian basins have been taken into account. In order to accelerate the process and to reach an order of magnitude of worldwide shale-oil reserves, we propose hereafter a method based on the Petroleum System principle as defined by Demaison and Huizinga (Demaison G and Huizinga B. 1991. Genetic classification of Petroleum Systems. AAPG Bulletin 75 (10): 1626–1643) and more precisely on the Petroleum System Yield (PSY) defined as the ratio (at a source-rock drainage area scale) between the accumulated hydrocarbons in conventional traps (HCA) and hydrocarbons generated by the mature parts of the source-rock (HCG). By knowing the initial oil reserves worldwide we can first derive the global HCA and then the HCG. Using a proxy for amount of the migrated oil from the source-rocks to the trap, one can obtain the retained accumulations within the shales and then their reserves by using assumptions about a possible average recovery factor for shale-oil. As a definition of shale-oil or more precisely LTO (light tight oil), we will follow Jarvie (Jarvie D. 2012. Shale resource systems for oil & gas: part 2 – Shale Oil Resources Systems. In: Breyer J, ed. Shale Reservoirs . AAPG, Memoir 97, pp. 89–119) stating that “shale-oil is oil stored in organic rich intervals (the source rock itself) or migrated into juxtaposed organic lean intervals”. According to several institutes or companies, the worldwide initial recoverable oil reserves should reach around 3000 Gbo, taking into account the already produced oil (1000 Gbo) and the “Yet to Find” oil (500 Gbo). Following a review of more than 50 basins within different geodynamical contexts, the world average PSY value is around 5% except for very special Extra Heavy Oils (EHO) belts like the Orinoco or Alberta foreland basins where PSY can reach 50% (!) because large part of the migrated oils have been trapped and preserved and not destroyed by oxidation as it is so often the case. This 50% PSY figure is here considered as a good proxy for the global amount of expelled and migrated oil as compared to the HCG. Confirmation of such figures can also be achieved when studying the ratio of S1 (in-place hydroca","PeriodicalId":55978,"journal":{"name":"Bulletin de la Societe Geologique de France","volume":"23 1","pages":"33"},"PeriodicalIF":0.0,"publicationDate":"2017-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84099231","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
V. Crombez, S. Rohais, F. Baudin, B. Chauveau, T. Euzen, D. Granjeon
The recent development of unconventional resources has triggered a regain of interest for source-rocks. The presence of hydrocarbons in these unconventional systems is generally associated with organic-rich sediments. This study aims at better understanding the factors controlling the accumulation of marine organic matter at basin scale, using a process-based approach. This work focuses on the Montney, Doig and Halfway Formations (Lower and Middle Triassic, Alberta and British Columbia, Canada). Recent studies show that the Triassic strata of the Western Canada sedimentary basin can be considered as a transitional period between the Paleozoic passive margin and the Jurassic foreland basin. Based on a 3D regional stratigraphic architecture and on a description of the organic rich interval distribution, a process-based numerical model (DionisosFlow and DORS) has been used to simulate the stratigraphic evolution of the Montney, Doig and Halfway Formations and reproduce the organic distribution in these formations. This modeling approach allowed us to test different scenarios of primary productivity and basin restriction and discuss the regional controls on organic matter accumulation such as dynamic of anoxia or dilution of organic matter by detrital sediments. The reconstruction of the stratigraphic architecture emphasizes a major drop of the water discharge in the basin. In the absence of any evidence supporting a link with a climate change, the drop in water discharge suggests a major modification of the drainage area of the basin, potentially associated with the early stage of the cordilleran orogeny and foreland basin evolution. The numerical simulation also shows that the primary productivity rates in the Montney and Doig Formations are characteristic of a coastal area and that a basin restriction is required to account for the level of anoxia observed in the studied Formations. Lastly, this study investigates the regional controls on organic matter accumulation and emphasizes the impact of regional paleogeographic and geodynamic evolution on the dynamic of anoxia and on the dilution.
{"title":"Controlling factors on source rock development: implications from 3D stratigraphic modeling of Triassic deposits in the Western Canada Sedimentary Basin","authors":"V. Crombez, S. Rohais, F. Baudin, B. Chauveau, T. Euzen, D. Granjeon","doi":"10.1051/BSGF/2017188","DOIUrl":"https://doi.org/10.1051/BSGF/2017188","url":null,"abstract":"The recent development of unconventional resources has triggered a regain of interest for source-rocks. The presence of hydrocarbons in these unconventional systems is generally associated with organic-rich sediments. This study aims at better understanding the factors controlling the accumulation of marine organic matter at basin scale, using a process-based approach. This work focuses on the Montney, Doig and Halfway Formations (Lower and Middle Triassic, Alberta and British Columbia, Canada). Recent studies show that the Triassic strata of the Western Canada sedimentary basin can be considered as a transitional period between the Paleozoic passive margin and the Jurassic foreland basin. Based on a 3D regional stratigraphic architecture and on a description of the organic rich interval distribution, a process-based numerical model (DionisosFlow and DORS) has been used to simulate the stratigraphic evolution of the Montney, Doig and Halfway Formations and reproduce the organic distribution in these formations. This modeling approach allowed us to test different scenarios of primary productivity and basin restriction and discuss the regional controls on organic matter accumulation such as dynamic of anoxia or dilution of organic matter by detrital sediments. The reconstruction of the stratigraphic architecture emphasizes a major drop of the water discharge in the basin. In the absence of any evidence supporting a link with a climate change, the drop in water discharge suggests a major modification of the drainage area of the basin, potentially associated with the early stage of the cordilleran orogeny and foreland basin evolution. The numerical simulation also shows that the primary productivity rates in the Montney and Doig Formations are characteristic of a coastal area and that a basin restriction is required to account for the level of anoxia observed in the studied Formations. Lastly, this study investigates the regional controls on organic matter accumulation and emphasizes the impact of regional paleogeographic and geodynamic evolution on the dynamic of anoxia and on the dilution.","PeriodicalId":55978,"journal":{"name":"Bulletin de la Societe Geologique de France","volume":"86 1","pages":"30"},"PeriodicalIF":0.0,"publicationDate":"2017-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90644375","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Baltzer, M. Reynaud, A. Ehrhold, J. Fournier, C. Cordier, Hélène Clouet
About a decade ago, a large field of pockmarks, covering an overall area of 36 km2 was discovered in water depths of , forming original benthic communities. Two complementary surveys were carried out in April 2011 (Pock and Ploops) and April 2014 (Pock and Tide), on the Oceanographic Vedette (O/V) Haliotis (Ifremer/Genavir), to map the limit of the pockmarks and Haploops fields with the maximum accuracy. The link between the presence of the Haploops nirae communities and the occurrence of pockmarks /gas was then established and the proposed hypothesis was that tidal cycles may provide a good candidate for a short-term (monthly) triggering mechanism of fluid expulsion (Baltzer A, Ehrhold A, Rigolet C, Souron A, Cordier C, Clouet H. 2014. Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders. Geo-Marine Letters, 34, 215–230). Due to the high-level precision (50 cm) of the positioning system (Magellan Aquarius Ixsea Hydrins) coupled with the RTK attitude system, these new bathymetric and imagery maps together with the sub-bottom Chirp profiles, allow us to compare the data sets from April 2011 and April 2014. The superimposition of the two data sets shows that the distribution of the pockmarks remains similar between these 2 dates (i.e. for 3 years), for the group of large, widely scattered pockmarks, which are deeply rooted in the Holocene palaeo-valley infills and for the group of pockmarks identified as the trawl-scour pockmarks, initiated by trawling action. Most of the pockmarks present very recent shapes without any infilling but sonar imagery reveals that some of them have been covered by a thin muddy layer, thereby reflecting, at least, a temporary cessation of expulsion or a different activity. Chirp profiles indicate some acoustic flares above the pockmarks, revealing gas/fluid expulsion. Different gas clues within the sedimentary column, such as acoustic turbidity, enhanced gas reflectors (EGR), chimneys pipes, occur at exactly the same places on the chirp seismic profiles from 2011 and 2014. Therefore, contrary to most examples described in the literature, this pockmarks field is still active.
大约十年前，在深海发现了一大片麻坑，总面积达36平方公里，形成了原始的底栖生物群落。2011年4月(Pock and Ploops)和2014年4月(Pock and Tide)在Oceanographic Vedette (O/V) Haliotis (Ifremer/Genavir)上进行了两次补充性调查，以最大精度绘制pockmarks和Haploops场的边界。随后建立了Haploops nirae群落的存在与坑点/气体的发生之间的联系，提出的假设是潮汐循环可能为短期(每月)触发流体排出机制提供了一个很好的候选机制(Baltzer a, Ehrhold a, Rigolet C, Souron a, Cordier C, Clouet H. 2014)。布列塔尼南部Concarneau湾一个活动麻子场的地球物理勘探及其对常驻悬浮喂食动物的影响。地球海洋学报，34,215-230)。由于定位系统(Magellan Aquarius Ixsea Hydrins)的高精确度(50厘米)加上RTK姿态系统，这些新的水深和图像地图以及海底Chirp剖面，使我们能够比较2011年4月和2014年4月的数据集。两组数据的叠加表明，在这2个日期(即3年)中，深植根于全新世古河谷填塞物的大而分散的麻子组和由拖网活动引发的拖网冲刷麻子组的麻子分布保持相似。大多数麻子都是最近形成的，没有任何填充物，但声纳图像显示，其中一些麻子被一层薄薄的泥泞层覆盖，因此至少反映出暂时停止了驱逐或不同的活动。啁啾谱显示，在麻子上方有一些声波耀斑，显示出气体/流体的排出。在2011年和2014年的啁啾地震剖面上，沉积柱中不同的气体线索，如声浊度、增强气体反射器(EGR)、烟囱管，出现在完全相同的位置。因此，与文献中描述的大多数例子相反，这个麻子场仍然活跃。
{"title":"Space-time evolution of a large field of pockmarks in the Bay of Concarneau (NW Brittany)","authors":"A. Baltzer, M. Reynaud, A. Ehrhold, J. Fournier, C. Cordier, Hélène Clouet","doi":"10.1051/BSGF/2017191","DOIUrl":"https://doi.org/10.1051/BSGF/2017191","url":null,"abstract":"About a decade ago, a large field of pockmarks, covering an overall area of 36 km2 was discovered in water depths of , forming original benthic communities. Two complementary surveys were carried out in April 2011 (Pock and Ploops) and April 2014 (Pock and Tide), on the Oceanographic Vedette (O/V) Haliotis (Ifremer/Genavir), to map the limit of the pockmarks and Haploops fields with the maximum accuracy. The link between the presence of the Haploops nirae communities and the occurrence of pockmarks /gas was then established and the proposed hypothesis was that tidal cycles may provide a good candidate for a short-term (monthly) triggering mechanism of fluid expulsion (Baltzer A, Ehrhold A, Rigolet C, Souron A, Cordier C, Clouet H. 2014. Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders. Geo-Marine Letters, 34, 215–230). Due to the high-level precision (50 cm) of the positioning system (Magellan Aquarius Ixsea Hydrins) coupled with the RTK attitude system, these new bathymetric and imagery maps together with the sub-bottom Chirp profiles, allow us to compare the data sets from April 2011 and April 2014. The superimposition of the two data sets shows that the distribution of the pockmarks remains similar between these 2 dates (i.e. for 3 years), for the group of large, widely scattered pockmarks, which are deeply rooted in the Holocene palaeo-valley infills and for the group of pockmarks identified as the trawl-scour pockmarks, initiated by trawling action. Most of the pockmarks present very recent shapes without any infilling but sonar imagery reveals that some of them have been covered by a thin muddy layer, thereby reflecting, at least, a temporary cessation of expulsion or a different activity. Chirp profiles indicate some acoustic flares above the pockmarks, revealing gas/fluid expulsion. Different gas clues within the sedimentary column, such as acoustic turbidity, enhanced gas reflectors (EGR), chimneys pipes, occur at exactly the same places on the chirp seismic profiles from 2011 and 2014. Therefore, contrary to most examples described in the literature, this pockmarks field is still active.","PeriodicalId":55978,"journal":{"name":"Bulletin de la Societe Geologique de France","volume":"32 1","pages":"23"},"PeriodicalIF":0.0,"publicationDate":"2017-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89425896","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Fluid seepage features on the upper continental slope offshore Congo are investigated using multi-disciplinary datasets acquired during several campaigns at sea carried out over the last 15 years. This datasets includes multibeam bathymetry, seismic data, seafloor videos, seafloor samples and chemical analyses of both carbonate samples and of the water column. Combined use of these datasets allows the identification of two distinctive associations of pockmark-like seabed venting structures, located in water depths of 600–700 m and directly above a buried structural high containing known hydrocarbon reservoirs. These two features are called spiders due to the association of large sub-circular depressions (the body) with smaller elongate depressions (the legs). Seismic reflection data show that these two structures correspond to amplitude anomalies located ca. 60–100 ms below seabed. The burial of these anomalies is consistent with the base of the methane hydrate stability domain, which leads to interpret them as patches of hydrate-related bottom-simulating reflection (BSR). The morphology and seismic character of the two structures clearly contrasts with those of the regional background (Morphotype A). The spider structures are composed of two seafloor morphotypes: Morphotype B and Morphotype C. Morphotype B makes flat-bottomed depressions associated with the presence of large bacterial mats without evidence of carbonates. Morphotype C is made of elongated depressions associated with the presence of carbonate pavements and a prolific chemosynthetic benthic life. On that basis of these observations combined with geochemical analyses, the spider structures are interpreted to be linked with methane leakage. Methane leakage within the spider structures varies from one morphotype to another, with a higher activity within the seafloor of Morphotype C; and a lower activity in the seafloor of Morphotype B, which is interpreted to correspond to a domain of relict fluid leakage. This change of the seepage activity is due to deeper changes in gas (or methane) migration corresponding to the progressive upslope migration of fluids. This phenomenon is due to the local formation of gas hydrates that form a barrier allowing the trapping of free gas below in the particular context of the wedge of hydrates.
{"title":"Spider structures: records of fluid venting from methane hydrates on the Congo continental slope","authors":"V. Casenave, P. Imbert","doi":"10.1051/BSGF/2017189","DOIUrl":"https://doi.org/10.1051/BSGF/2017189","url":null,"abstract":"Fluid seepage features on the upper continental slope offshore Congo are investigated using multi-disciplinary datasets acquired during several campaigns at sea carried out over the last 15 years. This datasets includes multibeam bathymetry, seismic data, seafloor videos, seafloor samples and chemical analyses of both carbonate samples and of the water column. Combined use of these datasets allows the identification of two distinctive associations of pockmark-like seabed venting structures, located in water depths of 600–700 m and directly above a buried structural high containing known hydrocarbon reservoirs. These two features are called spiders due to the association of large sub-circular depressions (the body) with smaller elongate depressions (the legs). Seismic reflection data show that these two structures correspond to amplitude anomalies located ca. 60–100 ms below seabed. The burial of these anomalies is consistent with the base of the methane hydrate stability domain, which leads to interpret them as patches of hydrate-related bottom-simulating reflection (BSR). The morphology and seismic character of the two structures clearly contrasts with those of the regional background (Morphotype A). The spider structures are composed of two seafloor morphotypes: Morphotype B and Morphotype C. Morphotype B makes flat-bottomed depressions associated with the presence of large bacterial mats without evidence of carbonates. Morphotype C is made of elongated depressions associated with the presence of carbonate pavements and a prolific chemosynthetic benthic life. On that basis of these observations combined with geochemical analyses, the spider structures are interpreted to be linked with methane leakage. Methane leakage within the spider structures varies from one morphotype to another, with a higher activity within the seafloor of Morphotype C; and a lower activity in the seafloor of Morphotype B, which is interpreted to correspond to a domain of relict fluid leakage. This change of the seepage activity is due to deeper changes in gas (or methane) migration corresponding to the progressive upslope migration of fluids. This phenomenon is due to the local formation of gas hydrates that form a barrier allowing the trapping of free gas below in the particular context of the wedge of hydrates.","PeriodicalId":55978,"journal":{"name":"Bulletin de la Societe Geologique de France","volume":"28 1","pages":"27"},"PeriodicalIF":0.0,"publicationDate":"2017-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89487019","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
P. Malié, J. Bailleul, F. Chanier, R. Toullec, G. Mahieux, V. Caron, B. Field, R. F. Mählmann, S. Potel
Analysis of offshore seismic lines suggests that a strong relationship exists between tectonic structures and fluid migration in accretionary prisms. However, only few field analogues of plumbing systems and their tectonic frameworks have been investigated in detail until now. The uplifted accretionary prism of the Hikurangi Margin (North Island, New Zealand) exposes early to late Miocene mudrocks in coastal cliffs of Cape Turnagain and in the Akitio syncline, south-east of the Pongaroa city. These outcrops display tubular carbonate concretions corresponding to complex subsurface plumbing networks of paleo-seeps within Miocene trench slope basins. We present here, new results on the spatial distribution of these tubular carbonate concretions, with particular attention to their relation to tectonic structures. In the Pongaroa area, tubular carbonate concretions in lower Miocene mudrocks occur along a N-S trend, while in middle Miocene strata they occur along a NNE-SSW direction. The N-S trend parallels a major fault zone (i.e. the Breakdown fault zone), which separates two wide synclines, the Waihoki and the Akitio synclines. During the Early-Middle Miocene, the Breakdown fault zone controlled the evolution of the Akitio trench slope basin constituting its western edge. The NNE-SSW strike parallels the axis of the Akitio syncline and is also parallel to the present-day subduction front. Our results therefore show that tubular concretions are parallel to post-Middle Miocene second order folding and thrusting in the northeastern limb of the Akitio syncline. In the Cape Turnagain area, tubular concretions occur in the western limb of the Cape Turnagain syncline, in the footwall of the major seaward-verging Cape Turnagain fault. This suggests that fluid migrations may occur not only in the crests of anticlines, as observed offshore for present-day plumbing system of cold seeps, but also in the footwalls of thrust faults. All these observations show that the spatial distribution of tubular concretions is controlled by regional tectonic structures with paleo-fluid migrations related to major deformation episodes of the accretionary prism. Thus, we distinguish three episodes events that likely triggered fluid migration leading to the formation of the tubular concretions: (1) In the Early Miocene, shortly after the onset of development of the Akitio trench slope basin, on its inner (western) edge; (2) During the late Middle Miocene, during an extensional deformation episode on the western limb of the Akitio trench slope basin; (3) At the end of the Late Miocene, during a second major shortening period at the footwall of major thrust fault, such as in the Cape Turnagain area.
{"title":"Spatial distribution and tectonic framework of fossil tubular concretions as onshore analogues of cold seep plumbing systems, North Island of New Zealand","authors":"P. Malié, J. Bailleul, F. Chanier, R. Toullec, G. Mahieux, V. Caron, B. Field, R. F. Mählmann, S. Potel","doi":"10.1051/BSGF/2017192","DOIUrl":"https://doi.org/10.1051/BSGF/2017192","url":null,"abstract":"Analysis of offshore seismic lines suggests that a strong relationship exists between tectonic structures and fluid migration in accretionary prisms. However, only few field analogues of plumbing systems and their tectonic frameworks have been investigated in detail until now. The uplifted accretionary prism of the Hikurangi Margin (North Island, New Zealand) exposes early to late Miocene mudrocks in coastal cliffs of Cape Turnagain and in the Akitio syncline, south-east of the Pongaroa city. These outcrops display tubular carbonate concretions corresponding to complex subsurface plumbing networks of paleo-seeps within Miocene trench slope basins. We present here, new results on the spatial distribution of these tubular carbonate concretions, with particular attention to their relation to tectonic structures. In the Pongaroa area, tubular carbonate concretions in lower Miocene mudrocks occur along a N-S trend, while in middle Miocene strata they occur along a NNE-SSW direction. The N-S trend parallels a major fault zone (i.e. the Breakdown fault zone), which separates two wide synclines, the Waihoki and the Akitio synclines. During the Early-Middle Miocene, the Breakdown fault zone controlled the evolution of the Akitio trench slope basin constituting its western edge. The NNE-SSW strike parallels the axis of the Akitio syncline and is also parallel to the present-day subduction front. Our results therefore show that tubular concretions are parallel to post-Middle Miocene second order folding and thrusting in the northeastern limb of the Akitio syncline. In the Cape Turnagain area, tubular concretions occur in the western limb of the Cape Turnagain syncline, in the footwall of the major seaward-verging Cape Turnagain fault. This suggests that fluid migrations may occur not only in the crests of anticlines, as observed offshore for present-day plumbing system of cold seeps, but also in the footwalls of thrust faults. All these observations show that the spatial distribution of tubular concretions is controlled by regional tectonic structures with paleo-fluid migrations related to major deformation episodes of the accretionary prism. Thus, we distinguish three episodes events that likely triggered fluid migration leading to the formation of the tubular concretions: (1) In the Early Miocene, shortly after the onset of development of the Akitio trench slope basin, on its inner (western) edge; (2) During the late Middle Miocene, during an extensional deformation episode on the western limb of the Akitio trench slope basin; (3) At the end of the Late Miocene, during a second major shortening period at the footwall of major thrust fault, such as in the Cape Turnagain area.","PeriodicalId":55978,"journal":{"name":"Bulletin de la Societe Geologique de France","volume":"26 1","pages":"25"},"PeriodicalIF":0.0,"publicationDate":"2017-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77455333","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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