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Palynological variations and sedimentary cycles in the Jurassic from the Paris basin

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Abstract

Two studies, one on Hettangian dolomitic cap rocks over “Gaz de France” gas storage reservoir rocks at Sologne, and the other on two separate sections in Dogger formations in Burgundy, conducted at the Palynology Laboratory of Strasbourg, illustrate that a palynological approach can lead to a considerable refinement of knowledge in sequence stratigraphy. The palynological study of the dolomitic cap rocks at Sologne dates them as Hettangian, distinguishes microfloristical assemblage types within the sequence, and establishes palynofacies variations with basin evolution. The evidence suggests that palynological variations in the Hettangian rocks are attenuated, reflecting restricted marine conditions and weak eustatic changes. The two sections in Dogger formations in Burgundy, one in Buffon with Callovian sediments deposited in an inner neritic environment, and the other in Varzy with Bathonian sediments deposited in an outer neritic environment, show an alternation between a sapropelic palynofacies, with abundant amorphous organic matter and numerous acritarchs, and a humic palynofacies, rich in dinoflagellate cysts, and spores and pollen. Such alternations indicate a cyclic palynological evolution even within the sedimentary parasequences (4th–6th order), as recorded in the section at Buffon.

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... The reservoirs are located at an average depth of approximately 1085 m (3560 ft) in Upper Triassic sandstones and Lower Jurassic shallow-marine limestones, both of which also cannot be studied in outcrops. A more extensive definition of the reservoir intervals and sedimentologic and stratigraphic reservoir characterization can be found in Grauls and Lafay (1979), Merzeraud (1992), Rauscher et al. (1992), Huault et al. (1995), Merzeraud et al. (1999), Merzeraud et al. (2000), Hamon and Merzeraud (2005). ...
... The reservoirs are located at an average depth of approximately 1085 m (3560 ft) in Upper Triassic sandstones and Lower Jurassic shallow-marine limestones, both of which also cannot be studied in outcrops. A more extensive definition of the reservoir intervals and sedimentologic and stratigraphic reservoir characterization can be found in Grauls and Lafay (1979), Merzeraud (1992), Rauscher et al. (1992), Huault et al. (1995), Merzeraud et al. (1999), Merzeraud et al. (2000), Hamon and Merzeraud (2005). ...
Article
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The fact that velocity models based on seismic reflection surveys commonly do not consider the near-surface geology necessitates filling the gap between the top of a velocity model and the surface of the Earth. In this study, we present a new workflow to build a shallow geologic model based exclusively on borehole data and corroborated by laboratory measurements. The study area is in Chemery (France), located at the southwestern border of the Paris Basin, where a large amount of borehole data is publicly available. The workflow starts with identifying lithologic interfaces in the boreholes and interpolating them between the boreholes. The three-dimensional (3-D) geometry of the lithologies then allows interpretation of the position, orientation, and offset of fault planes. Given the importance of the fault interpretation in the modeling process, a combination of different approaches is used to obtain the most reasonable structural framework. After creating a 3-D grid, the resulting 3-D structural model is populated with upscaled velocity logs from the boreholes, yielding the final near-surface P-wave velocity model. To better constrain the velocity model, we conducted laboratory measurements of P-and S-wave velocities in dry and water-saturated conditions on all lithologies in the model. The laboratory data were used to populate the 3-D near-surface model with V p/Vs ratio values. The presented workflow accounts for one-dimensional borehole data and is much more iterative and time-consuming than workflows based on two-dimensional seismic sections. Nevertheless, the workflow results in a robust 3-D near-surface model allowing for structural interpretations and revealing the 3-D seismic velocity field. Copyright © 2013. The American Association of Petroleum Geologists. All rights reserved.
... The reservoirs are located at an average depth of approximately 1085 m (3560 ft) in Upper Triassic sandstones and Lower Jurassic shallow-marine limestones, both of which also cannot be studied in outcrops. A more extensive definition of the reservoir intervals and sedimentologic and stratigraphic reservoir characterization can be found in Grauls and Lafay (1979), Merzeraud (1992), Rauscher et al. (1992), Huault et al. (1995), Merzeraud et al. (1999), Merzeraud et al. (2000), Hamon and Merzeraud (2005). ...
Article
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... Regarding the origin of sedimentary organic matter (OM), the main published contributions concern paleo-depositional environments and source variability based on palynological records (Fauconnier, 1995;Huault et al., 1995Huault et al., , 2003 and molecular geochemistry (Hautevelle et al., 2006(Hautevelle et al., , 2007 of Jurassic sediments. Disnar et al. (1996) showed correlations between palynofacies, saturated hydrocarbon extracts and Rock-Eval pyrolysis data from Jurassic sediments at the northern part of the basin, near the Ardennes massif. ...
Article
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... Also, the presence of conifer pollen and the increase in spore pollen abundance in the East of the Paris Basin during the Oxfordian were reported (e.g. Rauscher and Schmitt, 1990; Huault et al., 1995 Huault et al., , 2003). Palaeoflora described on emerged land bordering the Paris Basin during the Jurassic is quite consistent with typical Jurassic vegetation. ...
Article
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Vegetation, climate, and sea-level oscillations interact in complex ways. Intuition suggests that climatic variations are recorded in geological deposits by both fossil phytoclasts and sediments. Therefore investigation of palaeobotanical remains within a precise sedimentological framework may help to decipher this much-debated interaction. A database of 129 Jurassic (Bathonian–Oxfordian) fossil wood samples from France is used here as a case study. The palaeoenvironment (freshwater swamp, carbonate lagoon, shoreface, argillaceous offshore) and the second-order relative sea-level phases were determined for each datum. There is no correlation between the wood genera distribution and second-order relative sea-level cycles. However, 66% of the fossil wood samples were found within transgressive parts of cycles or at maximum flooding surfaces versus 34% in their regressive counterparts. By contrast, there is a close correlation between wood distribution and palaeoenvironment for Agathoxylon and Brachyoxylon, the two most common genera. Although both genera occur in the four main depositional systems, Agathoxylon is the more common in offshore facies, whereas Brachyoxylon predominates in protected lagoon environments. A model of wood deposition/preservation is proposed. It is shown that (1) the distribution of wood is not controlled by climate but by environmental processes related to relative sea-level changes; (2) the abundance of wood in transgressive deposits may be related to ravinement of previously emerged areas and/or to the high preservational potential associated with increased accommodation space; (3) carbonate platforms were probably inhabited by low-diversity Brachyoxylon xylofloras, whereas offshore environments received a more diversified, probably multi-source flora from emergent land areas. Periods of Bathonian, Callovian and Oxfordian subaerial exposure of carbonate platforms may not have been conducive to the establishment of a diverse, stable climax xyloflora.
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The Gaz de France natural gas reservoir of the Sologne region, in the southern part of the Paris Basin and their doiomitic and limestone covers have been studied both at the sedimentological and palynological points of view. Our palynological study had two purposes: (1) to confirm or define more accurately the Permo-Triassic stratigraphic age of the reservoir formations and the Rhaetian and Hettangian-Sinemurian ages of their dolomitic and limestone covers, respectively; (2) identify the successive environments and, using the sedimentological observations, define the evolution of the sediments in this portion of the Paris Basin. The study of various boreholes generally indicated a rich continental microflora where the Circumpolles are, by far, the most abundant, with less abundant marine microfossils such as acritarchs, dinocysts, foraminifera. Four successive palynological episodes were defined on the basis of the microfossil distribution through the boreholes. From bottom to top: (1) a fossil-poor basal episode with marine microfossils which is probably Rhaetian in age; (2) a fossil-rich Hettangian episode which corresponds to a coastal swamp environment protected or open to weak marine influences; (3) the dolomitic cover representing the third episode, also Hettangian in age and still under swampy coastal conditions, as indicated by abundant Circumpolles, however the third episode shows slight, but more regular marine characteristics; (4) the uppermost episode corresponding to the Sinemurian limestone with microfossil assemblages slightly poorer in Circumpolles and hence enriched in bisaccates pollen grains which indicated the increasing distance from the coastal line.Our study not only contributed to reconsidering the Lower Liassic stratigraphy of the Sologne region, but also identified the various characteristics of the successive environments which prevailed during this time.
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