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Eldrett J.S.,Royal Dutch Shell | Dodsworth P.,StrataSolve Ltd | Bergman S.C.,Royal Dutch Shell | Wright M.,Chemostrat Inc. | Minisini D.,Royal Dutch Shell
Climate of the Past | Year: 2017

The Late Cretaceous Epoch was characterized by major global perturbations in the carbon cycle, the most prominent occurring near the Cenomanian-Turonian (CT) transition marked by Oceanic Anoxic Event 2 (OAE-2) at 94.9-93.7 Ma. The Cretaceous Western Interior Seaway (KWIS) was one of several epicontinental seas in which a complex water-mass evolution was recorded in widespread sedimentary successions. This contribution integrates new data on the main components of organic matter, geochemistry, and stable isotopes along a north-south transect from the KWIS to the equatorial western Atlantic and Southern Ocean. In particular, cored sedimentary rocks from the Eagle Ford Group of west Texas ( 90-98 Ma) demonstrate subtle temporal and spatial variations in palaeoenvironmental conditions and provide an important geographic constraint for interpreting water-mass evolution. High-latitude (boreal-austral), equatorial Atlantic Tethyan and locally sourced Western Interior Seaway water masses are distinguished by distinct palynological assemblages and geochemical signatures. The northward migration of an equatorial Atlantic Tethyan water mass into the KWIS occurred during the early-middle Cenomanian (98-95 Ma) followed by a major re-organization during the latest Cenomanian-Turonian (95-94Ma) as a full connection with a northerly boreal water mass was established during peak transgression. This oceanographic change promoted de-stratification of the water column and improved oxygenation throughout the KWIS and as far south as the Demerara Rise off Suriname. In addition, the recorded decline in redox-sensitive trace metals during the onset of OAE-2 likely reflects a genuine oxygenation event related to open water-mass exchange and may have been complicated by variable contribution of organic matter from different sources (e.g. refractory/terrigenous material), requiring further investigation. © 2017 Author(s).

Thomka J.R.,University of Akron | Sullivan N.B.,Chemostrat Inc. | Brett C.E.,University of Cincinnati
Lethaia | Year: 2017

Unusual specimens of an ichnofossil, herein ascribed to Arthrophycus, are described from siltstone tempestites in the lower Silurian (Llandovery, Telychian) Estill Shale near Irvine, Estill County, central Kentucky, USA. The specimens display closely spaced, uncurved transverse annulations, but lack the medial ridge, rectangular cross-sectional shape and dense branching typical of common Silurian Arthrophycus ichnospecies. This combination of features, coupled with the straightness of burrows and alignment parallel to inorganic tool marks, closely approximates the appearance of moulds of partially articulated crinoid columns (= pluricolumnals) that were impressed into firm mud and cast by storm-deposited sediment. That these structures do not represent the external moulds of body fossils is suggested by minor width variations along the long axis of specimens, possible bioglyphs and the total absence of skeletal material, including isolated crinoid columnals, in this and other tempestites from this interval. More convincingly, the presence of very sparse branching indicates a trace fossil origin for these structures. These fossils are best interpreted as the products of preferential mining of inorganically generated tool marks by deposit feeders, owing to exposure of more organic-rich muds by erosional scouring. Thus, this occurrence represents a rare instance in which trace fossils display a strong alignment parallel to inorganic sedimentary structures. © 2017 The Lethaia Foundation.

Hildred G.,Chemostrat Inc. | Scmidt K.,Pioneer Natural Resources Inc. | Pearce T.,Chemostrat Ltd.
3rd EAGE Shale Workshop: Shale Physics and Shale Chemistry | Year: 2012

Over the few past years shale resource plays have become increasingly important hydrocarbon plays. In the USA, formations such as the Barnett Formation, the Haynesville Formation and the Eagle Ford Shale Formation have become major hydrocarbon exploration targets. However, understanding the controls on reservoir quality in these shale formations is still in its infancy, despite thousands of well penetrations. In this paper, the Eagle Ford Shale Formation is used to demonstrate how inorganic whole rock geochemical data that are primarily obtained to provide stratigraphic correlations can be used to help understand mineralogy, organic content and rock brittleness. The primary application of whole rock geochemical data is to provide a chemostratigraphic correlation, which is of primary importance for temporally and geographically constraining other reservoir characteristics. The Eagle Ford Shale Formation is divisible into 4 geochemical units, based on changing values of P, Th/U and Cr/Th. The top of formation is readily geochemically defined by a decrease in the values of U, Cr/Th and V. Placing the top of the Eagle Ford Formation with confidence in itself is an important aspect for the drilling of horizontal wells. Reservoir quality in shale resource plays is dependent on numerous factors, including mineralogy, terrigenous input, bottom water conditions on deposition and TOC values. Mineralogically, the Eagle Ford Shale Formation is relatively simple, comprising quartz (av. 13%), calcite (av. 50%) and clay minerals (illite, illite/smectite, kaolinite and chlorite ) (av. 27%), with lesser amounts of pyrite, apatite and plagioclase feldspar. TOC values are typically between 1% and 7%. Each of these mineral phases and the TOC contents are readily modeled from the same elemental dataset used to define chemostratigraphic correlation framework. Furthermore, consideration of redox-sensitive elements, such as V, Ni, Th, U and Co provides a means to determine the degree of anoxia during deposition. The mineralogy plays an important role in how readily the formation can be fractured and because the inorganic geochemistry is directly linked to mineralogy, it is possible to calculate the relative brittleness of the mudstones. The methodologies demonstrated here in the Eagle Ford Shale Formation to define chemostratigraphic correlations, determine mineralogy and better understand bottom water conditions are readily applicable to any shale gas resource play around the world.

Sullivan N.B.,Chemostrat Inc. | Jeffrey Over D.,College Circle | Chuluun M.,Mongolian University of Science and Technology | Myrow P.M.,Colorado College
Journal of Asian Earth Sciences | Year: 2016

New and published stratigraphic data are integrated herein to resolve the age and depositional setting of Middle Devonian strata in the Gobi-Altai Terrane of south-central Mongolia (Bayanhongar Province). The Tsagaankhaalga Formation (Emsian?-Eifelian) is composed primarily of tabular sandy carbonates; it is sharply overlain by dark, fine-grained volcanoclastic marine sedimentary rock of the Govialtai Formation (Givetian), which is comparatively unfossiliferous, except for sporadic, fossiliferous carbonate interbeds. The latter unit contains abundant tentaculites as well as a newly recovered collection of conodonts assigned to the lower Givetian varcus Zone (timorensis, rhenana, or ansatus). A positive shift in bulk magnetic susceptibility coincides with the Tsagaankhaalga/Govialtai contact, and elevated values within the Govialtai correspond to tuffaceous and rhyolitic strata.The transition from carbonate facies of the Tsagaankhaalga Formation to the volcanoclastic shales of the Govialtai Formation is interpreted as the result of local tectonic activity. This may be qualitatively described as a transition from a shallow marine carbonate shoal with an epibenthic fauna (Tsagaankhaalga), to a deeper, clastic dominated environment with a nektic fauna (Govialtai). The pulse of tectonism must predate the lower Givetian, and it echoes the "Tsakhir Event", which is recorded by the transition from Ordovician-Silurian carbonates to Lochkovian conglomerate. These findings provide useful biostratigraphic control and insight into the complex tectonic evolution of the Gobi-Altai Terrane and adjacent blocks on the Eurasian Craton. © 2015 Elsevier Ltd.

Wright M.,Chemostrat Inc. | Ratcliffe K.,Chemostrat Inc. | Hounslow M.,Lancaster University
Society of Petroleum Engineers - Unconventional Resources Technology Conference, URTeC 2015 | Year: 2015

Magnetic Susceptibility (MS) is an indicator of the concentration of magnetic particles in rocks. In pre-Quaternary sediments the magnetic susceptibility is often sourced in either Fe-rich clays (chlorites etc), or Fe-oxides (magnetite or hematite), and often shows a dilution-relationship with calcite which has a small negative MS. Mudrocks lend themselves readily to MS analyses, since MS often responds to the gross lithological variations, with a superimposed provenance or sometimes diagenetic signature. Two applications for magnetic susceptibility in shale resource plays will be considered in this paper: a) stratigraphic correlation and b) paleoflow determination. The first is carried out using data acquired from either small samples, measured in the laboratory, or by direct analysis of cores using a hand held MS meter. Paleoflow determinations utilise directional variation in magnetic susceptibility (Anisotropy of Magnetic Susceptibility- AMS) to make an interpretation of grain orientation. Re-orientation of the core is required to convert the preferred grain orientation into geographic coordinates. Direct measurement of core using a handheld magnetic susceptibility meter enables large, high resolution (5-10 cm spacing) datasets to be gathered rapidly. Typically, these data show marked cyclicity, which in a Miocene carbonate sequence from Mallorca will be shown to be controlled by sea level fluctuations. Furthermore, because high resolution measurements are available, parasequences can be imaged in the magnetic susceptibility data. Changes in the symmetry of the transgressive - regressive portions of parasequences allow variations in "stacking patterns" to be compiled, thereby providing input into sequence stratigraphic interpretations. This aspect will be demonstrated using core analysis from a US shale play. AMS measurements provide a rapid and precise determination of the three-dimensional orientation of grains in samples. When dealing with a shale play, any such grain-orientation data are difficult to determine using visual analyses. The AMS expresses the bedding-foliation, and the lineation (i.e. paleoflow direction) within the bedding plane. Hence, it can be used to infer structural information, as well as with-bedding preferred grain-orientation information. Here, we will show initial results from a European shale play that suggests AMS has the potential to be a powerful tool in paleoflow and sediment fabric analysis of mudrocks. Copyright 2015, Unconventional Resources Technology Conference.

Wright M.,Chemostrat Inc. | Ratcliffe K.,Chemostrat Ltd. | Mathia E.,Chemostrat Ltd.
Society of Petroleum Engineers - Unconventional Resources Technology Conference, URTeC 2015 | Year: 2015

Samples from two shale successions from two boreholes drilled in the Lower Permian strata of the Delaware Basin were investigated and assessed in terms of palaeoredox conditions during the sediment deposition, basin hydrography, and the application of the elemental data for the organic carbon content prediction. The strata, composed of the alternating calcareous shale/limestone and siliciclastic shale intervals showed to have only minor biogenic silica input, with the major biogenic component being of the carbonate source, and the siliciclastic fraction dominated by the terrigenous material. The authigenic enrichment in elements typical for anoxia and elevated palaeoproductivity (Ni, Cr, V, Cu, Mo, U) was revealed to be inconsistent throughout the shale sequence, with the major change in the trace element - Total Organic Carbon (TOC) correlation trends at TOC ca. 2%. For organic- lean rocks, low concentrations in all the redox elements, and their high normalized values as compared to reference shales, suggest deposition in anoxic/euxinic waters, but with the redox signature masked by the strong carbonate dilution. In the organic-rich rocks, multi-element covariations show patterns typical for anoxia, with elemental concentrations not affected by the slow background terrigenous sedimentation. The limited enrichment in Mo and U in the most organic-rich shales (up to 7% TOC) suggest that the hydrographic conditions had a critical effect on the limited deposition of the most conservative trace elements during the terrigenous sedimentation intervals. In such scenario, Nickel (Ni) proved to be a valuable proxy for the organic carbon deposition, with a sufficient predictive power to predict TOC in unknown shale successions. The evaluation of the Lower Permian Shale successions showed how critical the geochemical approach is when assessing shale sequences in relatively unknown basins, and their potential to store hydrocarbons. Copyright 2015, Unconventional Resources Technology Conference.

Hildred G.V.,Chemostrat Inc. | Ratcliffe K.T.,Chemostrat Inc. | Wright A.M.,Chemostrat Inc. | Zaitlin B.A.,BMO Nesbitt Burns Inc. | Wray D.S.,University of Greenwich
Journal of Sedimentary Research | Year: 2010

Incised valleys are a ubiquitous feature of the Lower Cretaceous Mannville Group of the Western Canada Sedimentary Basin. The Basal Quartz, a member of the Lower Mannville Formation, is present in two north-south-oriented subparallel valley-form systems in southern Alberta, the western valley form termed the Taber-Cutbank Valley and the eastern valley form termed the Whitlash Valley. This paper details the application of chemostratigraphy to discriminate between the three informal lithostratigraphic units of the Basal Quartz, namely the Horsefly, the Bantry-Alderson-Taber (BAT), and the Ellerslie units in these valley forms. In the Taber-Cutbank Valley, the Horsefly, BAT, and Ellerslie units each have unique chemostratigraphic signatures that enable them to be readily differentiated using inorganic geochemical data. The changes in elemental chemistry that allow this differentiation are inferred to reflect changes in sediment provenance, paleoclimate, and surface residence time. The whole-rock geochemistry of the Horsefly unit, the basal unit of the fill, is demonstrated to remain relatively constant longitudinally in the Taber-Cutbank Valley, therefore demonstrating, when compared to the vertical changes in geochemistry, that temporal or stratigraphic changes are of a greater magnitude than longitudinal changes within in a single valley form. The whole-rock geochemistry of the Horsefly unit in the two valley forms, which in previous studies has been demonstrated to be homotaxial by conventional stratigraphic techniques, is markedly different between the two valley forms. The geochemical differences suggest that the Horsefly unit was subjected to more prolonged and/or intense weathering in the eastern Whitlash Valley than in the western Taber-Cutbank Valley, and that the provenance of the Horsefly unit is fundamentally different between the two valley systems. The application of chemostratigraphy to the Basal Quartz of the Lower Mannville Formation demonstrates that the technique can be utilized as a viable correlation tool in low-accommodation incised-valley settings; however, it is most effective when the whole-rock geochemical data are placed in a regional context by integration with a detailed stratigraphic framework. Once chemostratigraphy is used in conjunction with these other data streams, the differences in geochemistry of the Horsefly unit between the Whitlash and Taber-Cutbank Valleys can be used to infer that the Horsefly unit deposited in the two valleys are two homotaxial, not coeval subunits. Copyright © 2010, SEPM (Society for Sedimentary Geology).

Ellwood B.B.,Louisiana State University | Wang W.-H.,Louisiana State University | Tomkin J.H.,Urbana University | Ratcliffe K.T.,Chemostrat Inc. | And 2 more authors.
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2013

Low-field, mass specific magnetic susceptibility (χ) and gamma radiation spectra (GRS) field and laboratory data sets for two Upper Cretaceous marine sedimentary sequences representing the Cenomanian-Turonian (C-T) boundary in the Western Interior Seaway in central Colorado have been measured, sections including the C-T Global boundary Stratotype Section and Point (GSSP) and a ~one kilometer distant co-eval sequence, the S Section. It has previously been established that both χ and GRS data sets are controlled by detrital/eolian fluxes into the marine environment, although the effect of weathering, if any, on these parameters when exposed in outcrop, is not well documented. In addition, these parameters are controlled by differing detrital/eolian components that may be derived from different sources, or be differentially concentrated within the marine system. Of great interest is (1) how well these data sets record climate cycles, (2) how the χ and GRS data are affected by weathering, and (3) how well the two methods support each other. Reported here are the results of a number of field and laboratory experiments designed to evaluate instruments and how χ and GRS data sets co-vary, and to test their usefulness as correlation tools in stratigraphy. Results show that both χ and 40K data sets from laboratory measurement on samples collected in the field, where rocks are relatively fresh, are generally well correlated, respond closely to climate and the corresponding detrital/eolian flux into the marine environment, and can be used for a rough proxy for each other. However, weathering appears to affect the 40K data by removing 40K, thus reducing correlation potential, although χ does not appear to suffer as badly in more weathered sections and good correlations between sections remain. Field GRS measurements produce a broadly smoothed data set that regionally can be very useful for correlation, although potential global correlations are not tested here. Using χ data reported here, and comparing these data to a ~100kyr cyclic, eccentricity (E1) climate model for the Upper Cretaceous at ~93.9Ma, shows that χ and 40K data, measured in the laboratory, can be used to build floating point time scales that produce essentially equivalent results as do other studies using absolute dates. © 2013 Elsevier B.V.

Sullivan N.B.,Chemostrat Inc. | Mclaughlin P.I.,Indiana University | Emsbo P.,U.S. Geological Survey | Barrick J.E.,Texas Tech University | Premo W.R.,U.S. Geological Survey
Lethaia | Year: 2016

Constraining the age of strata is a fundamental source of uncertainty in the study of sedimentary rocks, particularly in restricted basins that generally lack index fossils. An illustrative example of this is the evaporite-bearing Salina Group in the Michigan Basin. Our integrated study of facies, paleontology, and stable isotope geochemistry from the base of the Salina Group in Wisconsin addresses long-standing chronostratigraphic uncertainty surrounding these units. Conodont samples from the basal boundary interval (Racine-Waubakee formation contact) produced non-diagnostic 'disaster' and 'recovery' faunas typical of both the Mulde (Homerian) and Lau (Ludfordian) events. Strontium isotope analysis (87Sr/86Sr) of these conodonts from five horizons just below the boundary yield values between 0.70844 and 0.70850 confirming a Homerian age. Multiple carbon isotope profiles through this interval confirm the presence of a 2.5-3‰ positive excursion. Cumulatively these data constrain the base of the Salina Group in Wisconsin to the Mulde Excursion interval (late Homerian). This integrated study provides a sound initial step towards a deeper understanding of the processes of Silurian evaporite formation in the Michigan Basin. © 2016 The Lethaia Foundation.

Sano J.,University of Arizona | Sano J.,Chemostrat Inc. | Ganguly J.,University of Arizona | Hervig R.,Arizona State University | And 2 more authors.
Geochimica et Cosmochimica Acta | Year: 2011

We have determined the Nd3+ diffusion kinetics in natural enstatite crystals as a function of temperature, f(O2) and crystallographic direction at 1bar pressure and applied these data to several terrestrial and planetary problems. The diffusion is found to be anisotropic with the diffusion parallel to the c-axial direction being significantly greater than that parallel to a- and b-axis. Also, D(//a) is likely to be somewhat greater than D(//b). Diffusion experiments parallel to the b-axial direction as a function of f(O2) do not show a significant dependence of D(Nd3+) on f(O2) within the range defined by the IW buffer and 1.5log unit above the WM buffer. The observed diffusion anisotropy and weak f(O2) effect on D(Nd3+) may be understood by considering the crystal structure of enstatite and the likely diffusion pathways. Using the experimental data for D(Nd3+), we calculated the closure temperature of the Sm-Nd geochronological system in enstatite during cooling as a function of cooling rate, grain size and geometry, initial (peak) temperature and diffusion direction. We have also evaluated the approximate domain of validity of closure temperatures calculated on the basis of an infinite plane sheet model for finite plane sheets showing anisotropic diffusion. These results provide a quantitative framework for the interpretation of Sm-Nd mineral ages of orthopyroxene in planetary samples. We discuss the implications of our experimental data to the problems of melting and subsolidus cooling of mantle rocks, and the resetting of Sm-Nd mineral ages in mesosiderites. It is found that a cooling model proposed earlier [Ganguly J., Yang H., Ghose S., 1994. Thermal history of mesosiderites: Quantitative constraints from compositional zoning and Fe-Mg ordering in orthopyroxene. Geochim. Cosmochim. Acta 58, 2711-2723] could lead to the observed ∼90Ma difference between the U-Pb age and Sm-Nd mineral age for mesosiderites, thus obviating the need for a model of resetting of the Sm-Nd mineral age by an "impulsive disturbance" [Prinzhoffer A, Papanastassiou D.A, Wasserburg G.J., 1992. Samarium-neodymium evolution of meteorites. Geochim. Cosmochim. Acta 56, 797-815]. © 2011 Elsevier Ltd.

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