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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. Source


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. Source


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. Source


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. Source


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. Source

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