The hydrocarbon source rocks of the Pliensbachian (Early Jurassic) in the Asturian Basin (Northern Spain): Their relationship with the palaeoclimatic oscillations and gamma-ray response.

Abstract

Deposition of black shale facies, one of the main contributors for hydrocarbon production, is commonly assumed to be linked to Oceanic Anoxic Events (OAEs), which are supposed to be generated during warm palaeoclimates. This assumption could bias the exploration for hydrocarbon source rocks preferentially towards sediments deposited under warm palaeonvironments, as a preferential guide for hydrocarbon exploration. As a consequence, the establishment of the links between palaeotemperature and the formation of organic-rich deposits is of primary importance. For this purpose, the Upper Sinemurian, Pliensbachian and Lower Toarcian (Lower Jurassic) deposits of the Asturian Basin in northern Spain, including more than 100 m thick succession containing organic-rich and black shale deposits, have been studied. Correlation between palaeoclimatic data, previously obtained from oxygen isotopes, and the TOC content reveals that black shales and organic-rich sediments were deposited not only during warming intervals but also during a prominent cooling event that occurred at the Late Pliensbachian. This cooling period has been pointed out as one of the main intervals to have developed ice caps in the poles during the Jurassic. On the contrary, no black shales were generated during the postulated Early Toarcian OAE, which coincides with a superwarming interval. Additionally, the study of the facies cycles and the measurement of gamma-ray in outcrops and its correlation with TOC content provides data on the use of natural radioactivity and sea level changes as a proxy for preliminary organic matter richness evaluation.Keywords: Organic-rich deposits, Climatic change, Oxygen isotopes, Carbon isotopes, Gamma ray, Sea level changes ResumenNormalmente se asume que la sedimentación de las facies de black shales, uno de los principales contribuyentes a la generación de hidrocarburos, está ligada a los Eventos Oceánicos Anóxicos (OAEs) y que éstas facies se formaron durante paleoclimas cálidos. Esta suposición puede orientar a la exploración de rocas madre de hidrocarburos preferentemente hacia los sedimentos depositados bajo paleoambientes cálidos. En consecuencia, el establecimiento de la relación entre paleotemperaturas y la formación de depósitos ricos en materia orgánica resulta de gran importancia para encontrar argumentos sobre este tema, aplicables a la exploración de hidrocarburos. Con este propósito se han estudiado los sedimentos del Sinemuriense Superior, Pliensbachiense y Toarciense Inferior (Jurásico Inferior) de la Cuenca de Asturiana, que incluyen más de 100 m de espesor de depósitos conteniendo facies con materia orgánica y black shales. La correlación entre los datos paleoclimáticos, obtenidos previamente a partir de isótopos de oxígeno, y el contenido de Carbono Orgánico Total (TOC) revela que los black shales y los sedimentos ricos en materia orgánica se depositaron no solo durante los intervalos cálidos sino también durante un destacado evento frío que se desarrolló durante el Pliensbachiense Superior. Est...

Climatic and sea‐level control of Jurassic (Pliensbachian) clay mineral sedimentation in the Cardigan Bay Basin, Llanbedr (Mochras Farm) borehole, Wales

“…Interestingly, abundant illite and R1 I–S occurs in the same aged rocks in the Paris Basin (Bougeault et al ., ) and the Aquitaine Basin (south‐western France, Brunel et al ., ). As suggested by many authors, lowering of the sea‐level could be a consequence of ice formation at high latitudes by the end of the Pliensbachian (Gómez et al ., ,b; Korte & Hesselbo, ; Ruebsam et al ., ; Silva & Duarte, ; Suan et al ., ). It is possible that lowering of sea‐level led to the emergence of part of the continental shelf and that the kaolinite, and the small proportions of berthierine that are common in the spinatum Zone, were partially reworked from older Jurassic sediments (including incipient ironstones).…”

Stratigraphy and Biochronostratigraphy of the Lower Pliensbachian (Jurassic) from the Asturian basin (Northern Spain)

“…In the lower part of the Taylori Subchronozone, a significant transgressive episode took place, probably at a global scale, coinciding with the first episode of black shales. It is characterised by a clearly negative Carbon (δ 13 C) excursion that is well recorded in the ER section (Gómez et al, 2016b). Some authors postulate that it might have been caused by volcanic activity and the corresponding hydrothermalism associated with the Central Atlantic Magmatic Province (CAMP) and by the resulting opening up of the Hispanic Corridor (Price et al, 2016;Ruhl et al, 2016).…”

Do Paleozoico dos Picos da Europa ao Jurássico da Costa (Asturiana) dos Dinossauros

“…Um mar epicontinental (raso) que se desenvolveu intra-placa tectónica, bastante antes da abertura do Atlântico Central e Norte, quando a Ibéria e a porção ocidental do norte de África estavam ainda unidas ao continente Norte Americano. A ocorrência de amonoides, em grande parte da sucessão, é o garante de um bom controlo biostratigráfico (o mesmo é dizer, cronostratigráfico), onde é possível identificar, de modo detalhado, os três últimos andares do Jurássico Inferior: Sinemuriano, Pliensbaquiano e Toarciano 12,16,17 . Mas, a toda esta importância estratigráfica, fulcral no exercício da correlação entre bacias sedimentares e na construção dos tão valiosos mapas paleogeográficos, adicionam-se os múltiplos aspetos paleontológicos e sedimentológicos de assinalável relevância científica e didática que aqui podem ser analisados.…”

“…Damos aqui relevância aos sedimentos ricos em matéria orgânica dos margo-calcários do Pliensbaquiano, que concedem uma cor negra característica à paisagem (FIGURA 12A)). A concentração do conteúdo orgânico é de tal ordem 16 , assim como as condições de grande soterramento que estas rochas sofreram (a alguns quilómetros de profundidade), terão permitido mesmo a génese de hidrocarbonetos (FIGURA 12B)) 18 . Uma preservação da matéria orgânica que terá sido favorecida por um ambiente marinho particularmente deficitário em oxigénio.…”