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The Upper Cretaceous in the Tagus Basin (Central Spain): sequential analysis based on oil-well data and outcrop correlation

By
Manuel Segura
Manuel Segura
Departamento de Geología, Universidad de Alcalá, 28871 Alcalá de Henares, Spain (e-mail: manuel.segura@uah.es)
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Teresa Polo
Teresa Polo
Geolog SpA, Via Cario Porta 21, 20098 San Giuliano Milanese, Milano, Italy
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José F. García-Hidalgo
José F. García-Hidalgo
Departamento de Geología, Universidad de Alcalá, 28871 Alcalá de Henares, Spain (e-mail: manuel.segura@uah.es)
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Javier Gil
Javier Gil
Departamento de Geología, Universidad de Alcalá, 28871 Alcalá de Henares, Spain (e-mail: manuel.segura@uah.es)
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Beatriz Carenas
Beatriz Carenas
Departamento Química Agrícola, Geología y Geoquímica, Universidad Autónoma, 28049 Madrid, Spain
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Álvaro García
Álvaro García
Departamento Estratigrafía, Universidad Complutense, 28040 Madrid, Spain
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Published:
January 01, 2006

Abstract

The logs from six oil wells drilled between 1960 and 1980 in the Upper Cretaceous succession of the Tagus Basin were analysed and correlated with surface outcrops, to estimate sedimentary environments and to detect cyclicity in sedimentation. Log interpretation has been carefully checked with outcrop data, because high peaks in gamma-ray logs, usually considered to represent open marine facies, may correspond either to open marine or to muddy coastal deposits. The former correspond to the maximum flooding surfaces of depositional sequences, whereas the latter correspond to sequence boundaries. Three second-order depositional megasequences have been recognized (MS-2, MS-3 and MS-4). The basal megasequence onlaps older rocks, grading upwards from continental to marine deposits. The megasequence MS-3 shows a basal marly transgressive interval and a thick carbonate pile at the top, grading to the SE to sabkha environments. The top megasequence is hardly recognized in surface outcrops, but in well logs it is a thick evaporite-claystone unit of mainly sabkha environments that grade westwards to coastal deposits. Sedimentation of megasequences MS-3 and MS-4 extended farther west than previously considered, covering areas considered as part of the exposed Hesperian Massif. These were areas of marine and coastal sedimentation where organic matter accumulation could be potentially high, and can be considered potential areas for oil or gas exploration.

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Geological Society, London, Special Publications

Tectonics of the Western Mediterranean and North Africa

G. Moratti
G. Moratti
CNR-Institute of Geosciences and Earth Resources, Italy
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A. Chalouan
A. Chalouan
University of Mohammed V Agdal, Rabat, Morocco
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Geological Society of London
Volume
262
ISBN electronic:
9781862395107
Publication date:
January 01, 2006

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