El patrón de fracturación alpina en el sector suroccidental de los Pirineos Vascos

Abstract

Esta tesis se encuentra dentro de un proyecto que pretende construir una planta piloto de almacenamiento geológico de CO2, en Hontomín (Plataforma Burgalesa, norte de España). Los objetivos principales son construir un modelo 3D de la estructura de Hontomín y entender el patrón de fracturas de la Plataforma Burgalesa para aplicarlo a la zona de Hontomín. El modelo tridimensional se interpreta a partir de datos de sísmica 3D y es presentado en esta tesis, definiendo la estructura y las principales unidades reservorio y sello. El patrón de fracturas se ha analizado a través de análogos de campo. Concretamente, se ha estudiado la fracturación en las rocas cretácicas superiores de la Plataforma Burgalesa, en la zona de la falla de Ubierna y en el diapiro de Poza de la Sal (cuyos resultados han sido presentados en los tres artículos compilados en la presente tesis). El resultado de estos estudios ha sido integrado y se ha propuesto un modelo de evolución del patrón de fracturación de la zona. Asimismo, se han extrapolado estos datos de fracturación a la estructura de Hontomín dando las directrices del principal patrón de fracturación que se espera.

This thesis is part of a project funded by the Fundación Ciudad de la Energia (CIUDEN), which decided to build a plant science and technology for CO2 capture and storage development in Spain, in the Hontomín anticline (north of Burgos). Within this project have collaborated many research centers in Spain and specifically the "Institut de Recerca de la UB Geomodels" was given the task of characterizing both the threedimensional structure and the fracture pattern that affects the main reservoir and seal structure Hontomín. The purpose of this thesis is precisely to fill this need to characterize the structure together with the Hontomín fracturing, constraining the latter with the acquisition of regional data to characterize the deformation mechanisms responsible for their formation and evolution.

From the interpretation of a 3D seismic cube provided by CIUDEN (integrated with information of wells Hontomín-1,-2,-3 and -4), a 3D model of the Hontomín structure has been generated. This model shows a raised platform limited to the south and to the west by two monocline flexures that plunge Mesozoic materials below Cenozoic sedimentary fill of the Bureba basin. To the south, the ENE-WSW monocline culminated by tight E-W folds, which underlines the Quintanilla anticline, connecting with Ubierna Fault near the town of Montorio. Below Quintanilla anticline seismic lines show a strongly dipping fault (parallel to the anticline) dipping towards north, with mainly clockwise displacement (vertical displacement is almost zero or very low and slightly up the northern block). These structures were generated after the sedimentation of the continental Oligocene deposits. Previously of this sedimentation, the area was already affected by a prior Upper Jurassic to Lower Cretaceous structure. It involves developing a bending of carbonate platforms of the terminal Triassic to Middle Jurassic to the north and to the west associated with forced folds, formed mainly during the early stages of the syn-rift sedimentation of the Upper Jurassic - Lower Cretaceous (specifically contemporary the "Purbeck" facies), which show a growth strata geometry above the oldest bended Jurassic successions. Conversely, strike-slip and reverse faults are younger so they cut lower Cretaceous sequences. The culmination of this dome located in the center of the 3D seismic cube Hontomín shifted to the SE, appears below the angular unconformity that, in this area, separates the sub-horizontal Lower Cretaceous to the tilted succession of the terminal Triassic-Upper Jurassic to the north and to the west. This shift in the position of the dome culmination is innate in the folding of angular unconformities as shows the work of Suppe et al., 1992. Data analyzed in the Burgalesa Platform show that the fracture pattern depends primarily on: 1) The tectonic events that have affected the material; 2) the position of the rocks in relation to major faults affecting the area; and 3) the lithology of the rock where they developed. Jurassic and Lower Cretaceous materials are affected by the Upper Jurassic – Lower Cretaceous extension and by the subsequent Pyrenean compression. Thus, they present a complex pattern that includes NW-SE to E-O extensional faults and NNE-SSW and WNW-ESE joints. These fractures, mostly, were generated during the Upper Jurassic – Lower Cretaceous extension and were partly contractively reactivated during the compression event. The density and degree of deformation generated is fairly constant throughout the study area and increases considerably over Ubierna deformed bands and Oña where compressive Cenozoic deformation was more intense. Also, it appears to be higher around salt diapirs. The upper Cretaceous successions, only affected by compressive Pyrenean deformation are less fractured. In general, these sequences one can distinguish two patterns fracturing; one located along directional deformation bands that bounds the Burgalesa platform, and the other developed at the inside of the raised platform. The deformed Ubierna band is characterized by right strike-slip faults oriented WNW -ESE and by a penetrative network of NNW -SSE joints. In this zone thrust are oriented NE- SW and the left strike-slip faults are oriented NE-SW. Otherwise, in the deformed Oña band, is characterized by NS to NNW-SSE cleavages, WSW-ENE joints and faults WNW –ESE left strike-slip faults and subordinately, NE-SW right strike-slip faults. Out of these deformed bands, the deformation is less intense and is made especially for joints orientated WNW-ESE, WSW-ENE, NNW-SSE and NNE-SSO. Moreover, this area present also other types of fractures (although less abundant than joints) constituted by extensive faults oriented WNW-ESE, NNW-SSE and WSW-ENE, compressive faults (and cleavages) oriented ENE-WSW, WNW-ESE and NE-SW right strike-slip faults and WNW-ESE left strike-slip faults.