Minerales diagenéticos y paleotemperaturas en la Cuenca del Maestrazgo: relación con la etapa de rift y otros eventos térmicos regionales.

In the Lower Cretaceous sandstones of the Maestrat Basin are present several diagenetic minerals that provide information about their temperature of precipitation. Quartz cement (homogenisation temperatures with a mode of 145ºC), fibrous illite replacing feldspars (70-130ºC), the complete transformation of kaolinite to dickite (90-130ºC), the complete albitization of detrital feldspars (>105ºC) and barite and saddle dolomite cements (associated to hydrothermal fluids). Vitrinite reflectance of the organic matter in the intercalated marls revealed burial temperatures from 119 to 134ºC. However, these temperatures are not consistent with the thermal modelling of Penyagolosa sub-basin (Maestrat Basin). In the Penyagolosa Sub-basin model, the Lower Cretaceous sediments reached a maximum burial temperature of 65ºC, based on a thermal gradient of 30ºC/km, which was estimated by subsidence analysis, vitrinite reflectance and the stretching factor and thermal conductivities during syn-rift stage 2. Thus, in the Maestrat Basin existed conditions of relative higher temperature than predicted in a typical rift basin. These thermal anomaly is consistent with the existence other high-temperature events, such as alkali basaltic volcanism, metamorphism, thermal heating associated with high thermal gradients, and Hg-Sb bearing deposits have been reported in the Basque-Cantabrian Chain, the Pyrenean Mountains and the Iberian Chain.Las areniscas del Cretácico inferior de la Cuenca del Maestrazgo presentan varios minerales diagenéticos y transformaciones que proporcionan información directa o indirecta de las condiciones de temperatura: Cemento de cuarzo (temperaturas de homogeneización con moda 145ºC), ilita fibrosa reemplazando a feldespatos (70-130ºC), transformación completa de caolinita a dickita (90-130ºC), albitización completa de los feldespatos (>105ºC) y cementos de barita y dolomita saddle (asociados con fluidos hidrotermales). La reflectancia de la vitrinita en la materia orgánica de las margas intercaladas ha revelado temperaturas de enterramiento entre 119 y 134ºC. Sin embargo, los rangos de temperaturas que indican todos estos minerales y procesos diagenéticos no son consistentes con las temperaturas obtenidas a partir de la modelización en la subcuenca de Penyagolosa (Cuenca del Maestrazgo). Según este modelo, considerando un gradiente térmico de 30ºC/km, estimado a partir de cálculos de subsidencia, la reflectancia de la vitrinita, el factor de estiramiento que tuvo lugar y la conductividad térmica durante la etapa sin-rift 2, las areniscas del Cretácico inferior habrían alcanzado cómo máximo una temperatura de enterramiento de 65ºC. Por tanto, en la Cuenca del Maestrazgo se dieron condiciones de temperaturas más altas de las típicas de una etapa de rift. A escala regional, estaría relacionado con la presencia de otros eventos de temperatura elevada como son el volcanismo alcalino básico de la Cadena Vasco-Cantábrica, el metamorfismo de Pirineos y la Cadena Ibérica y las mineralizaciones de Hg-Sb de la Cadena Ibérica.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEMinisterio de Educación y Ciencia (MEC)pu

Diagenesis, carbonate cementation and resevoir quality evolution of Eocene deep-water marine turbidite sandstones of the Hecho Group, South-Central Pyrenees

Sandstone turbidites from the Hecho Group in the South-Central Pyrenees are considered exceptional examples for reservoir modelling and outcrop analogous studies. The Hecho Group is divided into four major tectosedimentary units (TSU-2 to TSU-5) and the sandstone composition varies from quartzarenites to arkoses (TSU-2), lithoarenites to hybrid arenites (TSU-3 and TSU-4), and hybrid arenites (rich in carbonate bioclasts; TSU-5). In TSU-2, the lowermost and most deformed unit, calcite cement precipitation was related to tectonic deformation. In the other turbidite systems (TSU-3, 4 and 5) eodiagenesis is evidenced by precipitation of dolomite cement and pyrite, which are locally abundant in all sandstones. Overall, compaction was more important than cementation in destroying porosity. However, the precipitation of dolomite overgrowth and intragranular mesogenetic ferroan calcite occluded nearly completely the remaining porosity and halted further compaction. Dissolution of calcite and dolomite cements has resulted in creation of minor amounts of secondary porosity

Influencia de la composición petrológica y procedencia en la albitización de las areniscas sinrift del Cretácico inferior, Cuenca del Maestrazgo, Cadena Ibérica

Lower Cretaceous sandstones from the Maestrat Basin are characterized by differences in detrital composition, provenance and degree of albitized feldspars (partial to complete). Petrological composition of Lower Cretaceous sandstones revealed a mixed provenance from plutonic and metamorphic source areas. A progressively major influence of granitic source rocks was detected from the lowermost Mora petrofacies toward the uppermost El Castellar and Camarillas petrofacies. Albitization of detrital feldspars occurred in all the studied sandstones, but feldspars were completely albitized in the lowermost Mora Fm. and only partially albitized in the overlying Castellar and Camarillas formations. However, all the Lower Cretaceous sandstones underwent, approximately, the same burial and thermal conditions (119-134ºC; based on vitrinite reflectance and thermal modelling). The decrease in replacive albite/detrital feldspars ratio from the lowermost Mora Fm. toward the uppermost Camarillas Fm. was related to differences in detrital composition and provenance of sandstones. Albitization was pervasive in the Mora Fm. sandstones because of its dominant metamorphic source areas, which implies a relative low abundance of K-feldspar compared to plagioclase. In contrast, the abundance of K-feldspars in Castellar and Camarillas formations prevented the complete albitization of feldspars. Thus, differences in original composition and provenance of sandstones influenced significantly the different degree of albitization for comparable burial and thermal rates.Las areniscas del Cretácico inferior de la Cuenca del Maestrazgo presentan diferencias en su composición detrítica, procedencia y grado de albitización de los feldespatos. La composición petrológica indica una mezcla de aportes de rocas metamórficas (dominantes en la Fm. Mora, Berriansiense-Valanginiense) y plutónicas (progresiva mayor contribución hacia las fms. Castellar y Camarillas, Valanginiense-Barremiense). La albitización de los feldespatos detríticos se observa en todas las areniscas, pero sólo la Fm. Mora presenta feldespatos completamente albitizados, mientras que las fms. Castellar y Camarillas la albitización es parcial. Uno de los factores que controla el proceso de albitización es la temperatura, sin embargo, todas las areniscas del Cretácico inferior estuvieron sometidas a condiciones de enterramiento y temperatura similares (119-134ºC; basadas en reflectancia de vitrinitas y modelización térmica). La disminución de grado de albitización hacia las fms. Castellar y Camarillas estaría relacionada con las diferencias en composición y procedencia de las areniscas. La albitización fue completa en la Fm. Mora debido a la mayor presencia de aportes de rocas metamórficas, lo que implica una mayor abundancia de plagioclasas. Por el contrario, la mayor abundancia de feldespato-K en las fms. Castellar y Camarillas impidió la completa albitización de los feldespatos. Por tanto, las diferencias en la composición y procedencia de las areniscas influyeron significativamente en el grado de albitización de los feldespatos en condiciones similares de enterramiento y temperaturaDepto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEMinisterio de Educación y Ciencia (MEC)pu

Diagénesis y evolución de la porosidad de las areniscas turbidíticas del Cretácico de la Cuenca de Vøring en el margen pasivo de Noruega

The Vøring Basin forms an integrated part of the passive margin off central Norway. Cretaceous sandstones are among the most important hydrocarbon exploration targets in the basin. One of the most significant features of the sandstone reservoirs is the excellent reservoir quality. The lack of early pervasive diagenetic cements and the high compositional maturity have contributed to the preservation of primary porosity. Reservoir quality evolution of the sandstones was equally controlled by cementation and compaction. Primary porosity was subjected to overall successive deterioration with increase in burial depth until the precipitation of post-compaction (mesogenetic) cements (quartz overgrowths, rhombic dolomite/ankerite, saddle dolomite and calcite). However, reservoir quality was improved through the partial to total dissolution of framework grains (mainly feldspars). The sources of acidic fluid to accomplish this dissolution are uncertain, but could be organic acids derived form thermal maturation of organic matter.La Cuenca de Vøring forma parte del margen pasivo de la costa Noruega. En esta cuenca, las areniscas Cretácicas constituyen uno de los más importantes objetivos en la exploración petrolífera de la zona. Estas areniscas destacan por su excelente calidad como reservorio. La ausencia de cementos eodiagenéticos y la elevada madurez composicional han contribuido a la preservación de la porosidad primaria. La evolución de la calidad del almacén estuvo controlada en la misma magnitud por la cementación y la compactación. La porosidad primaria disminuyó progresivamente con el enterramiento hasta la precipitación de cementos mesodiagenéticos (sobrecrecimientos de cuarzo, dolomita/ankerita rómbica, dolomita saddle y calcita). Sin embargo, la calidad como reservorio de las areniscas estudiadas mejoró debido a la disolución parcial a total de los granos del esqueleto (principalmente feldespatos). La fuente de los fluidos ácidos implicados en el proceso de disolución se desconoce, pero podría estar relacionada con la maduración térmica de la materia orgánica.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEEuropean Science FoundationMinisterio de Educación y Cienciapu

Composición y procedencia de los sedimentos detríticos del Cretácico Inferior de las subcuencas occidentales de la cuenca del Maestrazgo.

A thick succession of Lower Cretaceous, predominantly continental clastic sediments ("Weald" facies) crop out in the westernmost sub-basins of the Maestrazgo Basin (eastern Iberian Range, Spain). 62 sandstone samples from the Mora, Castellar, and Camarillas Fms. have been thin sectioned and studied using transmitted-light and scanining electron microscopy, and 4 1 samples were point counted (300-400 points per thin section). The sandstones of the Mora and Castellar Fms. are similar, having mean compositions of QH,FHRh and Q86F10R4, respectively. They are relatively rich in plagioclase and metamorphic rock fragments, lacking K-feldspar. The sandstones of the Camarillas Fm. have mean compositions of Q7gFuR8 (Pehagolosa subbasin) and Q75F,,R, (Calve andAliaga subbasins). They are relatively rich in K-feldspar and, locally, in plutonic rock fragments. In all the studied sandstones, the diagenesis has changed significantly the depositional abundances in feldspar, which is partly dissolved or replaced by kaolin and calcite. The sandstones of the Mora and Castellar Fms. derive in part from low- and medium-grade metamorphic terrains. In contrast, the sandstones of the Camarillas Fm. derive in part from crystalline terrains that were located to the W or NW

Caracterización geoquímica de las areniscas del Cretácico inferior de la cuenca del Maestrazgo, Cadena Ibérica Oriental: Nueva aportación al estudio de la procedencia

A geochemical characterization (major, trace and rare earth element abundances) of the Lower Cretaceous sandstones of the Maestrat Basin has been carried out, in order to discriminate samples from different formations and assess their provenance and weathering conditions. All the studied sandstones are arkoses and subarkoses characterized by a multi-elemental diagram showing enrichment in Rb, U, La, Nd, Zr and Y, and depletion in Ba, Nb, Sr, P and Ti, which is typical of calc-alkaline igneous rocks. This pattern suggests an input from igneous rocks directly from the source area or by recycling. The low chemical index alteration values do not reflect intense weathering conditions in the source region. Sandstones from the studied formations were discriminated using a Rb versus Y+Nb plot. This plot shows a progressive Rb enrichment and Y+Nb depletion toward the uppermost sandstones (Camarillas Tm). Rb versus K20 indicates a good positive correlation, which is consistent with the observed Kfeldspar abundance increase towards the Camarillas sandstones. The higher SREE and Y+Nb ratios in Mora Em. sandstones suggest a slight difference in provenance, which can be explained by the contribution of Paleozoic shales and metasediments