Control diagenético sobre la calidad de los reservorios de las areniscas “U” y “T” de la Fm Napo del Cretácico de la Cuenca Oriente, Ecuador. Modelización térmica y su relación con la generación de hidrocarburos.

La cuenca del Oriente Ecuatoriano, se encuentra en la parte oriental del Ecuador, corresponde a una cuenca de antepaís, que se estructura como resultados de esfuerzos transpresivos a partir del Cretácico superior, lo que provoca la emersión de la Cordillera Real en la parte occidental del Oriente y la formación de la cuenca de tras arco. Las Areniscas “U” y “T”, de la Fm Napo del Cretácico corresponden el área de estudio de este trabajo. La Fm Napo fue depositada en periodos de transgresión marina (HST), de sedimentos detríticos procedentes de la parte occidental de la cuenca. Sin embargo, algunos intervalos de areniscas “U” y “T” fueron depositadas en periodos de regresión (LST). Los espesores de las areniscas “U” y “T” decrecen, desde el este hacia el oeste. Hacia el Oeste las areniscas son de grano fino bien seleccionadas y más glauconíticas comparadas con las areniscas del este. Los análisis petrológicos y geoquímicos fueron realizados a 54 testigos de perforación, 28 correspondiente a la Arenisca “U” y los restantes para la Arenisca “T”. La composición modal media para la Arenisca “U” es: Q94.9 F2.6 FR2.5 y para la Arenisca “T” es Q97.6 F1.6 FR0.8. Los datos obtenidos sugieren que los sedimentos proceden del escudo Guyana (gneises y granitos), y de las antiguas formaciones de la cuenca (Jurásico- Cretácico Inferior). La ilita, clorita y caolinita son escasos y precipitaron en una primera etapa de eodiagénesis. Durante la mesodiagénesis la caolinita fue reemplazada por la dickita. Los clay rim de clorita e ilita han favorecido la preservación de la porosidad primaria retardando el sobrecrecimiento de cuarzo. La dickita ha producido microporosidad interlaminar secundaria. Los cementos de cuarzo son los procesos diagenéticos más importantes (promedio 7% a 23%) y son los que influyen principalmente en la pérdida de porosidad primaria. La medida de Th (temperatura de homogenización), indican que los Sobrecrecimientos de cuarzos empezaron a precipitar a partir de los 80ºC, teniendo su máximo desarrollo alrededor de 110-120ºC. Los cementos carbonáticos son escasos y muestran varias fases: siderita S1, calcita, dolomita-Fe, y siderita S2. Todos son post cementos de cuarzo y se presentan en todas las secuencias estratigráficas excepto siderita S1. Sin embargo, todos los cementos carbonáticos son más abundantes en las areniscas fluviales con influencia tidal. Los datos isotópicos indican un origen meteórico para la siderita S1. Los datos de la siderita S2 sugieren una relación con la descomposición de la materia orgánica que se encuentra en las lutitas intercaladas en las areniscas. El alto contenido en Fe y Mg en los cementos de dolomita-Fe/ankerita y los valores isotópicos menos negativos indican una precipitación en un amplio rango de temperatura. El índice de compactación (COPL= 13% para la arenisca “U” y 4% en “T”), y el índice de cementación (CEPL= 17% “U” y 23% para “T”) muestran claramente que la pérdida de porosidad se produce por cementación. Asimismo, la porosidad secundaria fue generada por la disolución de los cementos carbonáticos y feldespatos. La muestras de lutitas intercaladas en las areniscas reservorio, son ricas en materia orgánica de tipo II, III (continental a marino con influencia algal). Todos los análisis realizados en las muestras de lutitas indican reflectancias de vitrinita (<0.5%), sugiriendo que los reservorios no han entrado a ventana de hidrocarburo. Por otro lado, las muestras que fueron varias veces analizadas, logrando dejarlas libres de hidrocarburos, llegaron a dar valores de reflectancias de vitrinita aproximadas a 0,9%. Asimismo, los análisis realizados a los petróleos, en base al grado de isomerización de los esteranos, para conocer su madurez, arrojan valores aproximados de 0,9%, que son valores equivalentes a las reflectancias de la vitrinitas. Estos valores indican que las lutitas de la Fm Napo serían la roca madre de los hidrocarburos extraídos en las dos areniscas “U” y “T”. A pesar de todos éstos resultados cabe destacar que se debe hacer más análisis sobre las lutitas de la Fm. Napo, para corroborar los datos obtenidos en este trabajo. Los biomarcadores analizados en los petróleos de los diferentes pozos (hopanos, homohopanos, esteranos y terpanos) de las diferentes arenas reservorios, no presentan diferencias, lo que indica que los petróleos de las dos areniscas proceden de las misma área fuente. La secuencia de los procesos diagenéticos observada en las areniscas, ha sido interpretada según el modelo térmico de la cuenca, que determina que en la cuenca la generación temprana del petróleo comenzaría entre 7 a 16 Ma, Mioceno (Tortoniense – Serravaliense). [ABSTRACT] The Oriente Basin is located in the eastern part of Ecuador. This foreland basin resulted from transpressives stress during the Upper Cretaceous, which produced the emersion of the Cordillera Real in the western part of the Oriente and the formation of the back arc basin. The “U” and “T” sandstones from the Napo formation of the Cretaceous are the focus of this research. The Napo formation was deposited in periods of marine transgression (HST), detrital sediments came from the western part of the Oriente basin. However, some intervals of the “U” and “T” sandstones were deposited during regressive periods (LST). The thickness of the “U” and “T” sandstones decrease from east to west. Toward the west the sandstones are fine grained well selected and richer in glauconite than the east. Petrological and geochemical determinations were performed from 54 drilling cores of which 28 were in the “U” sandstones and the rest in the “T”. The modal composition average for the “U” sandstone is Q94.9 F2.6 FR2.5 and for the “T” sandstone is Q97.6 F1.6 FR0.8. The obtained data suggest the Guyana Shield (gneisses and granites) and less importantly, the old detrital formations in the basin (Upper Jurassic–lower Cretaceous) as source areas. Illite, chlorite and kaolinite are scarce and precipitated in the early diagenetic stage. Lately, during the mesodiagenesis, kaolinite was replaced by dickite. Illite and chlorite have favoured the preservation of primary porosity delaying the quartz overgrowths. Dickite has produced interlaminar secondary microporosity. Quartz overgrowths are the most important diagenetic process (ranging from 7% to 23% ) and having the major influence on the loss of primary porosity. The measured Th temperatures indicate that they began to precipitate around 80ºC and they had their maximum development around 110-120ºC. The carbonate cements are scarce and show several phases: siderite S1, calcite, dolomite-Fe, and siderite S2. All are postquartz cement and appear in all stratigraphic sequences except siderite S1. However, all carbonate cements are more abundant in fluvial sandstones with tidal influence. The isotopic data indicate a meteoric origin for S1. For S2 siderite, the data suggest a relationship with organic matter descarboxilation in the interbedded shales. The high content in Fe and Mg in the dolomite-Fe/ankerite and the less negative isotopic values indicate precipitation during a wide range of temperatures. Compaction index (COPL= 13% for “U” and 4% in “T”), and cementation index (CEPL= 17% in “U” and 23% “T”) show clearly the control of the cementation over compaction in the loss of porosity. Likewise, secondary porosity (ranging from 5% -11% “U” and 1% - 7% “T”) was generated by dissolution of the carbonate cements and feldspars. The interbedded shales are rich in organic matter type II and III (continental and marine with algal influence). Whole analyses of the R0 (<0.5%) indicate that the reservoir did not enter into the hydrocarbon window. However, samples free of hydrocarbon contamination measured R0 at nearly 0,9%. Likewise, the degree of isomerization of the steranes measured on the oil samples gave values of 0,9%, equivalents to the obtained R0. These data suggest that the interbedded shales in the Napo Formation are the source area of the hydrocarbon in the two reservoirs. The biomarkers analysed in the oils (hopanes, homohopanes, steranes and terpanes) did not display differences, suggesting the same provenance for oils in both reservoirs. The sequence for the diagenetic processes occurred in the reservoirs sandstones has been interpreted according to the thermal modelling for the basin, which suggests that the early generation of oil began between 7 and 16 Ma, Miocene (Tortonian - Serravalian)

Efecto de la diagénesis sobre las propiedades físicas del almacén de petróleo de la Formación Napo del Cretácico Inferior de la Cuenca Oriente, Ecuador

The Napo Formation of the Cenomaniense age is an importan reservoir-sandstone in the Oriente basin of Ecuador. It displays higher porosity values (average 2 1%) compared to other reservoir in the region. The sandstone are fine to coarse grained and moderately to well sorted quartz arenites (average Qt932 F6.1 L0.7). Quartz cement is the dominant porosity reducing agent in this reservoir but o ther factors such as the glaucon ite, other clay in traclastos, grain-coating chlorite, pyrite, early siderite, mesodiagenetic Fedolomite-ankerite, kaolin, and pyrrhotite also playa critical role. Intergranular volume IGV vs cement percentage, indicates that cementation is the predominant contributor to porosity loss in all the reservoir. Some intervals shows secondary porosity but it is of minore effect on the enhancement of porosity

Oil geochemical characterization in the Cretaceous Napo Formation reservoir (Ecuador) and its relationship with the interbedded shales as hydrocarbon source rock

The Napo Formation from the Cretaceous age is located in the Oriente basin in Ecuador and is composed of limestones, shales, and sandstones, the «U» and «T sandstones», which are the most important reservoirs in the basin. The formation was deposited in fluvial, transitional and marine environments, and their correlation with the sequence stratigraphy may be associated with the fluctuation of the eustatic sea level. This work presents the analyses of the organic composition and the thermal maturity of the shales interbedded in the sandstones reservoir. Furthermore, the geochemical characterization of oils from the «U» and «T» reservoirs was preformed. The analyses were performed to establish the contribution of the shale as a source rock in the generation of the hydrocarbons. The spatial trend of the TOC in the shales varies from 1,78% in the Eden-10 well, located in the eastern part of the basin to 0,56% in Jivino-1 well located in the center of the basin. The hydrogen index (IH) varies from 116 to 414 mg/g, and the thermal maturity expressed as Tmax shows values of 445ºC. Oil studies show the relationship between tricyclic terpanes (C20/C23) and trisnorhopane/trisnoneohopane (Ts/Tm) among others biomarkers. These data suggest a marine origin and the thermal maturity is compatible with foundin thats the shale shows (reflectance equivalent of 0,6% - 0,9%, deduced from the analyses of the regular steranes isomerization). The interpretation of all the analyses suggests that the interbedded shales in the «U» and «T» sandstones could have contributed to the source of the hydrocarbon oil in the Oriente basi

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

Evidence for surface uplift of the Atlas Mountains and the surrounding peripheral plateaux: Combining apatite fission-track results and geomorphic indicators in the Western Moroccan Meseta (coastal Variscan Paleozoic basement)

This work represents an initial attempt to link the evolution of the topography in relation to the general tectonic framework of western Morocco. For this purpose, in a section of the Western Moroccan Meseta different tools are combined in order to attain the general objective. Apatite fission-track (AFT) data of granitic rocks of the Rabat–Khenifra area give ages around 200 Ma with track length distributions which are compatible with the thermal models already established for the area. An inverse correlation between AFT ages and elevation is observed which is compatible with previous models indicating northward tilting of the whole Western Moroccan Meseta which is younger than 20–25 Ma. In order to test this possibility a detailed analysis of the topography at different scales in the estern Moroccan Meseta has been performed. Results indicate that two open folds with different amplitudes are recognized and that the one with wider wavelength could correspond to a lithospheric fold as previously stated by other authors on the basis of independent geological arguments. The northward tilting proposed based on the AFT data agrees with the results obtained in the analysis of the topography which reinforces the presence of a very open fold with a wavelength of 200–300 km in the north-western limb of the Western Moroccan Meseta.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEpu