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

Impact of diagenesis on the spatial and temporal distribution of reservoir quality in the Jurassic Arab D and C members, offshore Abu Dhabi oilfield, United Arab Emirates

This study is based on petrographic examination (optical, scanning electron microscope, cathodo-luminescence, backscattered electron imaging, and fluorescence) of 1, 350 thin sections as well as isotopic compositions of carbonates (172 carbon and oxygen and 118 strontium isotopes), microprobe analyses, and fluid inclusion microthermometry of cored Jurassic Arab D and C members from 16 wells in a field from offshore Abu Dhabi, United Arab Emirates. The formation was deposited in a ramp with barrier islands and distal slope setting. Petrographic, stable isotopic and fluid-inclusion analyses have unraveled the impact of diagenesis on reservoir quality of Arab D and C within the framework of depositional facies, sequence stratigraphy, and burial history. Diagenetic processes include cementation by grain rim cement and syntaxial calcite overgrowths, formation of moldic porosity by dissolution of allochems, dolomitization and dolomite cementation, cementation by gypsum and anhydrite, and stylolitization. Partial eogenetic calcite and dolomite cementation has prevented porosity loss in grainstones during burial diagenesis. Dolomitization and sulphate cementation of peritidal mud are suggested to have occurred in an evaporative sabkha setting, whereas dolomitization of subtidal packstones and grainstones was driven by seepage reflux of lagoon brines formed during major falls in relative sea level. Recrystallization of dolomite occurred by hot saline waters (Th 85-100\ub0C; and salinity 14-18 wt% NaCl). Anhydrite and gypsum cements (Th 95-105\ub0C; fluid salinity 16-20 wt% NaCl), were subjected to extensive dissolution, presumably caused by thermal sulfate reduction followed by a major phase of oil emplacement. The last cement recorded was a second phase of anhydrite and gypsum (Th 95-120\ub0C; 16-22 wt% NaCl), which fills fractures associated with faults

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

Diagenetic Evolution and Porosity Destruction of Turbiditic Hybrid Arenites and Siliciclastic Sandstones of Foreland Basins: Evidence from the Eocene Hecho Group, Pyrenees, Spain

International audienceThis study aims to unravel the impact of diagenetic alterations on porosity loss of foreland-basin turbiditic hybrid arenites and associated siliciclastic sandstones of the Eocene Hecho Group (south-central Pyrenees, Spain). In this succession, hybrid arenites and calclithites are extensively cemented by mesogenetic calcite cement (delta18O VPDB = –10.0 per thousand to –5.8per thousand ; Th, mode = 80° C; salinity mode = 18.8 wt% eq. NaCl), Fe-dolomite (delta18O VPDB = –8.5 per thousand to –6.3 per thousand ) and trace amounts of siderite. The extent of carbonate cementation is interpreted to be related to the amounts of extrabasinal and intrabasinal carbonate grains, which provided nuclei and sources for the precipitation and growth of carbonate cements. Other diagenetic alterations, such as pyrite and albitization, had no impact on reservoir quality. Scarce early diagenetic cements, coupled with abundant ductile carbonate and siliciclastic framework grains, have led to rapid porosity loss owing to compaction. Conversely, abundant quartz in the sandstones prevented rapid loss of porosity by mechanical compaction. Reservoir quality was affected by mesogenetic cementation by quartz overgrowths, calcite and dolomite intergranular pressure dissolution of quartz grains, and formation of fracture-filling calcite cement (delta 18O V-PDB values from –10.4 per thousand to –7.8 per thousand ; Th temperatures of circa 150° C), which are attributed to deep circulation of hot meteoric waters during extensional stages of tectonism. The results of this study illustrate that diagenetic evolution pathways of the arenites and sandstones are closely linked to the variation in detrital composition, particularly the proportion and types of extrabasinal noncarbonates, extrabasinal carbonates, and intrabasinal carbonate grains. These insights suggest that marine turbiditic hybrid arenites and calclithites of foreland basins are subjected to more rapid and extensive porosity loss owing to compaction and cementation than associated siliciclastic sandstones. Degradation of reservoir quality makes these hybrid arenites, calclithites, and sandstones suitable as tight gas reservoirs, but only if fracture porosity and permeability develop during tectonic deformation

Data for: METEORIC-WATER INCURSION INTOMARINE TURBDITIC SANDSTONES: EVIDENCE FROM THE ANDREWS FORMATION (PALEOCENE), UK CENTRAL GRABEN, NORTH SEA

This is a research paper and studies of petrographic, elemental and stable isotopic done on samples of the Paleocene deep-water marine turbidite sandstone reservoirs

Diagenesis of a limestone reservoir (Lower Cretaceous), Abu Dhabi, United Arab Emirates: Comparison between the anticline crest and flanks

Petrographic, stable-isotope and fluid-inclusion analyses were conducted on a Lower Cretaceous limestone reservoir, onshore Abu Dhabi, United Arab Emirates in order to compare the diagenetic processes and products in the oil zone (i.e. crest) versus water zone (i.e. flanks) of a giant oilfield anticline. The near-seafloor, shallow and intermediate burial (< 1 km) diagenetic processes across the anticline include micritization of allochems, mechanical compaction, cementation by calcite (rim, syntaxial overgrowths and equant spar) and rhombic dolomite, peloids dissolution, partial dolomitization, and incipient stylolitization. Diagenetic processes during tectonic compression of the foreland basin in Late Cretaceous and concomitant oil migration were mediated by basinal brines. These processes, which are more extensive in the flanks than the crest, include cementation by calcite, subordinate saddle dolomite, and minor dickite, fluorite, and sphalerite. Additional diagenetic processes subsequent to the main tectonic compression phase and oil migration and emplacement in the crest have occurred during deep burial (i.e. the flanks; present-day depth 2.5–3.5 km), and include extensive stylolitization and cementation by blocky calcite. These processes account for the poorer reservoir quality of limestones in the water zone in the flanks than the oil-saturated limestones in the crest. This paper demonstrates that variations in the role of diagenesis on distribution and evolution of reservoir quality across anticlinal structures of oilfields can be better understood in the light of: (i) the timing of generation, migration and emplacement/saturation of oil, and (ii) burial-tectonic evolution of the basin and related geochemical evolution of formation waters and flux of basinal fluids