This paper investigates whether diagenetic alterations in sandstones and resulting changes in reservoir quality are influenced by depositional environments and sequence stratigraphy. The study focusses on the Cretaceous U and T sandstone members of the Napo Formation in the Oriente Basin of Ecuador. The sandstones were deposited in fluvial, transitional and marine environments, and comprise Lowstand (LST), Transgressive (TST) and Highstand Systems Tract (HST) deposits. The data were obtained by detailed petrographic observations supported by microprobe, stable isotope, and fluid inclusion analyses. The sandstones consist of fine‐ to medium‐grained quartzarenites and subarkoses. Diagenetic events include cementation by chlorite, early and late kaolinite/dickite, early and late carbonates (siderite, Fe‐dolomite/ankerite), and quartz. Early (eogenetic) processes included formation of chlorite grain coatings, kaolinite pore filling, and siderite (SI) cementation. Chlorite is absent in TST sandstones but was found frequently in LST‐HST sandstones. Early kaolinite is not present in LST sandstones but occurred frequently in LST‐HST sandstones. The distribution of mesogenetic cements relative to sequence stratigraphy is different in the U and T units. In the U sandstones, calcite is frequent in LST deposits and absent in the LST‐HST. Fe‐dolomite/ankerite is abundant only in the TST. S2 siderite is present in the TST and LST, but absent in the LST‐HST. Quartz cement and kaolinite/dickite are equally distributed in all systems tracts. In the T sandstones Fe‐dolomite/ankerite is only abundant in the TST, whilst calcite, quartz and dickite have similar distributions in all the systems tracts. The distribution of kaolinite cement is interpreted to be the result of relatively more intense meteoric‐water flux occurring during sea‐level fall, whereas chlorite cement may have formed through burial diagenetic transformation of precursor clays e.g. berthierine which was precipitated in mixed marine‐meteoric waters in tidal channel and estuarine environments. Chlorite cement in the T and U sandstones appears to have retarded development of quartz overgrowths, and 12–13% primary porosity is retained. The T sandstones (LST‐HST) contain up to 4% chlorite cement. Little evidence for chemical compaction was found with the exception of occasional concave‐convex grain contacts. Eogenetic siderite appears to have helped to preserve reservoir quality through supporting the sandstone framework against further compaction, but mesogenetic calcite has considerably reduced primary porosity. Eogenetic siderite (SI) was partly replaced by later carbonate cements such as late siderite (S2) and Fe‐dolomite. Although there appears to be a relationship in the Napo Formation between the occurrence of siderite SI and sequence stratigraphy, the relationship may change when original volumes of siderite are considered. There is likewise partial replacement of early kaolinite and recrystalization to dickite which masks the amount of original early kaolinite....
Origin and nature of the aluminium phosphate-sulfate minerals (APS) associated with uranium mineralization in triassic red-beds (Iberian Range, Spain) Origen y naturaleza de los minerales fosfato-sulfato alumínicos (APS) asociados a la mineralización de uranio en los red-beds triásicos (Cordillera Ibérica, España) R. Marfil 1 , A. La Iglesia 2 , J. Estupiñan 3 ABSTRACT This study focuses on the mineralogical and chemical study of an Aluminium-phosphate-sulphate (APS) mineralization that occurs in a clastic sequence from the Triassic (Buntsandstein) of the Iberian Range. The deposit is constituted by sandstones, mudstones, and conglomerates with arenaceous matrix, which were deposited in fluvial to shallow-marine environments. In addition to APS minerals, the following diagenetic minerals are present in the clastic sequence: quartz, K-feldspar, kaolinite group minerals, illite, Fe-oxides-hidroxides, carbonate-sulphate cement-replacements and secondary uraniferous minerals. APS minerals were identified and characterized by optical microscopy, X-ray diffraction, scanning electron microscopy, and electron microprobe. Microcrystalline APS crystals occur replacing uraniferous minerals, associated with kaolinite, mica and filling pores, in distal fluvial-to-tidal arkoses-subarkoses. Given their Ca, Sr, and Ba contents, the APS minerals can be defined as a solid solution of crandallite-goyacite-gorceixite (0.53 Ca, 0.46 Sr and 0.01 Ba). The chemical composition, low LREE concentration and Sr > S suggest that the APS mineral were originated during the supergene alteration of the Buntsandstein sandstones due to the presence of the mineralizing fluids which causes the development of Ubearing sandstones in a distal alteration area precipitating from partially dissolved and altered detrital minerals. Besides, the occurrence of dickite associated with APS minerals indicates they were precipitated at diagenetic temperatures (higher than 80ºC), related to the uplifting occurred during the late Cretaceous post-rift thermal stage.Keywords: APS minerals, U-bearing sandstones, Diagenesis, Triassic Buntsandstein, Iberian Range, Spain. RESUMENEste estudio se centra en el estudio mineralógico y químico de una mineralización de fosfato de aluminio y sulfato (APS) que se produce en una secuencia clástica del Triásico (Buntsandstein) de la Cordillera Ibérica. El depósito está constituido por areniscas, lutitas y conglomerados con matriz arenosa, que fueron depositados en ambientes de fluvial a marino somero. Además de los minerals APS, los minerales diagenéticos siguientes están presentes en la secuencia clástica: cuarzo, feldespato potásico, minerales del grupo de la caolinita, illita, óxidos-hidróxidos de Fe, cemento sulfatado-carbonatado de sustitución y minerales secundarios uraníferos. Los minerales APS fueron identificados y caracterizados por microscopía óptica, difracción de rayos X, microscopía electrónica de barrido, y microsonda electrónica. Los microcristales APS reemplazan a minerales uraníferos, asociados con caolinita, ...
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