| 1. |
- El-ghali, Mohamed Ali Kalefa, et al.
(författare)
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Distribution of diagenetic alterations in fluvial and paralic deposits within sequence stratigraphic framework : Evidence from the Petrohan Terrigenous Group and the Svidol Formation, Lower Triassic, NW Bulgaria
- 2006
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Ingår i: Sedimentary Geology. - : Elsevier BV. - 0037-0738 .- 1879-0968. ; 190:1-4, s. 299-321
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Tidskriftsartikel (refereegranskat)abstract
- Elucidation of diagenetic alterations in the Petrohan Terrigenous Group (fluvial; highstand systems tract HST) sandstones and Svidol Formation (tide-dominated deltaic and tidal flat, transgressive systems tract TST and highstand systems tract HST, respectively) sandstones and calcarenite, Lower Triassic, NW Bulgaria was constrained within a sequence stratigraphic framework. Eogenetic alterations in the fluvial HST sandstones include (i) formation of grain-coating infiltrated clays as a result of percolation of mud-rich surface waters into underlying coarse-grained and permeable channel-fills and crevasse splay sandstones; (ii) formation of pseudomatrix by mechanical compaction of mud intraclasts that were incorporated into the coarse-grained channel sandstones during their lateral avulsion; and (iii) cementation by calcite (delta O-18(VPDB)=-6.5 parts per thousand to -3 parts per thousand; delta C-13(VPDB) =-5.1 parts per thousand to +0.6 parts per thousand) and dolomite (delta O-18(VPDB)=-6.1 parts per thousand to -0.3 parts per thousand 0; 81 3 CVPDB=-7.2 parts per thousand to -5.8 parts per thousand) in the crevasse splay and floodplain sediments. Mesogenetic alterations that are encountered in the fluvial HST sandstones include (i) illitization of grain-coating clays, mud intraclasts, and mica, possibly because of simultaneous albitization of feldspars; (ii) cementation by calcite (delta O-18(VPDB) = - 14.5 parts per thousand to - 8.4 parts per thousand; delta C-13(VPDB)=-7.7 parts per thousand to +0.6 parts per thousand) and dolomite (delta O-18(VPDB)=- 15.8 parts per thousand to -5 parts per thousand; delta C-13(VPDB)= - 7.9 parts per thousand to + 1.5 parts per thousand); and (iii) limited amounts of quartz overgrowths in the channel sandstones owing to occurrence of thick grain-coating clays. Conversely, the tide-dominated deltaic TST sandstones and the tidal flat HST calcarenite were pervasively cemented by calcite (delta O-18(VPDB)=-6.6 parts per thousand to -3.1 parts per thousand; delta C-13(VPDB)=-5.1 parts per thousand to +0.6 parts per thousand) and siderite (delta O-18(VPDB)=-7.2 parts per thousand to -5.7 parts per thousand; delta C-13(VPDB) =+0.3 parts per thousand to +0.9 parts per thousand) particularly below marine and maximum flooding surfaces, due to the presence of abundant bioclasts and prolonged residence time of the sediments under certain geochemical conditions along these surfaces. The remaining open pores were cemented during mesodiagenesis by calcite (delta O-18(VPDB)=-6.6 parts per thousand to -3.1 parts per thousand and delta C-13(VPDB)=-5.1 parts per thousand to +0.6 parts per thousand) and dolomite (delta O-18(VPDB))13=-6.6 parts per thousand to -3.1 parts per thousand. and delta C-13(VPCB)=-5.1 parts per thousand to +0.6 parts per thousand). This study shows that constructing a conceptual model for the distribution of diagenetic alterations is possible by integration of diagenesis with sequence stratigraphy. The model shows that tide-dominated deltaic TST sandstones and tidal flat HST calcarenite were pervasively cemented by carbonates during near-surface codiagenesis, owing to the presence of abundant bioclasts. Conversely, fluvial LST sandstones remained poorly cemented during near-surface eodiagenesis due to the lack of bioclasts, but were cemented by mesogenetic calcite, dolomite and quartz overgrowths instead.
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| 2. |
- El-ghali, Mohamed Ali Kalefa, et al.
(författare)
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Distribution of diagenetic alterations in glaciogenic sandstones within a depositional facies and sequence stratigraphic framework : Evidence from the Upper Ordovician of the Murzuq Basin, SW Libya
- 2006
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Ingår i: Sedimentary Geology. - : Elsevier BV. - 0037-0738 .- 1879-0968. ; 190:1-4, s. 323-351
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Tidskriftsartikel (refereegranskat)abstract
- The spatial and temporal distribution of diagenetic alterations has been constrained in relationship to depositional facies and sequence stratigraphy of the Upper Ordovician glaciogenic quartzarenite sandstones in the Murzuq Basin, SW Libya, which were deposited during the Haritanian glaciation when the basin was laying along the continental margin of Gondwana. Eogenetic alterations encountered include: (i) replacement of detrital silicates, mud matrix and pseudomatrix by kaolinite in paraglacial, tidedominated deltaic, in foreshore to shoreface (highstand systems tract; HST) and in post-glacial, Gilbert-type deltaic (lowstand systems tract; LST) sandstones, particularly below the sequence boundaries (SB). Kaolinite fort-nation is attributed to the influx of meteoric water during relative sea level fall and basinward shift of the shoreline. (ii) Cementation by calcite (61 80VPD,3 = - 3.1 parts per thousand to + 1.1 parts per thousand and delta C-13(VPDB)=+ 1.7 parts per thousand to +3.5 parts per thousand) and Mg-rich siderite in the paraglacial, tide-dominated deltaic and foreshore to shoreface HST sandstones, in the glacial, tide-dominated estuarine (transgressive systems tract; TST) sandstones and in the postglacial, shoreface TST sandstones is interpreted to have occurred from marine pore-waters. (iii) Cementation by Mg-poor siderite, which occurs in the post-glacial, Gilbert-type deltaic LST sandstones and in the paraglacial, tide-dominated deltaic and foreshore to shoreface HST sandstones, is interpreted to have occurred from meteoric waters during relative sea level fall and basinward shift of the shoreline. (iv) Pervasive cementation by iron oxides has occurred in the glacial, shoreface-offshore TST sandstones and postglacial, shoreface TST sandstones immediately below the maximum flooding surfaces (NUS), which was presumably enhanced by prolonged residence time of the sediments under oxic diagenetic conditions at the seafloor. (v) Formation of grain-coating infiltrated clays mainly in the glacial, fluvial incised-valley LST sandstones and in the post-glacial, Gilbert-type deltaic LST sandstones as well as, less commonly, in the paraglacial, foreshore to shoreface HST sandstones and in the tide-dominated deltaic HST sandstones below the SBs.Mesogenetic alterations include mainly the formation of abundant quartz overgrowths in the glacial, fluvial incised-valley LST sandstones, post-glacial, Gilbert-type deltaic LST sandstones and glacial, shoreface TST sandstones, in which early carbonate cements are lacking. Illite, chlorite and albitized feldspars, which occur in small amounts, are most common in the glacial, tidedominated estuarine TST sandstones and paraglacial, shoreface HST sandstones. This study demonstrates that the spatial and temporal distribution of diagenetic alterations and their impact on reservoir-quality evolution in glacial, paraglacial and post-glacial sandstones can be better elucidated when linked to the depositional facies and sequence stratigraphic framework.
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| 3. |
- El-ghali, Mohamed Ali Kalefa, et al.
(författare)
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The influence of transgression and regression on the spatial and temporal distribution of diagenetic kaolin in the Upper Ordovician paralic glaciogenic sandstones within sequence stratigraphic framework, the Murzuq Basin, S W Libya
- 2006
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Ingår i: Journal of Geochemical Exploration. - : Elsevier BV. - 0375-6742 .- 1879-1689. ; 89:1-3, s. 87-91
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Tidskriftsartikel (refereegranskat)abstract
- Reservoir quality prediction can be considerably improved with an integrated approach between sequence stratigraphy and diagenesis. Here, we demonstrate that the occurrence of low-porosity sandstones (pseudomatrix-rich, i.e. matrix formed by mechanical compaction and squeezing of soft clasts in intergranular pores) deposited in shallow marine settings can be predicted in a sequence stratigraphic framework. These sandstones occur as lags bounding parasequences and, thus may form baffles and barriers for vertical fluid flow, and reservoir compartments between two sandstone bodies of adjacent parasequences. The prediction of their occurrence and precise spatial distribution is, therefore, very useful for production of shallow marine reservoirs.
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