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首页> 外文期刊>Zeitschrift der Deutschen Gesellschaft fur Geowissenschaften: ZDGG >Diagenetic evolution of continental to shallow marine Mesozoic-Cenozoic deposits, Eastern Dahomey Basin, Nigeria: Implications for reservoir quality
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Diagenetic evolution of continental to shallow marine Mesozoic-Cenozoic deposits, Eastern Dahomey Basin, Nigeria: Implications for reservoir quality

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The Eastern Dahomey Basin is a continental to shallow marine hydrocarbon-bearing basin formed as a marginal pull-apart basin during the Late Mesozoic and Cenozoic. The heterogeneity of the sedimentary deposits filling the basin makes accurate location and prediction of reservoir rocks and their properties difficult. This difficulty is further complicated by the poor understanding of diagenetic processes and timing, and their influence on porosity. A proper understanding of the heterogeneity of diagenetic events promises a better assessment of the reservoir quality evolution. Sampling of outcrop ana-logues and petrographic analyses allows the identification of detrital and authigenic components, as well as pore types, and allows delineating reservoir quality controls. Outcrop and shallow well samples from the Cretaceous (Ise, Afowo, Cretaceous Benin Flank, Araromi formations) and Palaeogene (Ewekoro, Akinbo-Oshosun, Ilaro formations) were retrieved from the surface or a maximum depth of 35.4 m and generally exhibit poor to excellent optical porosity (2.7 to 39.7; avg.: 19.4; n = 66) for the siliciclastic rocks of the basin. Porosity distribution is largely controlled by the clay mineral matrix content and Fe-mineral cementation. The main pore types are intergranular and intragranular. The sampled near-surface siliciclastic sediments are within the early diage-netic realm where only iron oxide (FeOx) and iron oxyhydroxide (FeO(OH)) cementation, minor kaolinite and pyrite pre-cipitation, and feldspar and possible carbonate dissolution constitute the major diagenetic modifications. Furthermore, the occurrence of oil sands indicates that these shallow reservoir rocks have locally been charged. In comparison to shallow subsurface samples from the region, intense FeOx and FeO(OH) cementation is only observed in outcrops, while subsurface samples contain Fe-rich carbonate cements. Porosity reduction other than that associated with compaction (at greater depth) is predicted to be low unless carbonate cements are still preserved. Poor to fair optical porosity is present in the limestones, with values ranging from 0 to 6.3 (avg.: 2.6, n = 11). The wackestone and packstone samples have the highest optical porosity ranging from 0.3 to 6.3 (avg.: 4.2, n = 6). Sparry carbonate cements and lime mud in bioclastic wackestones and packstones reduced the available interparticle porosity (to 0). The cementstones only contain negligible optical porosity while sparstones retain some porosity in intercrystalline pores between dolomite rhombs (0 to 0.7).

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