The influence of transgression and regression on the spatial and temporal distribution of diagenetic kaolin in the Upper Ordovician glaciogenic sandstones within a sequence stratigraphic framework,Murzuq Basin,SW Libya

摘要

Linking diagenesis to depositional facies and sequence stratigraphy can provide a better understanding of some of the parameters that control the spatial and temporal distribution of diagenetic alterations and of their impact on reservoir quality.A study of the paralic,glaciogenic sandstones of the Melaz Shuqran and Mamunyiat formations(Late Ordovician)of the Murzuq Basin,SW Libya,reveals that the distribution of diagenetic kaolin can be constrained within depositional facies and sequence stratigraphy.Eogenetic kaolinite was formed by the dissolution of unstable detrital grains as a result of influx of meteoric waters into:(i)glacial,fluvial incised-valley lowstand systems tract(LST),glacial,tide-dominated estuarine transgressive systems tract(TST)during formation of overlying sequence boundary,(ii)paraglacial,tide-dominated deltaic highstand systems tract(HST),paraglacial,foreshore to shoreface HST during progradation and basinward shift of the shoreline and/or formation of overlying sequence boundary,(iii)postglacial,Gilbert-type deltaic LST sandstones during relative sea level fall.On the other hand,formation of kaolinite immediately below maximum flooding surfaces is attributed to dissolution of unstable detrital grains by organic acids that were presumably derived from thermal alterations of organic matter,possibly during mesodiagenesis.The transformation of eogenetic kaolinite into dickite during mesodiagenesis is probably a consequence of low alpha_K~+/alpha_H~+ ratio in the pore waters due to the scarcity of detrital K-feldspars.