Fe-Chlorite Diagenesis Though Transport Reaction Modeling

Abstract

Fe-chlorite is a well-known diagenetic mineral present in sandstone and has the property of hindering burial diagenesis, in particular quartz overgrowths precipitation. The effect is to preserve porosity at depth and thus to deepen the economic basement of a given sandstone play. Although deeply studied, a debate is still ongoing regarding the initial conditions of formation and the subsequent development of Fe-chlorite coatings during burial. Fe-chlorite forms through two main mechanisms. The first one relates to the presence in the sediments of volcanic clasts rich in iron and magnesium. They easily alter during burial and release the ions in the sediments, where they precipitate as Fe-chlorite. The other mechanism, object of this abstract, is the early formation of Fe-chlorite, or of a supposed precursor, in depositional environments where fluvial and seawaters mix (estuarine-tidal). This way of forming is not yet well determined in modern environments, even though studies carried out in estuaries demonstrated the presence of some chlorite forming due to various mechanisms as well as its ability to stick to the grains during transport. Moreover, a debate is still ongoing on the possibility of direct Fe-chlorite precipitation and on the growth-limiting factor during diagenesis. A transport-reaction approach demonstrated to be useful in trying to answer partially these issues. The activity comprised three steps. Screening on the ability of tidal cycles to accumulates Fe-rich precursor in the sediments due to repeated mixing of fluvial and seawaters Evolution of the precursor during the very early diagenesis, up to 40 °C and few hundred meters of burial, with 0D batch models; Fe-chlorite forms under these conditions in sizeable amounts Evolution of the sediment during a first fast burial phase and a second slow burial phase, up to 110 °C and 4000 m of burial over 70 My with 1D compaction models; Fe-chlorite continues to form provided the presence of suitable minerals in the sandstone and adjacent shales The burial and thermal history followed was real and related to a chlorite-bearing sandstone reservoir. The activity included the incorporation in the thermodynamic database of the code of the parameters related to reservoir Fe-chlorites. The approach demonstrated the possibility of forming Fe-chlorite very early during diagenesis of a Fe-rich precursor and to further growth chlorite during burial, the limiting factor being the availability of iron.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018