Facies-related diagenesis and multiphase siderite cementation and dissolution in the reservoir sandstones of the Khatatba Formation, Egypt's Western Desert

In the fluvio-deltaic, quartz-arenitic sandstones of the Jurassic Khatatba Formation in tile Salam field (Egypt's Western Desert), diagenesis and sedi mentary facies control reservoir quality. Fluvial channel sandstones have t he highest porosities (10-15%) and permeabilities (100-600 md), in part bec ause of siderite cementation, which tl) inhibited compaction and quartz cem ent and (2) was later dissolved, creating intergranular secondary porosity (1/4 of total porosity). Fluvial crevasse-splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolin ma trh and pervasive, shallow-burial Fe-dolomite cement, respectively. Siderite precipitation was multiphase and separated by distinct dissolution events. The earliest siderite precipitated near surface, within suboxic tr opical coastal swamps containing predominantly meteoric waters. Some influe nce of marine waters is indicated by Local enrichments in Mg and Ca, The ne xt major siderite generation shows a trend to decreasing Mn and Ca contents , and is of shallow-burial origin. The last major siderite phase is Mg rich and interpreted as deeper-burial in origin. Some dissolution occurred duri ng shallow burial related to climatically controlled meteoric water fluxing under unconformities. The most important dissolution, however, occurred du ring deep burial, resulting in ii) a major corrosion surface predating the last Mg rich zone, (2) selective dissolution of some earlier zones, and (3) secondary porosity. This burial dissolution is interpreted to have been ca used by cooling of compactional waters expelled from the basin along major faults. Other diagenetic phases observed include early-diagenetic pyrite, kaolin, q uartz, bitumen, and late-diagenetic barite and illite. Kaolinite precipitat ed at shallow depths (< 300 m) related to climatically controlled meteoric hushing, which later recrystallized to bloch kaolinite and/or dickite with increasing burial temperatures. Quartz is the predominant cement (6.5-16 vo l %) in matrix-free sandstones. Fluid-inclusion data indicate that tile ear liest quartz precipitated at elevated temperatures (> 130 degreesC) during and/or after rapid Late Cretaceous burial, Quartz developed in two phases, separated by oil migration. This paper illustrates that, in fluvio-deltaic quartz arenites deposited un der the influence of humid tropical climate, reservoir quality can be large ly controlled by the contrasting pathways of carbonate diagenesis followed by the different sedimentary facies, This paper also documents a case in wh ich siderite dissolution generated significant secondary porosity in reserv oir sandstones, and where both siderite cementation and dissolution took pl ace in multiple phases during different diagenetic stages, including early, shallow burial, and deep-burial diagenesis. The present study also shows t hat, in multiphase siderite cements, the earlier growth zones can be select ively dissolved and replaced by later siderite zones (i.e., recrystallized) during burial diagenesis, These findings contrast with the general thought that siderite cements are not susceptible to generation of significant sec ondary porosity by dissolution and that earlier-formed siderites are essent ially stable during diagenesis.