The major purpose of this study is to describe and quantify mineral reactions, reaction pathways, and mass transfer accompanying burial of passive margin sandstone-shale sequences, offshore Brazil. Four basins were investigated, encompassing a range of sandstone-shale compositions. Because these basins have similar geologic histories, the effect of original detrital mineralogy on diagenetic products could be ascertained. Standard light microscopy, X-ray diffraction, EM, SEM, and isotopic and chemical analyses provided the basis for interpretation of mineralogy, texture, and diagenetic reactions.

The initial mineral composition of the sediments was a major control of diagenetic products. Arkose and lithic arkose are the dominant sandstone types in these basins. Dioctahedral clay minerals, chlorite, and quartz characterize arkoses, whereas trioctahedral clays (saponite and corrensite) and zeolites are found in lithic sandstones. Dioctahedral smectite-rich shales exhibit the classical smectite/illite to illite burial pattern. However, mafic, trioctahedral clay-rich shales show a burial sequence of saponite to chlorite/saponite mixed-layer, a progressive increase of chlorite-rich phases with increasing burial depth.

The structural change of disordered to ordered interstratification of mixed-layer chlorite/saponite occurs in the temperature range of 60 to 80°C, and at vitrinite reflectance values around 0.7. Increasing substitution of silicon by aluminum in tetrahedral sites is the major chemical change accompanying transformation of saponite to chlorite via corrensite.

Material balance calculations indicate that sandstones lose less than 2% K⁺, which probably enters interstratified shales, and gain less than 3% H₂O, H⁺, and CO₂ during burial diagenesis. Therefore, the burial pathway of Brazilian sandstone

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diagenesis is nearly isochemical.

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