Genesis of siderite in the Upper Miocene offshore Sarawak: constraints on pore fluid chemistry and diagenetic history

A. R. Abdul Hadi and Astin, T. R. (1995) Genesis of siderite in the Upper Miocene offshore Sarawak: constraints on pore fluid chemistry and diagenetic history. Bulletin of the Geological Society of Malaysia, 37. pp. 395-413. ISSN 2637-109X

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Abstract

The cored sequence of the upper Miocene reservoir of Baram field, offshore Sarawak, consists of cyclic quartz-rich sandstones and mudstones interpreted to have been deposited during storm events in shallow to midshelfwater depths. The sequence is intercalated with minor tidal intervals. Authigenic siderite is common in sandstones throughout the sequence. Siderite cemented zones are up to 2 m thick. The cement is found in five different sandstone types: laminated sandstone, massive sandstone, bioturbated sandstone, heterogeneous sandstone, and in association with mud intraclasts and shell fragments horizon. Whole-rock XRD gives estimate of 20 to 40% of siderite in bioturbated and heterogeneous sandstones and 10 to 25% for the others. Petrographic analysis reveals that diagenetic siderite occurs in four different crystal morphologies: rhombic, “bundle”, acicular and cylindrical. The rhombic siderite, which commonly occurs in bioturbated and heterogeneous sandstone, has the most adverse effect on the poroperm characteristics of the sandstones, reducing porosity to 10% and permeability to 2 md. δ¹³C and δ¹⁸O plots show groupings based on morphology. Bundled and acicular siderite show ranges of δ¹³CPDB of -15 to -25 and δ¹⁸OPDB of 0 to -1. The late Miocene seawater δ¹⁸O estimate for the region is -0.8 PDB. This would give the bundle and acicular siderite a temperature of formation range of 25° to 30°C. The δ¹⁸O values are compatible with precipitation at shallow burial depth from unaltered seawater; δ¹³C values are inherited from sulfate reduction horizons. Rhombic siderite has ranges of δ¹³CPDB of -5 to -15 and δ¹⁸OPDB of -3 to -4. The range of δ¹³C indicates that siderite diagenesis occurred within both the shallow sulfate reduction zone and at deeper levels within the zone of decarboxylation. The maximum temperature of formation here is 38° to 48°C at depths of about -450 to -800 m, assuming precipitation from unaltered seawater. These results are consistent with those of ICP geochemical values which indicate that the rhombic siderite are Mg-rich relative to the acicular and bundle types. The cylindrical siderite shows isotope values between these two extremes. These results reveal the pathways of early pore water evolution and diagenetic history of the Upper Miocene succession of Baram Field.

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