Abstract: Genesis of Siderite in the Upper Miocene, Offshore Sarawak: Constraints on Pore Fluid Chemistry and Diagenetic History

Hadi Abdul, Abd Rahman, Timothy Robin Astin

The cored sequence of the Upper Miocene reservoir of Baram field, offshore Sarawak, consists of cyclic deltaic-related quartz-rich sandstones and mudstones interpreted to have been deposited during storm events in shallow to midshelf water 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 four different sandstone types: laminated sandstone, massive sandstone, bioturbated sandstone, and in association with shell fragments. Whole-rock XRD gives estimates of 20 to 40% siderite in bioturbated sandstones and 10 to 25% for the others.

Petrographic analysis reveals that diagenetic siderite occurs in four different crystal morphologies: rhombic, "bundles," acicular, and cylindrical. The rhombic siderite, which commonly occurs in bioturbated sandstone, has the most adverse effect on the poroperm characteristics of the sandstones, reducing porosity to 10% and permeability to 2 md.

^dgr¹³C and ^dgr¹⁸O plots show groupings based on morphology. Bundled and acicular siderite show ranges of ^dgr¹³C_PDB of -15 to -25 and ^dgr¹⁸O_PDB of 0 to -1. The late Miocene seawater ^dgr¹⁸O estimate for the region is -0.8 PDB. This would give the bundles/acicular siderite a temperature of formation range of 26° to 30°C. The ^dgr¹⁸O values are compatible with precipitation at shallow burial depth from unaltered seawater; ^dgr¹³C values are inherited from sulfate reduction horizons.

Rhombic siderite has ranges of ^dgr¹³C_PDB of -5 to -15 and ^dgr¹⁸O_PDB of -3 to -4. The range of ^dgr¹³C 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 was 38° to 48°C at depths of about 390-690 m, assuming precipitation from unaltered seawater.

The cylindrical siderite shows isotope values between these two extremes.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994