Lower Triassic oolites of the Nanpanjiang Basin, south China: Fades architecture, giant ooids, and diagenesis--Implications for hydrocarbon reservoirs.

Lower Triassic platforms in the Nanpanjiang Basin contain extensive oolites. Interior oolites are stacked in meter-scale cycles arranged into larger coarsening-upward sequences. Oolites thicken toward margins to include grainstones up to 50 m (164 ft) thick and contain giant ooids (up to 1 cm [0.4 in.]) and composite coated grains. Cross-bedding, ripples, and abraded ooids indicate deposition in high-energy shoals. Apparent layer-cake correlation across interiors indicates amalgamation of shoals. Thinner interior lenses represent spillover lobes. Ooids are interpreted to have originally been bimineralic with cortices of radial or micritic fabrics (high-magnesium calcite), alternating with coarse pseudospar or brickwork (originally aragonite). Distorted ooids formed by brittle compaction of micritic cortices around voids are interpreted to have been dissolved aragonite. Abundant potential nuclei indicate that limited supply was not a factor contributing to the large ooid size. High-energy and abnormally high-seawater CaCO₃ saturation are interpreted to be causes of the giant ooids. Most previous reports of high CaCCO₃ saturation caused by a minimal skeletal carbonate precipitation. We interpret similar seawater chemistry in the aftermath of the end-Permian extinction to explain the genesis of the giant ooids in the Early Triassic. The genesis of bimi-neralic ooids during an Early Triassic period of rapidly in-creasing pCO₂ and low SODue to image rights restrictions, multiple line equation(s) cannot be graphically displayed. indicates that an increasing Ca/Mg ratio was the primary mechanism driving the change from aragonite to calcite seas. The architecture, textures, and diagenesis of the Lower Triassic oolites of the Nanpanjiang Basin provide useful analogs for coeval reservoirs in Sichuan and the Middle East. [ABSTRACT FROM AUTHOR]