Dissolution of ooids in seawater-derived fluids - an example from Lower Permian re-sedimented carbonates, West Texas, USA

Selective dissolution of metastable minerals and certain pore types, such as moldic and oomoldic pores, are widely accepted to indicate diagenesis by meteoric fluids in carbonate rocks. In this study, oomoldic-rich re-sedimented carbonates from the Happy Field in the Midland Basin, which contain no evidence of subaerial environments, indicate that such pores may form in other settings. Ooid dissolution in the reservoir was evaluated through petrography, scanning electron microscopy, stable isotopes (delta O-18 and delta C-13), elemental geochemistry, and X-ray diffraction of bioclasts, ooids and associated cements. Cements are primarily dogtooth and equant low-magnesium calcite, poikilotopic celestine is present, and late-stage low-magnesium calcite occurs in lesser amounts. Equant low magnesium calcite cements are interpreted to reflect the initial stages of pore filling, synchronous with aragonitic ooid dissolution, thus implying that oomoldic pores and equant cements formed in similar fluids. Although the prominent equant low magnesium calcite cement textures mimic those often associated with meteoric fluids, geochemistry suggests a marine origin. The equant cements have delta O-18 (Vienna Pee Dee Belemnite) ratios between -2.0 parts per thousand and -3.5 parts per thousand, and delta C-13 (Vienna Pee Dee Belemnite) ratios between 4 parts per thousand and 5 parts per thousand. Magnesium/calcium ratios show values, ranging from 5 to 142 mmol/mol (n = 223, average 23.2 and SD = 20.1). Strontium/calcium ratios present values from 0.3 to 2.6 mmol/mol (n = 223, average 0.71 mmol/mol, SD = 0.35). Geochemical data coincide with typical low-latitude Permian marine carbonate values and further signify diagenesis within the marine phreatic realm. To explain this process, the results indicate that the dissolution of aragonite ooids and precipitation of early cements occurred in environments ranging between the seafloor to shallow burial dominated by undersaturated marine fluids. Undersaturation was triggered by organic matter decomposition occurring at the seafloor and in the shallow subsurface immediately below. These findings have broader implications for marine and burial carbonate diagenesis as well as diagenesis of re-sedimented deposits.

Automated summary

The study focused on the re-sedimented carbonates from the Happy Field in the Midland Basin, indicating that moldic pores may form in different environments than previously thought. The research findings suggest that equant low magnesium calcite cements were formed concurrently with aragonitic ooid dissolution, and the geochemical data imply a marine origin despite similarities to textures associated with meteoric fluids.