Abstract
The South Atlantic Aptian “Pre-salt” shrubby carbonate Formations of Brazil and Angola are of major interest for the oil industry due to their potential hydrocarbon accumulations. Although the general sedimentology of these deposits is widely recognised to be within saline, alkaline lakes in rift volcanic settings, the specific genesis of shrubby carbonate morphologies remains unclear. This study reports the first petrographically comparable shrubby carbonates amongst other carbonate microfacies from an Anthropocene limestone formed under hyperalkaline (pH 9-12) and hypersaline (conductivity 425-3200µS) conditions at ambient temperature (12.5-13 oC) (Consett, UK). This discovery allows us to capitalise on exceptional long-term hydrochemical monitoring efforts from the site, demonstrating that shrubby carbonates occur uniquely within the waters richest in calcium (~240mg/L) and with highest pH (~12) and consequently with very high levels of supersaturation. However, the physical distribution of shrubs is more comparable with estimated local kinetic precipitation rate than it is to thermodynamic saturation, indicating that the fundamental control on shrub formation arises from crystal surface processes. The shrubby carbonate we report grows in the presence of significant diatomaceous and cyanobacterial biofilms, despite the highly alkaline conditions. These biofilms are lost from the deposited material early due to the high solubility of organic and silica within in hyperalkaline settings, and this loss contributes to very high intercrystalline porosity. Despite the presence of these microbes, few if any of the fabrics we report would be considered as “boundstones” despite it being clear that most fabrics are being deposited in the presence of abundant extra-cellular polymeric substances. We are aware of no previous petrographic work on anthropogenic carbonates of this type, and recommend further investigation to capitalise on what can be learned from these “accidental laboratories”.
Highlights
The genesis and evolution of Lower Cretaceous, non-marine carbonate hydrocarbon reservoirs in the South Atlantic, with their voluminous shrubby and spherulitic carbonate deposits in a volcanic and alkaline lacustrine setting, remains an enigma (Mercedes-Martín et al, 2016, 2017; Wright and Tosca, 2016)
Within non-geothermal systems driven by mineral hydrolysis, all CO2 is provided from atmosphere and the carbonate system tends to be limited by the relatively slow kinetics of CO2(g) dissolution and hydrolysis to CO23−(aq) rather than the abundance of calcium (Andrews et al, 1997; Rogerson et al, 2017)
A sedimentological, mineralogical and geochemical study was conducted in human-induced carbonate deposits (Consett) including analysis of microfacies, X-ray powder diffraction (XRD), Fourier transform infrared (FTIR) analysis and hydrogeochemistry to shed light on the physico-chemical processes forming the analogous PreSalt Aptian non-skeletal carbonates
Summary
The genesis and evolution of Lower Cretaceous, non-marine carbonate hydrocarbon reservoirs in the South Atlantic, with their voluminous shrubby and spherulitic carbonate deposits in a volcanic and alkaline lacustrine setting, remains an enigma (Mercedes-Martín et al, 2016, 2017; Wright and Tosca, 2016). Two main mechanisms for spherulite formation have been presented, with formation within a transient Mg–Si gel (Wright and Barnett, 2015; Wright and Tosca, 2016; Tosca et al, 2018) or as a result of organic acid binding to growing crystal surfaces (Chafetz and Butler, 1980; Tucker and Wright, 2009; Spadafora et al, 2010; Mercedes-Martín et al, 2016, 2017) Both mechanisms suggest a saline, alkaline environment, and high metal fluxes are required to emplace these large Ca- and Mg-dominated precipitate bodies. Within non-geothermal systems driven by mineral hydrolysis, all CO2 is provided from atmosphere and the carbonate system tends to be limited by the relatively slow kinetics of CO2(g) dissolution and hydrolysis to CO23−(aq) rather than the abundance of calcium (Andrews et al, 1997; Rogerson et al, 2017)
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