Spherulitic Pedogenic Carbonate Nodules from the Lower Tuscaloosa; A Tool to Be Used in Paleogeochemical Exploration for Hydrocarbons?: ABSTRACT

Abstract

Geochemical exploration for hydrocarbons consists primarily of sampling soils and surface sediments for anomalous concentrations of a variety of materials resulting from vertical migration of hydrocarbon-contaminated subsurface waters. In addition to traces of light hydrocarbons, the anomalous materials include gases such as radon and other contaminants from oil, as well as substances concentrated by the physico-chemical properties of the vertically migrating waters. Iron concentrations are high because of the reduced condition of these waters, calcium, magnesium, and bicarbonate ions because of the acidity of the waters. Many authors have commented on the association of spherulitic aggregates of siderite in fossil soils and sedimentary facies such as levees and overbank splays in fluvial settings. Researchers have suggested that iron may have been derived by erosion from areas of lateritic weathering and transported into the area as coatings on clay minerals or complexed with organic polymers. It has been reported and illustrated that abundant dolomite spherulites in surface profiles of outcropping sediments of a variety of ages and mineral compositions from Kuwait, all from areas of surface hydrocarbon seeps or over known subsurface accumulations. Many of the spherulites are nucleated around hydrocarbon droplets, and the significance of the distribution is clear. Itmore » is hypothesizes that the dolomite spherulites are precipitated by the action of bacteria feeding on hydrocarbons, an idea supported by the work of Lalou, which demonstrated that spheroids are common products of bacterial carbonate precipitation. The authors suggest that some, at least, spherosiderites may also be related to vertically migrating oilfield fluids. Iron, calcium, and magnesium may all be supplied by these waters, while the bacterial action apparently required for spherulitic morphologies to form in the pedologic setting may result from hydrocarbons in the same rising waters.« less