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

Lester Williams, Brian E. Lock

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 highly 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 (sphaerosiderite) in fossil soils and sedimentary facies such as levees and overbank splays in fluvial settings. The process by which iron is concentrated in these environments has been debated; in the oxidized state, which is normal in the surface continental environment, iron has a very low solubility. 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. Seasonal flooding may then have carried the iron into overbank areas.

Recent papers by Ana Gunatilaka may be relevant to this issue. He has reported and illustrated 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. Gunatilaka 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.

We 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.

Spherulitic siderite (and dolomite) may have some exploration significance. In some cases (although not necessarily in all), the spherulites may represent contemporaneous geochemical anomalies. Additionally, since they are a component of soil profiles, they indicate a former exposure surface. If the host sediment is identified as marine or some other inappropriate depositional facies, the presence of spherulites would suggest an overlying unconformity within a few tens of feet.

Sphaerosiderites are relatively common in the Lower Tuscaloosa nonmarine strata, and examples are illustrated from the Hammon Oil and Refining Co. #1 Steptoe Heirs wildcat well, Amite County, Mississippi.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990