In situ determination of total carbon and evaluation of source rock

Abstract

A method for determining in situ the carbon content of a source rock comprises determining the carbon/oxygen elemental ratio of the formation via inelastic gamma spectroscopy, determining the porosity of the formation, obtaining the oxygen contents and densities of the fluid and minerals in said formation, and determining the carbon content from said carbon/oxygen ratio, and said oxygen contents and densities of the fluid and minerals. The density of the fluid may be assumed, as may the oxygen content of both the formation fluid and minerals. The mineral density may also be assigned a constant value based on lithology. An alternative total carbon content determination is made by additionally determining the fractional volume, density, and oxygen content of the organic matter in the formation, and the combined mineral-organic matter density, and combining this additional information with the previously-listed determinations. The oxygen content of the organic matter may be assumed and the density of the organic matter may be assigned a constant value. The fractional volume of organic matter is calculated by using the previously determined carbon content multiplied by a weight to volume conversion factor. The combined mineral-organic matter density is determined from the densities and volumes of the organic matter and minerals. The organic matter volume may be combined with the other information to provide the alternative total carbon content determination. By eliminating the carbon contributed to the total carbon determination by formation carbonates, total organic carbon may be determined and source rocks identified and evaluated.