The present invention relates to the determination, in situ, of the carbon content of source rocks of a formation. More particularly, the invention relates to methods for using the carbon/oxygen ratios produced by well logging tools to identify formation source rocks and to determine the quantity of organic matter located in the source rocks.
In the exploration for and production of hydrocarbons, it is generally valuable to analyze the geological basin involved. In determining the hydrocarbon generation potential of an area, source rocks (i.e. any rock capable of producing hydrocarbons) must be identified, along with volume (axial extent and thickness) of the rock and the quantities of organic matter contained therein. Indeed, the presence or absence of a source rock can be the determining factor in whether a drill operator drills deeper or abandons a well. Also valuable is a determination of the type of organic matter contained in the source rock as well as a determination as to whether the organic matter has matured sufficiently to produce oil or gas. Finally, it is desirable to determine whether the oil or gas has migrated out of the source rock into a reservoir.
Currently, an in situ identification of source rock is rarely undertaken due to the uncertainty of the results. Some have tried to equate a decrease in shale densities, an increase in uranium, or an increase in gamma ray activities as being proportional to the total organic carbon content of the shale. See Schmoker, J.W., "Determination of Organic Matter Content of Appalachian Devonian Shales from Gamma-Ray Logs" AAPG Bulletin, Vol. 65 p. 1285-1298, (1981). Others have inferred the presence of source rock from high gamma ray activity, low density, low sonic velocity, high porosity, and high resistivity. See Meyer, B.L. et al., "Identification of Source Rocks on Wireline Logs by Density/Resistivity and Sonic Transit Time/Resistivity Crossplots" AAPG Bulletin, Vol. 68 p. 121-129, (1984). The relationships hypothesized in the above-mentioned publications, however, appear to be of limited applicability, such as for the detection of relatively rich source rocks, and are not widely used.
Because source rock identifications have rarely been successfully accomplished in situ, source rock evaluation continues to be an expensive and time consuming undertaking. Source rock evaluation requires mud cuttings, or sidewall or core cuttings and a full lab analysis of each sample. If source rock is not identified in advance, non-source rock cuttings cannot be screened, and a full analysis of each cutting must be undertaken.
It is therefore evident that a method of rapidly identifying source rocks and categorizing them in terms of their carbon content would be valuable. It would not only provide information from which to conduct a basin evaluation including a determination of total hydrocarbon yield, but it would eliminate the need to analyze cutting and sidewall samples from noncarbonaceous formations, thereby vastly reducing the amount of laboratory analysis conducted.