Method for estimating the subsurface total organic carbon (TOC) from well-log data

摘要

The invention describes a procedure for determining the subsurface total organic content (TOC) from data obtained in the well by at least three well-logging tools measuring corresponding parameters. Three tools, namely sonic, density and deep resistivity are selected. The time interval signals from the sonic tool are converted to the P-wave velocity. The product of signals obtained from the sonic and density tools (P-wave velocity×Bulk density=Acoustic Impedance (AI)) responds in the same direction to a variation of the volume of water and organic matter (OM) volume of the rocks, whereas the third tool (Deep Resistivity) reacts very differently in response to a change of one or other of these same components, in a three-pole diagram, with rock matrix, OM and water as the three components onto an Acoustic Impedance vs resistivity ratio function plane. The resistivity ratio function is the square root of the ratio between the water resistivity and the measured formation resistivity. The position of the curved line with OM=0% by volume is fixed connecting the rock matrix pole with that of water pole. The slope of the matrix-water curve is controlled by the tortuosity factor ‘a’ that is a function of the rock pore structure, grain size and level of compaction. Iso-OM curves run parallel to this 0% OM reference curve. The data points to be analysed can be calibrated accordingly by changing the resistivity of water (Rw) and the tortuosity factor (a) parameters. In a graph where the parameters used depend, for example, on the sonic velocity in the rock, the rock bulk density and on the electric resistivity of the formations, the iso-OM lines form a set of parallel curved lines. The OM is derived from there corresponding to each pair of values of the parameters measured in the well. The obtained organic matter volume is converted to Total organic carbon (TOC) in gram percentage using a conventional relation.