Method for evaluation of hydrocarbon content of shale

Abstract

The invention relates to the evaluation of hydrocarbon gas or liquid deposits, or condensate, in a shale formation. From relatively few log inputs, together with assumed or estimated or known values for density or porosity of kerogen, a single mathematical process involving the solution of a number of simultaneous equations, provides a value for both kerogen volume and total porosity. Additional checks and balances may be used to provide corrections to the result, for example based on pyrite volume or water saturation.

1 . A method for evaluating the volume of hydrocarbon gas or liquid in a shale deposit, the method comprising combining known kerogen density and/or kerogen porosity values with log data in a mathematical analysis to derive directly values for kerogen volume, total porosity and water content. 
     
     
         2 . The method according to  claim 1  wherein said mathematical analysis comprises the solution of simultaneous equations incorporating said known kerogen density and/or kerogen porosity values and log data. 
     
     
         3 . The method according to  claim 1  wherein said log data includes:
 (i) Log measurements of bulk density (RHOB) 
 (ii) Log measurements indicative of porosity. 
 
     
     
         4 . The method according to  claim 3 , wherein said log measurements indicative of porosity comprise either or both of neutron log measurements and slowness measurements. 
     
     
         5 . The method according to  claim 2 , wherein said log data includes resistivity, providing an indication of water saturation. 
     
     
         6 . The method according to  claim 1 , wherein said mathematical analysis incorporates at least one known, assumed or estimated parameter from the group comprising: solid matrix grain density, matrix slowness, neutron response of solid matrix and slowness of solid kerogen. 
     
     
         7 . The method according to  claim 6  wherein all said known, assumed or estimated parameters are incorporated in said mathematical analysis. 
     
     
         8 . The method according to  claim 1  further comprising adjusting said derived value for kerogen volume to be consistent with a value for pyrite and/or marcasite volume based on an empirically derived relationship between kerogen volume and pyrite and/or marcasite volume. 
     
     
         9 . The method according to  claim 8  comprising an iterative process. 
     
     
         10 . The method according to  claim 8  wherein said value for pyrite and/or marcasite volume is obtained from an X ray diffraction analysis. 
     
     
         11 . The method according to  claim 6  wherein, if said derived value for kerogen volume is below a threshold value, then either:
 (a) an input value for matrix slowness is iteratively increased until said derived value for kerogen volume is above said threshold value; or 
 (b) if said log data includes neutron log measurements then, if said derived value for kerogen volume is below said threshold value, an input value for solid matrix grain density is iteratively increased until said derived value for porous matrix volume is above said threshold value. 
 
     
     
         12 . The method according to  claim 11  wherein, in terms of porous kerogen, said threshold value is between 0.01 and 0.05, preferably about 0.03. 
     
     
         13 . The method according to  claim 6  wherein a value for porosity of mineral matrix is derived from said mathematical analysis and wherein, if said derived value for porosity of mineral matrix is below a threshold value, then either:
 (a) an input value for matrix slowness is iteratively decreased until said derived value for porosity of mineral matrix is above said threshold value; or 
 (b) if said log data includes neutron log measurements then, if said derived value for porosity of mineral matrix is below said threshold value, then solid matrix neutron response is iteratively increased until said derived value for porosity of mineral matrix is above said threshold value. 
 
     
     
         14 . The method according to  claim 13  wherein said threshold value is between 0.01 and 0.05, preferably about 0.03. 
     
     
         15 . The method according to  claim 1  further comprising adjusting values derived from mathematical analysis so that they are consistent with one or more log data inputs selected from bulk density, sonic slowness and neutron data. 
     
     
         16 . The method according to  claim 6  wherein either (i) a value for solid matrix density RHO sm  and solid matrix neutron response NP sm  is derived from the difference between porosity derived from the RHOB log and the neutron log or alternatively (ii) a value for solid matrix density RHO sm  and solid matrix slowness DT sm  is derived from the difference between the RHOB porosity log and porosity derived from the slowness log, and said derived value for solid matrix neutron response NP sm  or said derived value for solid matrix slowness DT sm  is incorporated in said mathematical analysis. 
     
     
         17 . The method according to  claim 16  wherein a value Vclay ND  for clay volume is computed from said difference, RHO sm  and either NP sm  or DT sm  then being calculated from the Vclay ND  value.