Evaluating a reservoir formation

1. A method for evaluating a volcanic reservoir formation having volcanic rock forming elements and a borehole penetrating the volcanic reservoir formation, comprising:

controlling chlorine concentration in an environment of the borehole and a tool operation, by at least maintaining salinity within the borehole to be less than a pre-determined level, to enable neutron capture spectroscopy measurements of at least aluminum, potassium, and sodium of the volcanic rock forming elements;

obtaining neutron capture spectroscopy data relating to the at least aluminum, potassium, and sodium of the volcanic rock forming elements from borehole logging tools;

determining a lithology of the volcanic reservoir formation based on at least the neutron capture spectroscopy data; and

generating an evaluation of the volcanic reservoir formation based on at least the lithology.

2. The method of claim 1 , wherein determining the lithology comprises:

determining absolute element concentrations of the volcanic rock forming elements from the neutron capture spectroscopy data using a custom oxides closure model; and

generating a total alkalis versus silica (TAS) classification of the volcanic reservoir formation based on the absolute element concentrations of the volcanic rock forming elements.

3. The method of claim 2 , wherein determining the lithology further comprises:

obtaining measured absolute element concentrations of the volcanic rock forming elements from core sample measurements;

calculating relative yields of the volcanic rock forming elements from the absolute element concentrations of the volcanic rock forming elements; and

generating the custom oxides closure model representing the measured absolute element concentrations of the volcanic rock forming elements as a function of the relative yields of the volcanic rock forming elements, wherein the custom oxides closure model comprises empirical coefficients unique to the volcanic reservoir formation.

4. The method of claim 2 , wherein determining the lithology further comprises:

generating a compositional description of the volcanic reservoir formation based on the TAS classification;

generating a textural description of the volcanic reservoir formation based on a borehole micro-resistivity image and a nuclear magnetic resonance (NMR) transversal relaxation time (T2) distribution; and

integrating the compositional description and the textural description using a neural network-based approach.

5. The method of claim 1 , further comprising displaying the evaluation.

6. The method of claim 1 , further comprising using the evaluation to obtain at least one selected from a group consisting of a fluid and a mineral associated with the volcanic reservoir formation.

7. A computer readable medium storing instructions for evaluating a volcanic reservoir formation having volcanic rock forming elements and a borehole penetrating the volcanic reservoir formation, the instructions comprising functionality for:

generating a custom matrix property transform unique to the volcanic reservoir formation, wherein the custom matrix property transform comprises relationships between matrix nuclear properties of the volcanic reservoir formation and absolute element concentrations of the volcanic rock forming elements;

determining porosity of the volcanic reservoir formation using at least the custom matrix property transform, comprising:

controlling chlorine concentration in an environment of the borehole, by at least maintaining salinity within the borehole to be less than a pre-determined level, to enable neutron capture spectroscopy measurements of at least aluminum, potassium, and sodium of the volcanic rock forming elements; and

generating an evaluation of the volcanic reservoir formation based on at least the porosity.

8. The computer readable medium of claim 7 , wherein generating the custom matrix property transform comprises:

obtaining grain density and absolute element concentrations from core sample measurements;

determining matrix nuclear properties from the grain density and the absolute element concentrations using nuclear parameter modeling; and

generating empirical equations representing the matrix nuclear properties as a function of the absolute element concentrations of the volcanic rock forming elements, wherein the empirical equations comprise empirical coefficients unique to the volcanic reservoir formation.

9. The computer readable medium of claim 7 , wherein determining porosity of the volcanic reservoir formation comprises:

obtaining neutron capture spectroscopy data relating to the volcanic rock forming elements from borehole logging tools;

determining absolute element concentrations of the volcanic rock forming elements from the neutron capture spectroscopy data;

calculating a matrix density from the absolute element concentrations of the volcanic rock forming elements using the custom matrix property transform; and

calculating the porosity using the matrix density, a bulk density, and a magnetic resonance porosity (MPR) of the volcanic reservoir formation.

10. The computer readable medium of claim 7 , wherein the instructions further comprise functionality for displaying the evaluation.

11. The computer readable medium of claim 7 , wherein the instructions further comprise functionality for using the evaluation to obtain at least one selected from a group consisting of a fluid and a mineral associated with the volcanic reservoir formation.

12. The computer readable medium of claim 7 , wherein the instructions further comprise functionality for

partitioning a pore volume of the volcanic reservoir formation into a plurality of fractions, wherein each of the plurality of fractions is associated with a pore class, the pore class being at least one selected from a group consisting of a microporous class, a mesoporous class, and a macroporous class;

estimating the permeability using at least one of the plurality of fractions, and

generating the evaluation of the volcanic reservoir formation further based on the permeability.

13. The computer readable medium of claim 12 ,

wherein at least one of the plurality of fractions is associated with the macroporous class, and

wherein the permeability is estimated based on substituting a siliciclastic rock parameter in a carbonate reservoir permeability equation.

14. The computer readable medium of claim 7 , wherein the instructions further comprise functionality for

determining an orientation of a fracture in the volcanic reservoir formation;

determining an aperture of the fracture; and

estimating the permeability further based on the orientation of the fracture, the aperture of the fracture, and matrix permeability of the volcanic reservoir formation.

15. The computer readable medium of claim 7 , wherein the instructions further comprise functionality for

obtaining a transform relating capillary pressure to a first nuclear magnetic resonance (NMR) transversal relaxation time (T2) distribution;

determining a capillary pressure curve using the transform and a second NMR T2 distribution;

determining a buoyancy pressure distribution associated with the volcanic reservoir formation;

determining the fluid saturation by applying the buoyancy pressure distribution to the capillary pressure curve; and

generating the evaluation of the volcanic reservoir formation further based on the fluid saturation.

16. The computer readable medium of claim 15 , wherein the transform is calibrated using a core mercury injection capillary pressure (MICP) curve.

17. The computer readable medium of claim 15 , wherein the second NMR T 2 distribution is associated with the volcanic reservoir formation and corrected based on a gas saturation associated with the volcanic reservoir formation.

18. A system for evaluating a volcanic reservoir formation having volcanic rock forming elements and a borehole penetrating the volcanic reservoir formation, comprising:

a surface unit for collecting neutron capture spectroscopy data relating to at least aluminum, potassium, and sodium of the volcanic rock forming elements from borehole logging tools, chlorine concentration in an environment of the borehole and a tool operation being controlled, by at least maintaining salinity within the borehole to be less than a pre-determined level, to enable neutron capture spectroscopy measurements of the at least aluminum, potassium, and sodium of the volcanic rock forming elements;

a modeling tool operatively linked to the surface unit, the modeling tool comprising functionality for generating an evaluation of the volcanic reservoir formation from the neutron capture spectroscopy data relating to the volcanic rock forming elements using a custom oxides closure model; and

a drilling system operatively linked to the surface unit for retrieving target fluid in the volcanic reservoir formation, the drilling system being selectively adjusted based on the evaluation.

19. The system of claim 18 , wherein the modeling tool further comprises functionality for:

determining absolute element concentrations of the volcanic rock forming elements from the neutron capture spectroscopy data using the custom oxides closure model; and

generating a total alkalis versus silica (TAS) classification as the evaluation of the volcanic reservoir formation based on the absolute element concentrations of the volcanic rock forming elements.

20. The system of claim 18 , wherein the modeling tool further comprises functionality for:

obtaining measured absolute element concentrations of the volcanic rock forming elements from core sample measurements;

calculating relative yields of the volcanic rock forming elements from the absolute element concentrations of the volcanic rock forming elements; and

generating the custom oxides closure model representing the measured absolute element concentrations of the volcanic rock forming elements as a function of the relative yields of the volcanic rock forming elements, wherein the custom oxides closure model comprises empirical coefficients unique to the volcanic reservoir formation.