Identifying oil and gas reservoirs with oxygen isotopes

1. A method comprising:

receiving a plurality of carbonate samples from a wellbore formed in a geologic formation;

determining an oxygen isotope ratio and carbon isotope ratio present within each of the plurality of carbonate samples;

determining a mineral composition of each of the plurality of carbonate samples;

mixing a solution comprising a red dye, potassium ferricyanaide, water, and hydrochloric acid;

placing thin slabs made from each of the plurality of carbonate samples in the solution at 25° C. for 1-2 minutes;

identifying one or more minerals within each of the thin slabs based on a degree of color change that occurs while each of the plurality of the thin slabs is in the solution;

creating a plot showing the determined oxygen isotope ratios versus a depth from where within the wellbore each of the plurality of carbonate samples was obtained;

identifying, based on the plot, one or more negative oxygen isotope shifts;

determining natural carbonate cement levels within one or more of the plurality of carbonate samples that correspond to the one or more negative oxygen isotope shifts identified in the plot; and

determining one or more production sweet spots based on the identified negative oxygen isotope shifts and the determined natural carbonate cement levels.

2. The method of claim 1 , wherein the oxygen isotope ratio and the carbon isotope ratio is determined with an isotope ratio mass spectrometer.

3. The method of claim 1 , wherein substantially no color change indicates a presence of dolomite within the thin slab.

4. The method of claim 1 , wherein determining the natural carbonate cement levels comprises visually checking porosity of one or more of the plurality of samples with a microscope.

5. The method of claim 1 , wherein the plurality of samples comprises one sample per every one-foot interval within the wellbore.

6. The method of claim 1 , further comprising pulverizing at least a portion of each of the plurality of samples into a powder.

7. The method of claim 6 , wherein the portion of each of the plurality of samples that is pulverized into powder produces substantially 300 micrograms of powder.

8. The method of claim 6 , wherein the powder does not include cement.

9. The method of claim 6 , further comprising preparing the powdered samples, the preparation for each sample comprising:

drying the powder at substantially 50° C. for substantially 8-12 hours;

transferring the dried powder to a reaction vial;

vacuum-sealing the reaction vial;

introducing H 3 PO 4 to the powdered sample;

producing CO 2 gas in response to introducing the H 3 PO 4 ; and

determining a carbon isotope ratio and an oxygen isotope ratio simultaneously in response to producing the CO 2 gas.

10. The method of claim 1 , further comprising planning wellbore completion operations at least partially based upon the one or more determined production sweet spots.

11. The method of claim 1 , further comprising:

comparing the determined oxygen isotope ratios and the determined carbon isotope ratios; and

verifying the determined oxygen isotope ratios based on the comparison.