Identifying hydrocarbon production zones

1. A method of determining a paleo-temperature of a hydrocarbon reservoir, the method comprising:

obtaining a core sample with carbonate veins from a well formed in the hydrocarbon reservoir;

obtaining formation water samples from the well;

analyzing mineralogy of carbonate in the carbonate veins;

determining an oxygen isotope ratio in the formation water samples from the well and an oxygen isotope ratio in the carbonates;

determining a formation paleo-temperature value based on the determined oxygen isotope ratio using a model that relates the formation paleo-temperature value and the oxygen isotope ratio;

based on the formation paleo-temperature, determining that the core sample was obtained from a sweet spot in the hydrocarbon reservoir; and

producing hydrocarbons from a portion of the hydrocarbon reservoir from which the core sample was obtained.

2. The method of claim 1 , wherein analyzing mineralogy of carbonate in the carbonate veins comprises determining a presence of dolomite or calcite in the core sample.

3. The method of claim 1 , wherein the carbonate is dolomite, and wherein determining the formation paleo-temperature value based on the measured oxygen isotope ratios using a model that relates the formation paleo-temperature value and 0 - isotopes comprises determining the formation paleo-temperature using the following equation:

10 3 lnα dolomite-water =3.14×10 6 T −2 −3.14,

wherein, “T” is formation paleo-temperature, and “α dolomite-water ” is the oxygen isotope fractionation between dolomite and water.

4. The method of claim 1 , wherein the carbonate is calcite, and wherein determining the formation paleo-temperature value based on the measured oxygen isotope ratios using a model that relates the formation paleo-temperature value and 0 - isotopes comprises determining the formation paleo-temperature using the following equation:

10 3 lnα calcite-water =2.789×10 6 T −2 −2.89,

wherein, “T” is formation paleo-temperature, and “α calcite-water ” is the oxygen isotope fractionation between calcite and water.

5. The method of claim 1 , further comprising measuring the oxygen isotope ratio of vein carbonate in the core sample using an isotope ratio mass spectrometer.

6. The method of claim 1 , further comprising determining that a location from which the core sample was obtained is not a gas sweet spot based on the formation paleo-temperature being less than 100° C. or greater than 350° C.

7. The method of claim 1 , wherein the hydrocarbon reservoir is an oil reservoir, and wherein the method further comprises determining that a location from which the core sample was obtained is an oil sweet spot based on the formation paleo-temperature being substantially 100° C.

8. The method of claim 1 , wherein the hydrocarbon reservoir is a gas reservoir, and wherein the method further comprises determining that a location from which the core sample was obtained is a gas sweet spot based on the formation paleo-temperature being between 150° C. and 250° C.

9. The method of claim 1 , wherein the core sample is a first core sample, the well is a first well formed at a first location in the hydrocarbon reservoir, the formation paleo-temperature value is a first formation paleo-temperature value, and wherein the method further comprises:

obtaining a plurality of core samples, each with one or more carbonate veins, from a respective plurality of wells formed at respective, different locations in the hydrocarbon reservoir, the plurality of core samples comprising the first core sample, the plurality of wells comprising the first well;

for each core sample, determining a respective formation paleo-temperature based on a respective oxygen isotope ratio in formation water samples obtained from each respective well and a respective oxygen isotope ratio in carbonates in carbonate veins in each core sample resulting in a plurality of formation paleo-temperature values; and

generating a three-dimensional formation paleo-temperature map of the hydrocarbon reservoir based on the plurality of formation paleo-temperature values and the formation paleo-temperature value.

10. The method of claim 9 , further comprising determining maturity of the hydrocarbon reservoir and sweet spots in the hydrocarbon reservoir for hydrocarbon production based on the formation paleo-temperature map.

11. The method of claim 1 , further comprising purging the well prior to obtaining the formation water sample.

12. The method of claim 11 , wherein purging the well comprises flowing water multiple times out of the well.

13. The method of claim 1 , further comprising extracting fluid inclusion from vein carbonate to obtaining the formation water sample.

14. The method of claim 13 , further comprising analyzing oxygen isotopes of the formation water.