Carbon and oxygen isotope ratios to identify source rocks
The disclosure relates to methods for identifying desirable (e.g., the best) depth intervals in a formation using carbon and/or oxygen isotope ratio information. The desirable depth intervals in the formation correspond to regions in the formation with source rocks and relatively high (e.g., the highest) total organic carbon (TOC).
1 . A method of determining a depth of source rocks within a formation, the method comprising:
measuring a carbon isotope ratio and an oxygen isotope ratio from samples taken from a formation at a plurality of depths within the formation;
generating a data set comprising: i) the measured carbon isotope ratio versus the depth of the formation; and ii) the measured oxygen isotope ratio versus the depth of the formation; and
using the data set to determine a depth of the source rocks within the formation;
wherein using the data set to determine the depth of the source rocks in the formation comprises:
identifying a positive shift in the measured carbon isotope ratio followed by a local maximum value in the measured carbon isotope ratio as the depth decreases,
wherein the local maximum value in the measured carbon isotope ratio as the depth decreases corresponds to the depth of the source rocks.
2 . The method of claim 1 , wherein a first negative shift in the measured carbon isotope ratio occurs after the local maximum in the measured carbon isotope ratio as the depth decreases.
3 . The method of claim 2 , wherein a second negative shift in the measured carbon isotope ratio occurs before the positive shift in the measured carbon isotope ratio as the depth decreases.
4 . The method of claim 1 , wherein using the data set to determine the depth of the source rocks within the formation further comprises:
identifying, as the depth within the formation decreases, a negative shift in the measured oxygen isotope ratio followed by a period of relatively consistent value in the measured oxygen isotope ratio,
wherein:
the negative shift in the measured oxygen isotope ratio occurs at a depth within the formation corresponding to the positive shift in the measured carbon isotope ratio as the depth decreases; and
the period of relatively consistent value in the measured oxygen isotope ratio occurs at a depth corresponding to the local maximum in the measured carbon isotope ratio as the depth decreases.
5 . The method of claim 4 , wherein:
a positive shift occurs after the period of relatively consistent value in the measured oxygen isotope ratio as the depth decreases; and
wherein the positive shift in the measured oxygen isotope ratio occurs at a depth correspond to the first negative shift in the measured carbon isotope ratio as the depth decreases.
6 . The method of claim 1 , wherein using the data set to determine the depth of the source rocks within the formation further comprises:
identifying, as the depth within the formation decreases, a negative shift in the measured oxygen isotope ratio followed by a period of relatively consistent value in the measured oxygen isotope ratio.
7 . The method of claim 6 , wherein:
a positive shift occurs after the period of relatively consistent value in the measured oxygen isotope ratio as the depth decreases.
8 . The method of claim 1 , wherein:
the method further comprises measuring a strontium isotope ratio of the samples; and
the data set further comprises the measured strontium isotope ratio versus depth of the formation.
9 . The method of claim 1 , wherein:
the method further comprises measuring redox sensitive element content of the samples; and
the data set further comprises the measured redox sensitive element content versus depth of the formation.
10 . The method of claim 1 , wherein:
the method further comprises measuring a carbonate content of the samples; and
the data set further comprises the measured carbonate content versus depth of the formation.
11 . The method of claim 1 , further comprising, producing a hydrocarbon from the depth determined using the data set.
12 . One or more machine-readable hardware storage devices comprising instructions that are executable by one or more processing devices to perform operations comprising the method of claim 1 .
13 . A system comprising:
one or more processing devices; and
one or more machine-readable hardware storage devices comprising instructions that are executable by the one or more processing devices to perform operations comprising the method of claim 1 .
14 . The method of claim 1 , wherein:
the method further comprises measuring a clastic content of the samples; and
the data set further comprises the measured clastic content versus depth of the formation.
15 . A method of determining a depth of source rocks within a formation, the method comprising:
measuring a carbon isotope ratio from samples taken at a plurality of depths from a formation;
generating a data set comprising the carbon isotope ratio versus depth; and
using the data set to determine a depth of the source rocks within the formation;
wherein using the data set to determine the depth of the source rocks within the formation comprises:
identifying a positive shift in the measured carbon isotope ratio followed by a local maximum value in the measured carbon isotope ratio as the depth decreases,
wherein the local maximum value in the measured carbon isotope ratio as the depth decreases corresponds to the depth of the source rocks.
16 . The method of claim 15 , wherein a negative shift in the measured carbon isotope ratio occurs after the local maximum in the measured carbon isotope ratio as the depth decreases.
17 . The method of claim 16 , wherein a negative shift in the measured carbon isotope ratio occurs before the positive shift in the measured carbon isotope ratio as the depth decreases.
18 . One or more machine-readable hardware storage devices comprising instructions that are executable by one or more processing devices to perform operations comprising the method of claim 15 .
19 . A system comprising:
one or more processing devices; and
one or more machine-readable hardware storage devices comprising instructions that are executable by the one or more processing devices to perform operations comprising the method of claim 15 .
20 . The method of claim 15 , wherein at least one of the following holds:
the method further comprises measuring a strontium isotope ratio of the samples and the data set further comprises the measured strontium isotope ratio versus depth of the formation;
the method further comprises measuring redox sensitive element content of the samples and the data set further comprises the measured redox sensitive element content versus depth of the formation;
the method further comprises measuring a carbonate content of the samples and the data set further comprises the measured carbonate content versus depth of the formation; or
the method further comprises measuring a clastic content of the samples and the data set further comprises the measured clastic content versus depth of the formation.