Method for determining subsurface hydrocarbon fluid properties of reservoired hydrocarbons

1. A method for determining the fluid properties of subsurface reservoired hydrocarbons having a hydrocarbon seep, the method comprising the steps of:

a) locating a hydrocarbon seep at a seabed location where hydrocarbons are actively flowing out of the seabed;

b) collecting a sample of hydrocarbons from the hydrocarbon seep;

c) determining the fluid properties of subsurface reservoired hydrocarbons from the sample of hydrocarbons, the fluid properties selected from the group consisting of physical, transport and thermodynamic properties; and

d) determining whether the seep is at a steady state, indicative of reservoired hydrocarbons, by

(i) determining at least two temporally spaced compositions of the hydrocarbon seep, the at least temporally spaced compositions selected from the group consisting of isotopic compositions, molecular compositions, and combinations thereof,

(ii) determining a temporal variance between the at least two temporally spaced compositions; and

(iii) when the temporal variance falls with a predetermined temporal tolerance classifying the hydrocarbon seep as being indicative of the presence of reservoired hydrocarbons.

2. The method of claim 1 , wherein step c) comprises constructing a phase envelope from the sample of hydrocarbons.

3. The method of claim 1 , wherein the sample is isobaric.

4. The method of claim 1 , wherein gas and oil samples are independently collected, and the gas and oil samples are recombined based on gas and oil volumetric flow rates prior to determining the fluid properties.

5. The method of claim 1 , wherein the sample comprises oil, gas and combinations thereof.

6. The method of claim 1 , wherein the at least two compositions comprises at least two temporally spaced isotopic compositions selected from the group consisting of isotopologues of methane, ethane, propane, butane, carbon dioxide, hydrogen sulphide, and combinations thereof.

7. The method of claim 1 , wherein the sample is analysed to measure a fluid property selected from the group consisting of viscosity, gas:oil ratio, dew point, bubble point, molecular composition, isotopic composition, API gravity, density and combinations thereof.

8. The method of claim 1 , further comprising the step of determining a formation volume factor for oil.

9. The method of claim 1 , wherein step c) is determined in-situ, remotely or a combination thereof.