How Does an Underground Oil Pocket Affect Gravity Measurements?

[tavo0116]

Problem:
The center of a 1.30 km diameter spherical pocket of oil is 1.30 km beneath the Earth's surface.
Estimate by what percentage g directly above the pocket of oil would differ from the expected value of g for a uniform Earth? Assume the density of oil is 8.10x10^3 kg/m^3

Should I use the equation: F = G (m1 + m2) / r^2 ?
If not, which equation should I use?

Please help me to get started.

Thank you.

 

[Spinnor]

Does this seem right way to find g and g'?