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How can gravity surveys help identify geological structures that trap oil?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

Gravity surveys are a vital tool in petroleum exploration because they detect local density variations in the Earth's subsurface, which often indicate geological structures capable of trapping oil and gas.12

While the geoid represents an irregular equipotential surface influenced by global mass distribution, local gravity surveys focus on identifying specific anomalies caused by subsurface features. These surveys utilize a gravimeter to measure minute differences in gravitational acceleration.231

Geological structures are identified through these density contrasts:

Positive Anomalies: Uplifts that bring denser rock formations closer to the surface create areas of increased gravitational force.12
Negative Anomalies: Salt domes, which are less dense than surrounding sedimentary rocks, produce a decrease in gravitational pull.12

Because folds, faults, and salt domes frequently act as stratigraphic or structural traps for petroleum, mapping these gravity anomalies allows geologists to pinpoint promising locations for further exploration before committing to more expensive methods like seismic surveying.12