Detection of reservoirs from normalized full gradient of gravity anomalies and its application to Shengli oil field, east China

Abstract

We present how to detect reservoirs by the 3‐D normalized full gradient (NFG) of gravity anomalies constrained by seismic and drilling data. The normalized full gradient represents the full gradient of the gravity anomaly at a point divided by the average of the full gradient at the datum. The closed minima on an NFG map indicate the occurrence and horizontal locations of the centers of mass of density anomalies related to oil reservoirs. This information can be used to select well locations in an oil field. On a cross‐section, the closed minima can be used to estimate the depth to centers of mass of possible hydrocarbon reservoirs. Some characteristics of the NFG are calculated for a hypothetical case of an anticline saturated with oil and gas. The relationship of these characteristics to the geometry and physical parameters of the anticline is studied. Modeling studies show that the NFG largely depends on the number of terms in the Fourier series used to calculate it, and it closely related to the length of the gravity profile or the size of the study area. Applying the NFG method to the Shengli oil field, i.e., using closed minima of NFG alongside drilling data, showed an effectiveness of up to 70%. These results were used to select well locations with the result that some high‐production gas‐bearing strata were discovered by drilling.