Abstract

A systematic geophysical investigation is necessary to determine whether the Punjab Platform of Pakistan's proven Infracambrian petroleum system extends into the BikanerNagaur Basin of India. To support this, the National Geophysical Research Institute (NGRI) carried out gravity and magnetic surveys, and collected 1,027 stations spaced one kilometre apart along roads and tracks. The complete bouguer anomalies range from a minimum of -55 mGal to a maximum of -10 mGal, reflecting notable subsurface density variations. The residual anomaly map highlights lateral density heterogeneities within the shallow sedimentary formations. In the IGRF-corrected total magnetic intensity anomaly map, a variation of about 540 nT was observed, ranging from a low of -407 nT to a high of +133 nT. The circular magnetic anomalies are attributed to acidic and basic igneous intrusions, while the linear trends are interpreted as the result of structural features and variations in basement magnetization. The residual Bouguer anomaly and IGRF corrected total intensity magnetic anomaly data have been interpreted with the assumption that they reflect the buried features up to the Precambrian basement. The density of the Precambrian basement rocks (quartzite and granite-gneiss) is considered to be 2.8 g/cm3 (Farah et al., 1977; Ram Gopal, 2007). The 2.5D density model derived from residual gravity anomalies using constraints from seismic information indicates deepening of the sedimentary layers from East to West. In contrast, the shallow sedimentary layers are more or less horizontal. Further modelling of gravity and magnetic anomalies has revealed structural insights, undulations in the basement, and delineated key subsurface features relevant to hydrocarbon exploration. Maximum depth to basement is found to be about 8.5 km in the two delineated basement depressions, which shallows to about 1.5 km along the Delhi- Sargodha ridge.