Two-dimensional gravity modelling (Talwani et al., 1959) of the observed free-air gravity data along deep seismic line PC/9074-86 (which is running through well PakCan-1 in the NE/SW direction in Indus offshore basin) predicted different depth models by assuming different background regional anomalies e.g., (0, 0), (40, -0.4762) and (60, -0.7143). However, the model produced in Figure 2 (right half) with background anomaly value (40, -0.4762) shows a best match between the observed and calculated anomalies, except for minor residuals, and is preferred to others. These residuals could be attributed to the overall 2D assumption of the earth model.
As discussed above that observed free-air gravity shows generally a linear negative gravity field westward of the present coastline. To the southwest of well PakCan-1 the area is characterised by a steep positive gravity above the thick sedimentary strata (greater than 10 Km) with an approximate gradient of 1.8 mgal Km-1. It starts rising from Km mark 22 steeply to +40 mgal (right half of Figure 2). Comparison of bathymetry with the gravity field further to the present shelf break and beyond suggests the true scale speculative depth model of the line PC/9074-86 (left half of Figure 2). The model from 0 to -50 Km mark is projected from the right half using same depths for each layer except for the seabed configuration which is plotted using the true values taken from the bathymetric map. It appears from this model that the positive gravity gradient close to the break could be partly attributed to the changing upper mantle and water depths (edge effect - Worzel, 1965; Walcott, 1972; and Naveed, 1986 and 1987). This Edge Effect Anomaly is characterised by three parts: (1) The initial edge effect anomaly (2) The positive anomaly to load and (3) The broad negative anomaly to compensation for that load partly produced by the density contrast between basement and sediments and partly by the displacement of mantle material.
Beneath the mantle rise, the crustal thickness which is approximately 24.5 Km towards northeast is reduced to 6.5 Km, and it may be the area of the transitional crust types. The deep seismic line (15 sec twt) does not show typical lower crustal reflections in the extensional regime of the Indus Offshore, which is possibly due to extra-thick sedimentary sequence above the acoustic basement, and weak energy source used in the survey. Therefore, the predicted gravity anomaly between 0 to -50 Km mark which is due to the typical shelf edge and shallowing of mantle boundary explains as to why gravity starts to become strongly positive from Km mark 22 south-west. Clearly, more gravity data beyond the present shelf break is required to strengthen the argument. Furthermore, the isostatic anomaly calculation in this region would show as to how much gravity is due to the edge effect. Near elimination of free-air gravity i.e. zero isostatic anomaly would mean the positive gravity is simply due to edge effect.
The crustal model also shows the magnitude of the crustal thinning as one moves towards the present shelf break region (south-west). The whole crustal attenuation can be measured with respect to its initial thickness (30 Km). Maximum attenuation has been observed beneath Km marks 12 to 26 (Figure 2) and this area appears to be the indication of the transitional crustal regions.
The magnetic profiles seem to support the gentle north-eastward rise in the basement. The two relatively short wavelength magnetic anomalies located above Km marks 60 and 75 appear to be due to supra-basement features. It is difficult to appreciate any feature related to this on the seismic line owing to the presence of long period multiples. The depth to basement analysis using magnetic profile (Naveed, 1987) suggests that the acoustic basement reflector (possible Base Tertiary) is close to the deduced depths of basement surface.
Bombay High (a paleo-high) and Cambay graben are two major oil producing regions of India and are characterised by high geothermal gradients attributed to the shallowness of the mantle as inferred from seismic refraction experiments. To the north of the study area, the onshore Badin Block (Pakistan) produces oil and gas on a commercial scale from the Cretaceous Lower Goru deltaic sandstones. Also, in the study area steep positive gravity gradient above the thick sedimentary strata (greater than 10 Km) towards southwest of the area is attributed to prominent rise in the mantle. Therefore, it is interpreted that the younger sediments in the offshore area may have attained maturity, due to thinning of the crust and consequent rise of mantle in the outermost shelf regions, to the stage of producing hydrocarbons.
Figure 2. Depth model and total gravity effects
Speculative true scale depth model (bottom) and total gravity effects (top), by assuming 40 mgal (40, -0.4762) regional background anomaly, along the PC/9074-86 (Indus Offshore Basin). The model from 0 to –50 km mark is projected from the right half except for the seabed configuration, which is plotted using true values. Note the predicted gravity anomaly between 0 to –50 km mark, which is due to the typical shelf edge, and shallowing of mantle boundary (Mohorovicic discontinuity). This explains as to why gravity starts to become strongly positive from km mark 22 southward.