Deep Crustal Studies of Indus Offshore Basin (Pakistan) Using Seismic and Gravity Data
Abstract
Deep crustal studies of Indus Offshore basin, using gravity and deep seismic reflection data along the line 9074-86, prognose a speculative crustal picture beneath the seismic line. Steep positive gravity gradient towards south-west of the line, above thick sedimentary strata (greater than 10 Km) in the offshore depression area in the North East, is attributed to a prominent rise in the mantle. Intuitively, extending the depth model beyond the present day shelf break and computing its gravity indicates that the overall gravity effect is one of the typical shelf edge ("Edge Effect anomalies. The crustal thickness which is approximately 24.5 Km thick towards north-east is reduced to 6.5 Km on the mantle rise and it may be the area of transitional crust. The deep seismic line (15 sec twt) does not show typical lower crustal reflections as shown by COCORP (Consortium for Continental Reflection Profiling) in USA and BIRPS (British Institutions Reflection Profiling Syndicate) in UK. The absence of such reflections in the extensional regime of the Indus Offshore is possibly due to extra-thick sedimentary sequence above the acoustic basement and weak energy source used in the survey. Presence of thick sedimentary rocks of varying lithologies which commonly form source-reservoir-seal trilogies, optimum geothermal gradients, analogies •with other basins, oil/gas shows in and around the offshore area, number of structural traps including reefs and "Bright Spots" in sediments ranging in age from Cretaceous to Early Miocene show positive evidence to the presence of hydrocarbons. Furthermore, it is speculated that the younger sediments in the offshore area may have attained maturity, due to thinning of the crust as a consequence of mantle rise in the outermost shelf regions, and may produce hydrocarbons. Lack of data, absence of new technology and equipment, remote and difficult area, high risk and highly expensive zone, and formation pressure hazard may be the main reasons of non-discovery of this hidden.
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