Introduction
Summary Confident seismic imaging below high acoustic, sporadically distributed, considerably thicker & deep seated dolerite sill in parts of Mahanadi onshore basin viz. Cuttack depression in east coast of India, remained as a major challenge to Oil India Limited (OIL), A Govt. of India Undertakings, through out its decades long exploration venture for oil and gas in the region. To surmount the problem, OIL adopted an approach of integrated seismic, gravity and magnetic modelling with constrains from the available well information. The present paper discusses the constraints to the seismic data quality in the area, the method followed in imaging the sill & the basement configuration through 2D integrated seismic, gravity and magnetic modelling and the results that helped OIL in meeting the objectives.
Mahanadi Basin (Fig.1) developed in response to the extensional break-up of East Gondwana. The basin developed along a northeast-trending rift system that nucleated along the older Eastern Ghats mobile belt. The basin was subjected to volcanism, possibly in several phases, during Early Cretaceous period (Rajmahal eruption). The Cuttack depression is an onshore sub-basin of the Mahanadi Basin. It is bounded in north by basement rocks of Chandikhol ridge and in south by that of Bhubaneswar ridge. The Up. Jurassic to Lr. Cretaceous sediments (Athagarh Sandstone), similar to those of Krishna-Godavari and Cauvery basins outcrops over a small area near the western margin of the depression. Dolerite sills appear to intrude the Upper Jurassic to Lower Cretaceous sediments and are overlain by basalt, tuffs and intertrappean sediments. The basement rock in the Archaean -Proterozoic Eastern Ghats belt is exposed to the southwest, west and north of the depression. The depression is filled by Up. Jurassic - Lr Cretaceous sediments and to its south & east the basement as well as younger rocks are buried beneath Miocene - Recent sediments and alluvial plains of the Bay of Bengal. The depression was first identified from surface gravity data and was subsequently confirmed by Aeromagnetic, Seismic and DSS survey. Evidence from seismic data (1982-83 vintage: 12 fold), however, was ineffective in delineating the depression due to the inherent limitation of poor imaging below high acoustic volcanics. The only well drilled within the depression revealed two major acoustic barriers i.e. about 100 m of basalt at about 914 m and about 200 m of dolerite at about 1861 m. Subsequently, the area was revisited in the year 2003 with acquisition of 60-fold CDP seismic data. Processing / reprocessing of these data though could help in marginal improvement in quality but delineation of the basin configuration remained still inconclusive. Lack of continuity in sill signature, sporadic high amplitude events, crosscutting of reflector in seismic sections led to various ambiguities in achieving any meaningful inference. Although the regional geology suggested the possible presence of Permotriassic sediments (a potential source rock for hydrocarbon generation) below the dolorite sill but the acceptance of the same was not possible due to poor seismic imaging.