Exploring sub-basalt reservoirs using object-oriented imaging: A case study from Kutch–Saurashtra area, India

Abstract

The hydrocarbon strike has been reported above and below basalt (Mesozoic sequences) in shallow water of Kutch–Saurashtra basin of Indian west coast. The evaluation of entrapment conditions within Mesozoic (below basalt) is extremely problematic because of poor quality images, of available data which are not amenable for subsurface mapping. The precise sub-basalt imaging in Kutch–Saurashtra basin is crucial for successful exploration activities in Mesozoic. The challenges for imaging the streamer data are two folds: first, poor data quality, due to poor signal penetration through basalt section and second, masking of signals with multiples and converted waves (PSPP and PSSP). Modelling using Norsar software, reveals that PSSP waves (noise), generate from an offset of about 3000 m from basalt top. We followed a new imaging strategy for achieving good sub-basalt images by proper multiple attenuation, elimination of converted waves, and adequate noise cleaning in different domains with correct estimation of velocity and signal boosting, below basalt. The new images clearly exhibit the sub-basalt horizons like Base of Basalt, Bhuj Formation and Mundra Formation, etc., which are amenable for mapping and pre-stack inversion, a powerful tool for reservoir characterization. The acoustic impedance of gas pay, drastically drops to 8000 unit from background value or 14000 units, delineated lateral extension of pay sand in present study. The new imaging exhibits presence of Graben, half Graben and anticlinal structures formed during Mesozoic and some reverse faults which are key components to unravel the basin tectonics and to aid derive Mesozoic petroleum system.