The BSE benchmark Sensex today fell by over 75 points in opening trade . The 30-share index, which had gained 48.61 point on Thursday, fell by 75.07 points, or 0.40 per cent, to 18,684, with stocks of oil and gas, IT and banking sectors succumbing to profit-booking.Similarly, the wide-based National Stock Exchange Nifty Index moved down by 23.20 points, or 0.40 per cent, to 5,682.