Want to know how much further oil prices might fall? Don’t ask Wall Street. Analysts at big banks have been scrambling to lower their oil-market forecasts after prices plunged to 12-year lows below $30 a barrel in January. As recently as November, banks were expecting the global Brent contract for oil to average $57 a barrel in 2016. Now, that number has fallen to $50 and could fall again. J.P. Morgan Chase & Co. reduced…