Analisis pengaruh harga minyak, arus kas operasi, cadangan terbukti dan produksi minyak dan gas bumi terhadap kinerja perusahaan : studi empiris pada perusahaan minyak dan gas bumi periode 2007-2010 = Analysis of the effect of oil price, operating cash flow, oil and gas proved reserves and production to firm performance : an empirical studies on oil and gas company period 2007-2010