LONDON, Jan 3 With crude prices at 11-year lows, the world's biggest oil and gas producers are facing their longest period of investment cuts in decades, but are expected to borrow more to preserve the dividends demanded by investors. At around $37 a barrel, crude prices are well below the $60 firms such as Total, Statoil and BP need to balance their books, a level that has already been sharply reduced over the past 18 months.