WTI crude oil futures in portfolio diversification: the time-to-maturity effect

The aim of the paper is to analyze the diversification effect brought by crude oil Futures contracts, the most liquid commodity Futures, into a portfolio of stocks. The studies that have documented the very low- and essentially negative-correlations between commodities and equities typically rely on normally distributed returns, which is not the case for crude oil Futures and stocks indexes. Moreover, the particular time-to-maturity chosen for the Future contract used as an investment vehicle is an important matter that needs to be addressed, in presence of forward curves switching between backwardation and contango shapes. Our goal in this paper is twofold: (a) we introduce copula functions to have a better representation of the dependence structure of oil Futures with equity indexes; (b) using this copula representation, we are able to analyze in a precise manner the “maturity effect” in the choice of crude oil Future contract with respect to its diversification benefits. Our finding is that, in the case of distant maturities Futures, e.g., 18 months, the negative correlation effect is more pronounced whether stock prices increase or decrease. This property has the merit to avoid the hurdles of a frequent roll over while being quite desirable in the current trendless equity markets. Empirical evidence is exhibited on a database comprising the NYMEX WTI crude oil Futures and S&P 500 index over a 15 year-time period.