Looking at energy sector seasonality, the end of May represents what is, historically, the sector’s best stretch while June is usually unkind to oil stocks. In fact, June is the worst month of the year for the Energy Select Sector SPDR (NYSEArca: XLE), the largest equity-based energy ETF by assets.

Oil supply is abundant, OPEC is not doing anything about that and plenty of market observers are calling for near-term declines for crude prices. Those factors could bode well for refining equities and the currently small number of ETFs that offer robust refiners exposure, namely the PowerShares Dynamic Energy Exploration & Production Portfolio (NYSEArca: PXE) and the PowerShares DWA Energy Momentum Portfolio (NYSEArca: PXI). [Why These Energy ETFs Impress]

“This higher exposure to oil refining and marketing firms has helped boost the relative performance of PXI and PXE over the past three and five years,” according to a recent PowerShares research note.

PXI and PXE have refiners weight of 23.6% and 33.8%, respectively, according to PowerShares data. By comparison, XLE’s refiners exposure is just 9.4% while the iShares U.S. Energy ETF (NYSEArca: IYE) has a scant refiners allocation of 7.7%.

The higher refiners weights have led to higher returns for PXI and PXE over the trailing three- and five-year periods. Over the past three year, PXE and PXI are up 13.7% and 9.9% while XLE and IYE are higher by an average of 8.9%. Over the trailing five-year period, PXE and PXI are up an average of 13.1% while the average gain for XLE and IYE is 9.9%. [Why These Energy ETFs Impress]

“Refiners’ share prices have been rising because issuers’ margins, known as crack spreads, are also elevated. A crack spread is the difference between the cost of crude oil and the price charged for refined products, such as gasoline and distillate fuel. The 3:2:1 crack spread for West Texas Intermediate Crude oil was $17.96 on June 1, and has been at the top end of the range since December 2013,” notes PowerShares.