Oil and gas related stocks continued to slide on Monday after OPEC decided not to lower the rate of oil production on Thursday, causing crude prices to tumble below $70 a barrel, their lowest in over four and a half years.Afren suffered the biggest hit Monday afternoon, declining 9%, while Enquest, Ophir Energy and Soco International also fell.Oil and gas exploration company Afren has suffered substantially today on the back of recent reports that its management was corrupt. The firm's shares are down by 71% so far this year at their lowest level since June 2009.JPMorgan Cazenove was also dampening sentiment in the sector as it lowered its recommendations for Afren and Ophir to 'underweight' and cut Enquest to 'neutral'.Meanwhile, Balfour Beatty rose 5.18% amid Monday's reports that it has been offered £1bn by John Laing Infrastructure Fund for its investment arm. Balfour Beatty responded: "The Board of Balfour Beatty will review the proposal once received from JLIF. A further announcement will be made in due course, as appropriate."Croda International also felt a boost as Deutsche Bank upgraded its shares to a 'buy' rating in its research note handed out to investors on Monday. Analysts said the stock's valuation is now "relatively attractive" after a recent de-rating. FTSE 250 - RisersBalfour Beatty (BBY) 193.30p +5.46%Croda International (CRDA) 2,562.00p +4.40%Morgan Advanced Materials (MGAM) 303.00p +3.84%SSP Group (SSPG) 274.00p +3.67%William Hill (WMH) 347.00p +3.58%Spire Healthcare Group (SPI) 321.40p +2.36%Brit (BRIT) 252.00p +1.33%Premier Oil (PMO) 189.40p +1.28%Fenner (FENR) 241.20p +1.13%Wood Group (John) (WG.) 598.00p +1.10%FTSE 250 - FallersAfren (AFR) 47.15p -8.89%EnQuest (ENQ) 46.70p -5.79%Vedanta Resources (VED) 689.00p -5.55%Hochschild Mining (HOC) 89.15p -4.96%Bodycote (BOY) 613.00p -4.52%Lancashire Holdings Limited (LRE) 519.00p -3.89%FirstGroup (FGP) 108.60p -3.89%Bwin.party Digital Entertainment (BPTY) 103.90p -3.62%Ophir Energy (OPHR) 140.10p -3.58%Soco International (SIA) 273.40p -3.43%