* LLS sells for $13.60 over WTI, off 40 cents * Mars trades at premium of $10.50, down 80 cents * Transatlantic spread settles 48 cents wider * Seaway pipeline reversal could be having impact HOUSTON, May 11 (Reuters) - U.S. cash crude differentials weakened on Friday despite a wider transatlantic spread, and traders said they suspected plans to reverse Seaway pipeline later this month were a drag, although the first Seaway cargo put on offer was pricey. Light Louisiana Sweet sold for $13.60 a barrel over West Texas Intermediate, off 40 cents. Mars sour traded at a premium of $10.50, weaker by 80 cents. The transatlantic spread widened 48 cents to $16.13 in favor of Brent, from $15.65 Thursday, which should have lifted cash crude premiums. Differentials weakened, however, and traders said planned reversal of Seaway pipeline starting May 17 to bring the first crude directly from Cushing, Oklahoma, to the Gulf Coast could be having an effect. Cushing crude is expected ultimately to weaken Gulf Coast grades as it gains on Brent. Nevertheless, the first offer of domestic sweet crude on the Gulf Coast via Seaway on Friday was at 50 cents under Brent, which one trader said was pricey given the fact that LLS traded $2.05 weaker than the arb's Friday settlement. "WTI should be weaker than LLS," he said. Traders identified BP as the offerer of two 500,000-barrel Seaway-delivered batches to the Houston area. BP declined comment. Domestic sweet has been discounted sharply from Brent because it has been unable to reach the world market at the Gulf Coast. When that changes, it is expected to strengthen WTI and weigh on cash crudes. "The arb strengthened. What's funny is grades have not. It's even going into sours a little. It could be the result of Seaway," trading consultant John Troland said. On futures markets, June WTI settled at $96.13 a barrel, down 95 cents. June Brent ended regular trading at $112.26, down 47 cents.