Since Goldman Sachs last advised investors to buy Royal Dutch Shell (LON:RDSB) earlier this month, shares in the oil and gas supermajor rallied nearly 10 percent, in part due to yesterday's upbeat quarterly report.

However, according to the investment bank's estimates, the stock is still trading at an 11 percent discount to the sector.

Consequently, Goldman repeated its "conviction buy" recommendation for Shell and raised its 12-month target price by two percent to 2,900 pence, a premium of nearly 25 percent to yesterday's closing price of 2,330 pence.

Today's report followed the third quarterly figures released by Shell on Thursday, which showed that earnings on a current cost of supply basis were US$7.2 billion for the three months to September 30 compared with US$3.5 billion for the same period last year.

David Fuller's view Royal Dutch Shell (monthly, weekly & daily) is the best performing share in my personal long-term investment portfolio this year to date and has been my second largest position since I first purchased it on 28th June 2010, after it had been knocked down by fallout from BP's Gulf of Mexico disaster. I had been waiting to get back into the energy market - a Fullermoney secular theme - and the yield was attractive at 6.25% for the B-shares.

Shortly thereafter I increased the position on at least two occasions and I also bought a little more on 29th September. I would certainly consider adding to my overall holding in RDSB on setbacks, should the yield rises back above 5% from its current level of 4.5%. Therefore, I am less inclined to buy today, despite yesterday's new all-time high in GBP, because the share is temporarily overbought and near its highest level against the MA in recent years, albeit within a broad range. I expect a consolidation of recent gains followed by somewhat higher levels over the medium term.

Today's Lex column from the Financial Times provides some additional reasons for retaining an investment in Royal Dutch Shell.