Oil and gas producers were among the heaviest fallers in London on Tuesday after Brent crude dropped to below $100 a barrel the previous session.Royal Dutch Shell was providing a drag on the sector, down 1.6%, while BP lost 0.7% and Tullow fell 0.4%Whilst slightly off the low of $99.36 seen on Monday - its weakest since 1 May 2013 - Brent was still trading below $100 at $99.86 on Tuesday afternoon, down 0.34% on the day. The price has now fallen 10.5% so far this quarter."Sellers continue to drive the price of crude lower in the face of building stocks and lower global growth prospects," said analyst Angus Campbell from Fxpro.Declines have been spurred after Monday's data from China showed a second straight monthly fall in imports in August, while figures on Friday indicated that hiring in the US slowed down to its weakest pace this year.Smaller companies in the UK-listed oil and gas sector were also under pressure in afternoon trade, including Aminex, Xplorer, 3Legs Resources, Solo Oil and Leyshon Energy.Top performing sectors so far todayBeverages 14,367.57 +0.97%Industrial Metals & Mining 1,384.38 +0.96%Oil Equipment, Services & Distribution 22,900.89 +0.79%Mobile Telecommunications 4,658.81 +0.75%Electronic & Electrical Equipment 3,760.89 +0.71%Bottom performing sectors so far todayElectricity 9,294.49 -1.37%Oil & Gas Producers 8,361.28 -1.02%Forestry & Paper 11,342.24 -0.94%Real Estate Investment Trusts 2,844.10 -0.87%Mining 17,168.04 -0.56%