decrease included lower fuel costs of $920 million. Also contributing to the decrease were lower expenses for construction, repair and maintenance of $300 million, contract labor of $270 million, and research, technical and professional services of $200 million.

Operating, selling, general and administrative expenses increased $642 million between 2014 and 2013. The increase included higher employee compensation and benefit costs of $360 million, primarily related to a buyout of a legacy pension obligation. Also contributing to the increase was higher transportation costs of $350 million, primarily reflecting the economic buyout of a long-term contractual obligation, and higher environmental expenses related to a mining asset of $300 million. Partially offsetting the increase were lower fuel expenses of $360 million.

Exploration expenses in 2015 increased from 2014 mainly due to higher charges for well write-offs largely related to project cancellations. Exploration expenses in 2014 increased from 2013 mainly due to higher charges for well write-offs, partially offset by lower geological and geophysical expenses.

Depreciation, depletion and amortization expenses increased in 2015 from 2014 mainly due to impairments of oil and gas producing fields of about $3.5 billion in 2015 compared with $900 million in 2014. Also contributing to the increase were higher depreciation rates and higher production levels for certain oil and gas producing fields. The increase in 2014 from 2013 was mainly due to higher depreciation rates and impairments for certain oil and gas producing fields, and the impairment of a mining asset.

Taxes other than on income decreased in 2015 from 2014 mainly due to lower crude oil and refined product prices. Taxes other than on income decreased in 2014 from 2013 primarily due to a decrease in duty expense in South Africa along with lower consumer excise taxes in Thailand, reflecting lower sales volumes at both locations.

Effective income tax rates were 3 percent in 2015, 38 percent in 2014 and 40 percent in 2013. The decrease in the effective tax rate between 2015 and 2014 primarily resulted from the impacts of jurisdictional mix, one-time tax benefits, foreign currency remeasurement, equity earnings and a reduction in statutory tax rates in the United Kingdom, partially offset by the effects of valuation allowances recognized on deferred tax assets and the sale of the company's interest in Caltex Australia Limited.

The rate decreased between 2014 and 2013 primarily due to the impact of changes in jurisdictional mix and equity earnings, and the tax effects related to the 2014 sale of interests in Chad and Cameroon, partially offset by other one-time and ongoing tax charges.

Liquidity and Capital Resources

Cash, Cash Equivalents, Time Deposits and Marketable Securities Total balances were $11.3 billion and $13.2 billion at December 31, 2015 and 2014, respectively. Cash provided by operating activities in 2015 was $19.5 billion, compared with $31.5 billion in 2014 and $35.0 billion in 2013. Cash provided by operating activities was net of contributions to employee pension plans of approximately $0.9 billion, $0.4 billion