China's major oil and gas giants expect their net profits and net incomes to surge for the first half of this year, benefiting from the revival in crude oil prices, but analysts believe high earnings growth will be hard to maintain in the second half as oil prices are unlikely to rise much above current levels.

China National Petroleum Corp, the nation's largest oil and gas producer by annual output, expects its net profit to more than double for the first six months of this year, rising by between 13.5 billion yuan ($1.98 billion) and 15.5 billion yuan compared with the first half of 2017, up 107 percent to 122 percent from the same period a year ago.

Based on a net profit of 12.67 billion yuan in the first half of 2017, PetroChina's net profit is forecast to be 26.17 billion yuan to 28.17 billion yuan in the period, it said in a filing to the Shanghai Stock Exchange.

It is the highest six-month profit for the company as well as the best quarterly result since the second quarter of 2015.

PetroChina's domestic rival China Petroleum and Chemical Corp, or Sinopec, said it expects to deliver its best quarter since 2013. It expects first-half net profit to rise by 50 percent from 27.1 billion yuan in the same period a year ago thanks to favorable downstream refining business.

"Global crude oil prices increased year-on-year in the first six months, and the company's upstream business improved substantially," the company said in a statement.

Analysts said they believe the upsurge of net profit and net income is a result of cost optimization and increased prices of natural gas, crude oil, refined oil and other products.

Min Na, a senior analyst for oil and gas at Bloomberg New Energy Finance, said rising oil prices have played a fundamental role in driving the upstream sector, and both companies have seen payoffs from great efforts on cost management as well as improving efficiency in previous years when oil prices collapsed.

"In the downstream sector, we have seen both companies adjusting their product structure to reflect the change in demand side by increasing the production of kerosene while reducing diesel output," she said.

"This has helped their balance sheets as well."

Wang Lu, an Asia-Pacific oil and gas analyst at Bloomberg, echoed her views. She said PetroChina's earnings may have surged by 107 percent to 122 percent year-on-year to 1 billion yuan in the first half due to improving exploration and production margins on the back of the oil price rally.

"The company's oil-output decline eased and gas-production growth accelerated in the first six months, and we expect losses from importing pipeline gas and LNG have expanded in the period as domestic gas prices adjustment lagged behind the rise of imports' costs," she said.

"Inventory gains partly masked downstream weakness, where strong competition has been eroding margins."

Wang said OPEC's demonstration of a higher-than-expected compliance rate in the output-cut deal, helped by supply issues in Angola and Venezuela, as well as a decline in Iran's oil exports after US President Donald Trump reinstated sanctions, have all pushed oil prices higher in the first half of this year.

China's three top oil and gas behemoths－PetroChina, Sinopec and China National Offshore Oil Corp－will publish their final first-half results in August.

Wang said considering the fact that oil prices are unlikely to rise much above current levels, it's difficult for oil giants like PetroChina to repeat such high earnings growth in the second half of 2018.

"Profit growth in the second half of this year will be driven by higher gas prices on the back of the residential citygate prices rise on June 10," she said.