Global oil and gas resources from mature conventional, unconventional and offshore fields rose by 151 billion bbl over a four-year period through 2017, almost 17% higher than the total amount produced over the period, according to Rystad Energy.

By the end of last year, the total estimated remaining liquids resources for the mature assets increased to 1.227 trillion bbl.

“Growth in mature fields can be driven by many factors, such as increased infill drilling, improved understanding of the reservoirs, enhanced oil recovery projects and technology improvements,” said Rystad’s Espen Erlingsen, senior vice president of upstream research.

“These developments are more challenging to track compared to new discoveries and often are difficult to measure.”

Of the 151 billion bbl increase, 68 billion bbl are conventional onshore resources, 43 billion bbl are shale/tight resources and 40 billion bbl are from the offshore.

“For shale, the growth is primarily driven by better well performance and more target benches in the Permian Basin,” according to Rystad.

Most of the conventional onshore growth is driven by Saudi Arabia on “increased infill drilling and better investment terms,” which lifted the estimate for the remaining resources in the country. The offshore uplift was driven by the Middle East, Russia and Europe.

“The effect of these developments can be estimated by comparing the total amount of remaining resources in 2013 and 2017,” analysts said. “The number was at 1.186 trillion bbl at the end of 2013. This was reduced through production by 132 billion bbl of oil and natural gas liquids. Over the same period, 22 billion bbl of new liquid resources were discovered.”

Based on the update, the numbers indicate 1.076 trillion bbl should remain, but Rystad estimates are based on bottom-up research. That puts the count at 1.227 trillion bbl at the end of 2017, implying the 151 billion bbl growth from mature assets.

“Combining resource revisions in mature fields with volumes from new discoveries, 173 billion bbl were added to global resources over the last four years,” Erlingsen said. “The contribution from producing fields shows the importance of investing in mature assets.”