Since Shell first entered the shales oil and gas business in 2005, we have matured by focusing our portfolio, through continuous improvement and embedding an unconventional culture. Today, we are competitive against our regional and international peers through our nimbleness of an independent as well as the strength of a major. Our business is well positioned for strong growth in North America and beyond. Together with our partners and through Shell operated ventures, we continue to optimize our existing assets and look for new, attractive opportunities, including stacked pay potential in existing assets, opportunistic bolt-on additions near existing positions; and entries into new emerging basins.
Since 2013, Shell has reduced capital investment by 50% and streamlined its shales portfolio by 50%, while increasing our discovered and prospective reserves1? by 10%. We now have approximately 12 billion barrels of discovered and prospective resource within the portfolio, which could underpin a sizable business in the future.
Our activity today is focused on six key assets and eights plays, all of which are material. We have expanded our portfolio beyond the shales heartland in North America, to Argentina. Our portfolio comprises of 25% liquids and 75% gas.
Shell invests $2-3 billion per annum in our Shales business, prioritizing profitable light tight oil (LTO) production growth and targeting investment toward the Permian and Fox Creek assets. Gas assets, such as Groundbirch and Appalachia, present longer-term option value. We are actively appraising acreage so that resources can be brought onto production at the right moment in time. We expect to double total production of shale oil and gas from about 250,000 barrels of oil equivalent a day (boe/d) in 2017 to about 500,000 boe/d by the end of the decade.
Shell has relentlessly driven nimbleness in its shales activities. Our unit operating costs have decreased by 40%, drill and complete costs by 50% and above asset costs by 75%. At the same time, we have focused on improving delivery efficiency in both drilling and completions. Our drilling and completion times have reduced and learning rates improved. The number of wells per pad has grown exponentially and the quality of wells has significantly improved. 83% of the wells drilled are Top Quartile for cost. We have turned drilling from an art to a science, designing the drilling parameters for our wells on a foot-by-foot basis. We have also increased both the length and completion intensity of wells, significantly increasing the ultimate recovery per well. With improved efficiency, we have driven down the break-even point (BEP) in sweet spots. Our production has increased by 60%, from 173,000 boe/d in 2013 to approximately 250,000k boe/d in 2017, exceeding target in each year.
While pursuing nimbleness, Shell has also successfully leveraged its strengths as a major: technology, integration and scale. We have pursued technological solutions that not only offer short-term efficiency gains but have the potential to transform the way shales field is build and operated – our iShale is actively developing and testing new innovations in automation and digitalization. We have created real-time Drilling Automation and Remote Technology (DART) Centers to manage integrated workflows and rapidly move technical innovations and well design improvements between assets. Meanwhile, we have leveraged Shell global supply chain and developed key supplier relationships. We have also driven speed and simplicity across the entire Shell. For example, we have rationalized well material specifications to ensure they are fit for purpose for shales. At the same time, we have placed safety and societal acceptance at the forefront of our activities.
1Discovered + prospective resources shown at year end 2016, consistent with the Society of Petroleum Engineers 2P (Proved + Probable Reserves), 2C (Contingent Resources) and 2U (Prospective Resources) definitions.
2013 volumes exclude divested assets