"More than 80% of the cumulative $17.5 trillion in upstream oil and gas spending is required to compensate for decline at existing oil and gas fields ... Gradual depletion of the most accessible reserves forces companies to move to develop more challenging fields; although offset in part by technology learning, this puts pressure on upstream costs and underpins an oil price that rises to reach $128/barrel in real terms by 2035 ... In the case of tight oil, the way that costs are projected to evolve differs inside and outside the United States. In the United States, on the basis of the current estimates of total recoverable resources, high production rates mean resources are rapidly depleted, with a corresponding rise in costs per barrel as operators move out of the sweet-spots to areas where the recovery per well is lower. This explains the peak and then the subsequent decline in US tight oil production after 2025 in our projections. In the rest of the world, only a low level of tight oil production is expected (compared to the resources)."