Experimentally determined pressure effect on vitrinite reflectance at 450 °C

► Pressure (P) effect on VR at 400-450 °C depends on VR magnitude with regard to VRi. ► Increasing P retards vitrinite maturation for VR < VRi = 1.34 ± 0.29%. ► Increasing P promotes vitrinite maturation for VR > VRi = 1.34 ± 0.29%. ► Retarding effect of P on vitrinite maturation decreases with increasing VR. ► Promoting effect of P on vitrinite maturation increases with increasing VR.