def stoiip(area=40, res_height=20, porosity=0.25, avg_water_saturation=0.4, oil_vol_factor=1.30):
    """Returns the estimate for stock tank oil initially in place (STB) given the area (acres), reservoir height (ft),
    porosity (fraction), average water saturation (fraction), and the oil formation volume factor (RB/STB)."""
    return (7758*area*res_height*porosity*(1-avg_water_saturation))/oil_vol_factor