def unf_bo_below_pb_m3m3(rho_oil_st_kgm3=820, rs_m3m3=100, rho_oil_insitu_kgm3=700, gamma_gas=0.8):
    """
        Oil Formation Volume Factor according McCain correlation for pressure below bubble point pressure

    :param rho_oil_st_kgm3: density of stock-tank oil, kgm3
    :param rs_m3m3: solution gas-oil ratio, m3m3
    :param rho_oil_insitu_kgm3: Oil density at reservoir conditions, kgm3
    :param gamma_gas: specific gas  density(by air)
    :return: formation volume factor bo, m3m3

    ref1 book Mccain_w_d_spivey_j_p_lenn_c_p_petroleum_reservoir_fluid,third edition, 2011
    """
    # коэффициенты преобразованы - смотри описанию в ноутбуке
    bo = (rho_oil_st_kgm3 + 1.22044505587208 * rs_m3m3 * gamma_gas) / rho_oil_insitu_kgm3
    return bo