Undocumented Offset Registers mpu9150

Hello,

I accidentally came across what seem to be accel and gyro offset registers (0x06-0x0B) in the mpu9150 when I was supposed to be accessing the AK8975 registers in the mpu9150. The values stored in these seem to be coming back to a default value when power is reset but ever since I accessed register 0x0A accidentally (apparently the most significant byte for accelerometer Z axis offset), My Z axis for my devices seem to be off by quite a substantial amount (>25% deviation from gravity when laid flat).

Is there any way to permanently correct this offset?

Any help would be much appreciated,
Kind Regards,
Cyrus