I have in my possession price and time of different trade from an auction. The price series isn't stationnary so I work with the log return series. I'd like to forecast the evolution of the log return series thanks to models such as AR, MA and ARMA. To identify the parameters of AR and MA models, I used the ACF and PACF plots. However, I don't understand their signification because they're almost identical. I know how to find the p and q parameters from these plots but in this case, I don't know how to do it and which of the models can be use.
Here is a picture of the ACF and PACF plot https://i.stack.imgur.com/PhbFj.jpg