Show portfolio posts from a specific portfolio type (category)

Here is the code which I think controls the portfolio posts which are displayed on the page

<?php
        // Grab the number of posts per page
        $posts_per_page = zilla_get_option('portfolio_posts_per_page');
        if( empty($posts_per_page) ) { $posts_per_page = get_option('posts_per_page'); }

        // Query has to be called $wp_query to work with pagination
        $wp_query = new WP_Query();
        $wp_query->query("post_type=portfolio&posts_per_page=$posts_per_page&paged=$paged&orderby=menu_order&order=ASC");  

        if( $wp_query->have_posts() ) :
        $i = 1;
    ?>

I think “$wp_query->query…;” part is the part that actually controls which posts are retrieved

My question is, what do I insert into that code which would allow me to specify which category is displayed? (portfolio categories are called Portfolio Type in Volumes: Responsive Portfolio WordPress Theme – ThemeForest if that makes a difference)

THANK YOU! I really appreciate the help