Why does my custom Query not return the chossen post type only?

    <?php if( have_posts() ) : while ( have_posts() ) : the_post(); ?>
        <h1><?php the_title(); ?></h1>
        <?php the_content(); ?>
    <?php endwhile; endif; ?>

    <?php $args = array('post_type', 'portfolio'); ?>
    <?php $query = new WP_query($args); ?>
    <?php if( $query->have_posts() ) : while ( $query->have_posts() ) : $query->the_post(); ?>
        <div class="entry__container cf">
            <div class="blog__featured-image--archive">
            </div>
            <h2 class="entry__mainheader"><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
            <?php the_excerpt(); ?>
        </div>
    <?php endwhile; endif; ?>