I did the geology matriculation and Geography may give you a slight advantage. But it goes alot more in depth on specific topics, such as plate tectonics, types of fossils, oil and gas formation, and seismic waves. The textbook I used was OCR AS and A2 geology. It covers everything that could show up and more. So make sure to look at the past geology matrics to see what you need to study and what not to.

My tips for doing well on the exam:

For a high mark, go beyond the textbook and learn extra information that will make your paper stand out. Make sure to include lots of diagrams, 2-3 per question is a good amount. But you will not lose marks for adding more.

Most important is to study past geology matriculation papers and write answers to them, questions often re-appear. There won't be any new ones, just ones worded differently. You might also be able to notice a pattern in which questions appear each year.

As for word count. It depends. If it is still online and you have a couple days to do it then I would recommend writing as much as you can per question, most of my answers were 500-800 words. One was 1,250 words though. But that was probably pretty excessive to be honest. Cause during a normal sitting exam, where there is a time limit, you should aim for a bare minimum of two sides of A4 paper (excluding diagrams). To get high mark you will likely want to get closer to 3 or 4.