How can the choice of "where to publish" affect the potential number of citations? I study petroleum engineering; however, I want everybody from different fields to express their opinion based on their personal experience.

Let's assume you have the following options: publishing in a journal that is not open access and has a high impact factor, publishing in a conference, and finally publishing in a journal that is open access and has a low impact factor. The only option that has ever been appealing to me is to publish in a reputable journal, simply because I don't think the other two are challenging enough and also the whole process of modifying your paper based on peer reviews will help improve the quality of your paper. However, I have recently stumbled upon some papers that had me rethink about the whole thing.

It seems to me as if for some reason open access publishing increases the potential number of citations. The straightforward explanation is that many more people will have the chance to see your paper and there is a huge chance that some of them cite your paper at least in their literature review. (I understand that if they don't have access to high quality journals, they might not be currently in the right setting to necessarily publish a good work, but in this context a citation is a citation even if it's by a poor quality paper)

Also, I do have a feeling that depending on the field you're working in, the number of conference publications can hugely exceed the number of journal publications. For example, petroleum engineering is extremely industrialized and  a huge number of fellows in the industry are only willing to publish in conferences. This is simply because publication in a conference is easier, it is fun to travel for free (!) and in general networking is very important to many of them. Also, I keep thinking that there a chance that some of these people, for some reason that I don't know of, have exclusive access to conference papers, but no access to journal papers or maybe they simply don't have the willingness to spend time on reading journal papers because they don't need to! What if my paper gets exposed to this huge number of people if I publish it in a conference, but only to a small number of researchers in the academia if I publish it in a journal.
A prior question would be why you care about the number of citations. There are valid career-related reasons, and I assume that you know that citation-gathering businesses do not always include "self-publication" and conference proceedings. That applies both to the article being cited, and the citing article. So there is a difference between actual citations in the abstract sense, and enumerated citations. That could support the refereed respected journal approach, depending on your motivation. Also, there are numerous open-access respectable peer-reviewed journals, so since you are looking for non-overlapping categories, I assume you specifically mean unrefereed repositories of various kinds, in referring to "open access".

I would be surprised if there is any evidence that open-access results in an increase in citations, because in the subset of people in the world publishing academic papers, I doubt that the factor of having to pay to read an article has a significant effect on whether a researcher can actually read the paper. Via my university, I get to read articles in any arbitrary journal, and request a free copy if they don't subscribe. An opposite expected trend is that an article in a large, open repository may more easily be missed, if say they get a hundred submissions a week, and if authors are unaware of your work, they won't cite it.

In my field (linguistics), conference proceedings were traditionally just a minor notch below peer-reviewed journals in stature, due to there being few journals and a major publication lag, but now conference proceedings are in third place (behind book chapters). But in some sub-areas (computational), conference proceedings are a major publication venue. Correspondingly, CL conference proceedings go through a serious wringer.

So your basic distinction centered around peer-review is correct, it's just that peer-review standards vary considerably for journals and conference proceedings. It may be that more people will have access to your paper if it is in an open repository, but they will also have access to thousands of other papers, and to generate a citation by the subset of the public that writes scientific papers, you would need to do something extra to draw attention to your paper.

I suggest you first scrutinize the notion of "citation", to determine what citation engine(s) you care about, and whether they scoop up citations of un-reviewed works that appear in other un-reviewed works.