The nice thing is that nothing that you apply to the stain can damage the color, unlike any other fiber. the only thing though is to make sure that you are dealing with 100% Olefin, not an Olefin blend. The previous comment was correct, try dish washing liquid first.
The rule of the thumb when removing stains from carpet is to begin with mild neutral or close to neutral ph cleaners and then gradually move to the stronger stuff. Also, you are dealing with a difficult stain being that the grease from the chair adheres to the oil based Olefin. Odds are that you can definitely lighten it though. Your stain is probably more on the surface than others that I have seen such as oil and vinegar salad dressing.
If the dish washing solution doesn't help to your satisfaction I would recommend a grease remover. Professionally I use one called POG which stands for paint, oil and grease remover. Customers also tell me that the product that they use that works the same is called AFTA and can be purchased in most hardware stores. If these don't quite do the trick you are going to need to call in a professional.