Pecans according to USDA SR-24 are 72% fat, primarily monounsaturated fats and polyunsaturated or "healthy" fat. There are approximately 19 shelled pecans in 1 ounce.

Wolfram|Alpha says there are 22 grams of fat in 30 grams of Pecans. So, figure a pecan is roughly two-thirds fat. Can't really tell you how much an average single pecan weighs though.