GEOLOGICAL REASONING FOR THE PETROLEUM SOURCE ROCKS OF KNOWN FIELDS

Abstract

Geological reasoning for the petroleum source-migration-accumulation relationships centres around petroleum composition and its variability, water composition and its variability, and stratigraphy. For example, a field with several pools of petroleum of different compositions is likely to have been sourced from several distinct source rocks that are stratigraphically associated with the reservoir rocks. If water compositions are also variable, the conclusion is reinforced. A field with several pools of petroleum of similar quality was sourced either from similar source rocks that are stratigraphically associated with the reservoirs, or from a single source that is removed from the accumulations. There are also considerations of wax content, environment of deposition of the reservoir sequence, sand/shale ratios, and faulting.

Geological reasoning does not always lead to the same conclusions as geochemical reasoning. Such cases are particularly important for petroleum geology because they should lead us to a better understanding of the source-migration-accumulation relationships. Some of the remaining giant oil accumulations of the world may be in areas that would be discarded on geochemical evidence. Most of the crude oil remaining to be discovered will be in relatively few giant fields, so misunderstanding could jeopardize our future supplies.