Petroleum accumulation history

Even if all of the elements described so far have been present within a sedimentary basin an accumulation will not necessarily be encountered. One of the crucial questions in prospect evaluation is about the timing of events. The deformation of strata into a suitable trap has to precede the maturation and migration of petroleum. The reservoir seal must have been intact throughout geologic time. If a leak occurred sometime in the past, the exploration well will only encounter small amounts of residual hydrocarbons. Conversely, a seal such as a fault may have developed early on in the field s history and prevented the migration of hydrocarbons into the structure. 

The transformation of porphyrin precursors to porphyrins, as well as the occurrence of these compounds in possible petroleum source materials and In petroleum, have considerable geochemical significance In the history of the origin and accumulation of petroleum. 

Furthermore, the undersaturated nature of the petroleum is suggested not to be primarily controlled by the maturity of the source rock, but rather by the cap rock properties (Sales 1997) and the burial (pressure) history of the accumulation. The same mechanism is suggested to cause more shallow fields in the area to maintain a black oil preference as lower boiling compounds are rapidly leaking from the relatively poor seals. 

Also, the chemical composition of iodine in waters of petrohferous areas has a number of features pointing to a marine origin, and the high iodine accumulation in the bed waters is a general geochemical phenomenon related to the history of petroleum formation (Vinogradov, 1939). 

Figure I. Elements of a Petroleum System. All petroleum systems contain 1. at least one formation of organic-rich sediments that has been buried to a sufficient depth by overburden rock such that petroleum is generated and expelled, 2. Pathways (permeable strata and faults) that allow the petroleum to migrate, 3. Reservoir rocks with sufficient porosity and permeability to accumulate economically significant quantities of petroleum, and 4. Sealing rock (low permeability) and structures that retain migrated petroleum within the reservoir rock. The top and bottom of the oil window is approximated as a function of burial depth. In actual basins, these depths are not uniform and vary as a function of organic matter type, regional heat flow from basement, in thermal conductivity of the different lithologies, and burial history (e.g., deposition rates, uplift, erosion, and hiatus events).

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