Accumulation history, reservoir

Hillebrand, T. Leythaeuser, D. 1992. Reservoir geochemistry of Stockstadt oilfield compositional heterogeneities reflecting accumulation history and multiple source input. Organic Geochemistry, 19, 119-131. 

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. 

Oil accumulations can continue to undergo thermal evolution, depending on the depth of the reservoir and subsequent geothermal history. As noted in Section 4.5.2, oil becomes susceptible to thermal cracking at temperatures above 160-200 °C. For example, paraffinic-naphthenic oils (Fig. 4.22) are degraded to aromatic-naphthenic oils (with moderate S content, <1%), and aromatic-intermediate oils degrade to aromatic-asphaltic oils (with high S content, >1%). 

Agricultural workers may have the greatest exposure rate to the aforementioned compounds, but one must remember that these chemicals can reside on clothing that is brought back into the household and are found in many household exterminator preparations and garden products. Accumulation of these compounds in the skin after repeated exposures sometimes creates reservoirs of pesticide in the stratum corneum that effectively increase their concentration and their irritant capacity, as well as potential toxic effects. A good investigative history will usually help verify a relationship between possible pesticide exposure and dermatitis. 

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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