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From Source to Reservoir – the Generation and Migration Process

Barry J. Katz
Energy Technology Company, ChevronTexaco, Bellaire, TX

Hydrocarbon charge is considered one of the four primary exploration risk elements. The lack of hydrocarbon charge is very often cited as a reason for failure in all but mature exploratory provinces (i.e., those with a long and successful exploration history). An understanding of the origin of hydrocarbons is, therefore, considered key to a successful exploration program. An examination of the origin of petroleum must satisfactorily explain its distribution and complex geochemical character. Since the beginning of the modern petroleum era both an organic and an inorganic origin for oil have been proposed. These different origins would lead to very different geographic distributions of hydrocarbons, geochemical compositions, and approaches to exploration.

There is little doubt that “inorganic” hydrocarbons exist, forming where superheated fluids circulate through iron- and chromium-rich rocks under high pressure. These conditions exist at such as within mid-oceanic ridges and can produce methane, ethane, and some propane. Methane has also been detected in volcanic emanations. However, the nature of these hydrocarbons, including the lack of biomarker compounds and their distribution is largely inconsistent with known commercial petroleum accumulations. This has lead to the conclusion that these commercial deposits have an organic origin. An organic origin satisfies the known distribution and the very complex compositions of petroleum. An organic origin focuses exploration to basins where thick sedimentary accumulations are present and where the potential for organic-rich sediments exist. This typically directs exploration away from areas where mantle degassing and hydrothermal circulation is occurring. The organic origin relies on the accumulation and subsequent burial of significant amount of organic matter converting biopolymers to geopolymers and eventually oil and natural gas, the expulsion of these hydrocarbons from discrete sedimentary units, and migration of these fluids along a carrier network.

The processes associated with the organic origin of petroleum are, in general, inefficient. For example, typically less than 10% of surface water productivity accumulates in the sediment to form petroleum precursors. Differences in the nature of the accumulating organic matter and the sedimentary matrix lead to the complex, but systematically consistent, oil compositions. Lacustrine source rocks tend to produce a highly paraffinic crude, while marine source rocks tend to produced a more naphthenic crude oil. Argillaceous source rocks tend to produce low sulfur crudes, while those associated with carbonate source rocks result in sulfur-rich oils. The conversion from sedimentary organic matter to petroleum is also limited in its efficiency and is dependent on organic matter type. Laboratory simulations have shown that both the product and rate of generation is kerogen dependent, with the nature of the products evolving during the generation process. Both bulk and molecular compositions evolve in a regular manner with increasing thermal maturity of the source. Once generated, these hydrocarbons are expelled from the source. There is evidence that the mechanisms for expulsion differ for oil and gas. Oil-prone hydrocarbon source rocks require that the source rock’s pore network become saturated with oil for expulsion to occur. In contrast, primary migration in gas-prone systems is largely driven by diffusion. Diffusion does not require that the pore system become saturated but only that a concentration gradient be present. The combined expulsion and migration processes also appear to be rather inefficient. A comparison of the volumes of hydrocarbons retained in the source with that in known accumulations suggests average efficiencies of about 10%, with maximum values approaching only 35%. After entrapment various processes including biodegradation, water washing, thermal cracking, and phse segregation may act on the trapped oil altering its original composition but maintaining compositions distinct from those that would be anticipated from an abiogic origin.