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Can Hydrocarbons Be Generated on Oceanic Crust?

Abstract

Embedded in regional exploration workflows is generally a strong preference for prospects to be located on the continental crust. The main logic in this rule of thumb is that continental crust generates 10–100 times more radioactive heat per volume than oceanic crust and, therefore, ensures sufficiently warm thermal history of source rocks within sedimentary section to mature and generate hydrocarbons. Neglected in this reasoning is a fact that thermal history of a basin is a function of not only basement heat flow history, but also sediment thermal properties and depositional history. Although overall thermal history of sediments deposited on the oceanic crust tend to be cooler than over the continental crust, it does not automatically prevent source rock maturation. In this study we investigate a number of crustal type, crustal and sedimentary thickness scenarios and their impact on hydrocarbon generation. We consider a basin with a rifting period during 100–90 Ma and continuous siliciclastic sedimentation to present-day. The results of basin modeling indicate, for example, that over the thinned continental and oceanic crusts the potential source rocks would reach their maturity of 1% VRo (mid-range of oil window) at present day ~3-km and ~5-km depths, respectively. These estimates support presence of sufficient heat flow for hydrocarbon generation in areas of thick sedimentary deposits over the oceanic crust. Although exceptional, well-known examples of the basins with sedimentary thickness exceeding 5 km over the oceanic crust are the Foz do Amazonas and Niger Delta. Exploration geologists commonly use rules of thumb to help them quickly screen a basin. Limitations and uncertainties of such rules are often underestimated and not considered. Moreover, initial incorrect perceptions about basin history may persist even though later basin models may indicate a different story. This happens because basin model is typically built later in the exploration workflow after “all the geology” is put together. Our example illustrates the need for performing a numerical integration early in, and throughout, the exploration workflow and keeping in mind that rules have their exceptions.