Basic Principles of Migration and Hydrocarbon Exploration

Diffusive transport and volume flow of hydrocarbon may coexist and transform each other in the heterogeneous formation. Diffusive transport is helpful to hydrocarbon expulsion from source rock and directly contributes to oil and gas accumulation by transform of flow pattern. The calculated rates of the diffusive and volume flow in the tight mudstone, with the flow velocity of 4-18 m/Ma and 3-15 m/Ma respectively, show little difference, which indicates that Darcy's law is suitable for describing the flow behavior in the dense mudstone. Hydrocarbon migrates in the buoyancy-driven flow or the seepage flow through the subsurface porous media, but it moves upward in discontinuous phase in the course of buoyant ascent. Hence, the buoyant ascent does not relate with critical migration saturation and relative permeability, and can't obey the Darcy's law. Two concepts on hydrocarbon migration have been discussed, i.e. dominant migration pathway and effective migration space, the former implies main direction of hydrocarbon migration and the latter refers to the space in which the hydrocarbon migration really occurred. About 70% oil and gas reservoirs are situated in the dominant migration pathway, while the effective migration space occupies only about 5%-10% of total porous space in a carrier bed. There are three types of traps according to the different combinations of seal strength and trap closure, which are the key controlling factors on the distribution of oil and gas. On the basis of research and statistics, the half-life of petroleum reserve is inferred to be about 29 Ma, and the average age of a giant oil field is about 35 Ma. According to the calculated results of hydrocarbon microseepage fluxes, the average natural age of a median-giant oil and gas field is thought to be about 50-100 Ma.