Vertical migration in theory and in practice

Abstract

We use the term “vertical migration” to describe the mechanism of mass transport that moves trace petroleum hydrocarbons from subsurface accumulations to the surface. Vertical migration is the heart of geochemical exploration for petroleum. While our use of geochemical exploration for petroleum has been accelerating, our understanding of the mechanism that makes the process successful lagged behind. Early vertical migration models were based on a molecular process called diffusion. Diffusion remained the accepted mechanism during the early years of geochemical exploration even though the diffusion mechanism could not explain many observations, including migration that appeared predominately vertical. About 30 years ago buoyancy driven migration mechanisms were modeled. The models predicted how gases below the water table could migrate vertically as a gas phase with buoyancy providing a mechanism for predominately vertical gas migration. Buoyancy models explained how surface expressions observed from vertical migration measurements related to petroleum reservoirs including small lateral offsets. Buoyancy models also explained gradients and data contrasts observed in many surface geochemical features including what they mean and predicted fast vertical migration rates that have been verified by field observations. In addition, buoyancy models predicted how petroleum reservoirs could be mapped using gas data and how surface expressions of faults and fractures could be mapped using liquid data. Those data can be obtained from direct measurement of trace hydrocarbons in near-surface fluid samples. Since reservoir rock samples are not required, vertical migration provides a way to obtain information about a reservoir prior to exploratory drilling. Characteristics of these migrating fluids and their surface patterns are the foundation of modern geochemical exploration for petroleum.

Geological processes mediate a microbial dispersal loop in the deep biosphere

“…Recurrent cyclical dispersal facilitates this scenario, consistent with a “microbial conveyor belt” framework for understanding microbial biogeography where the same environment features as both the origin and eventual destination for migrating populations ( 8 ). Upward dispersal of spores originating in ~2000 mbsf petroleum reservoirs in this region ( 40 ) would require tens to hundreds of years, based on seepage rates on the order of 10 cm day −1 for hydrocarbons ( 41 , 42 ). Spores dispersed by geofluids exiting the subsurface (see movie S1) and entering the marine environment ( Fig.…”

Controversial Issues of Hydrocarbon Field Formation and the Role of Geomagnetic Fields

“…The reservoir fluids move from the highest pressure areas to the lowest ones along the path of least resistance. Such pathways can be vertical disjunctive faults in the sedimentary cover [63][64][65][66]. Following this, the zones near faults may be the source of discrete geomagnetic fields, which in turn may influence the petroleum generation processes due to the spin magnetic effects during the transformation of organic matter.…”

Analysis of acidolysis hydrocarbon anomaly in the Chaoshan Depression of the Northern South China Sea and its response to reservoir properties

“…As a result, adsorbed hydrocarbon anomalies with positive correlation with underlying oil-gas reservoirs are formed in the surface soil above the oil-gas reservoirs, Hydrocarbon anomalies formed by oil and gas reservoir leakage can be detected in seafloor sediments. Based on the positive correlation between latent oil-gas reservoirs and the hydrocarbon concentration of adsorbed gas in seafloor sediments, the characteristics of hydrocarbon internal composition can reliably predict the properties of migrated hydrocarbons, predict the development of oil-gas reservoirs, and distinguish the properties of underlying oil-gas reservoirs [15][16][17][18][19][20][21][22][23][24][25]. Marine oil and gas geochemical exploration mainly uses the analysis of gaseous alkanes (C 1 -C 5 ) in seabed sediments to predict the underlying oil-gas reservoirs [14].…”