The Vertical Migration Model and Recent Advances in Geochemical Exploration

摘要

Petroleum geochemical exploration is the science of using geochemical measurements to assess information about petroleum reservoirs before drilling. Vertical migration is the foundation of all geochemical exploration methods. Vertical migration provides the geochemical communication link between subsurface reservoirs and the surface. Vertical migration is the foundation of petroleum geochemical exploration. Geochemical exploration techniques began about 85 years ago in Russia and about 10 years later in the USA. Horvitz (1939) described the measurement of petroleum gases in shallow soil samples in Harris, Brazoria, and Galveston counties, Texas. Since that time, geochemical exploration methods have proven successful both onshore and offshore in the Gulf Coast area. Brooks et aL (1986) reported that they achieved the most success for sampling liquid hydrocarbons offshore, including the Gulf of Mexico, over the surface expressions of seismically determined faults. The faults were thought to provide conduits for petroleum to travel from reservoir depths to ocean-bottom sediments. Although surface expressions of vertical migration were known and documented throughout the last 75 years, the mechanisms by which hydrocarbons could move from reservoirs to the surface were not known (Horvitz, 1978). Diffusion and other vertical migration models could not explain the surface hydrocarbon expressions nor could they explain why migration was primarily vertical. About 20 years ago the industry developed a buoyancy mechanism (Fig. 1) modeled by Klusman and Saeed (1996). Below the water table, gases migrate vertically as a gas phase with buoyancy providing a mechanism for predominately vertical migration. The buoyancy model explains the gradients and data contrasts observed in many surface geochemical features. Arp (1992) quantified the buoyancy-driven migration mechanism in a form that calculated vertical migration rates. Vertical migration rates predicted by the model were verified by field measurements. The buoyancy model therefore explained most of the observations from vertical migration data. When the vertical migration model was extended to petroleum liquids, we found that liquid hydrocarbons could migrate similarly to gases, but slower and through larger fractures and faults. Each phase (gas or liquid) provides information about reservoir composition. Different surface expressions of these two migrating phases form the basis of modern geochemical exploration. An added benefit from liquid hydrocarbon geochemical exploration data is reservoir petroleum fluid characterization from exploration measurements prior to drilling. A seismic/geochemical integration project reported by Belt and Rice (1996) in the Main Pass area, shallow offshore Louisiana, was instrumental in understanding the liquid hydrocarbon migration mechanism. A cross section of the Main Pass data in Figure 2 shows gas concentration data in green represented by ethane C_2) concentration. Liquid hydrocarbon concentrations, measured by a fluorescence technique (3-ring C_(14) to about C_(18)) are in magenta. While petroleum liquids appeared to migrate up faults, gases migrated up visible faults and also between faults. Most of this gas migration would have been through fractures too small to be visible on the seismic section. As predicted by the vertical migration model, gas migration is possible about everywhere over a petroleum reservoir.