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AAPG Southwest Section Annual Convention

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Interpreting Geochemical Exploration Data Using the Vertical Migration Model

Abstract

Vertical migration is the basis of near-surface geochemical exploration. Below the water table gases migrate vertically in a gas phase and upward movement is due to buoyancy. The surface expression of hydrocarbon gases rising vertically from reservoirs makes possible detecting and mapping petroleum reservoirs. Extending this model to include liquid hydrocarbons explained the observed differences between gas and liquid surface expressions. This general vertical migration model is used for nearly all geochemical exploration data interpretation. Many of these modern interpretation concepts were developed in oil fields located in the Southwest Section area. A review of selected oil fields on the Eastern Shelf of the Midland Basin shows how the vertical migration model developed to correctly interpret data at multiple locations. Wages Dreamfield (Tannehill Ss) in King County, Texas, included five producing wells surrounded by dry holes. Geochemical interpretation integrated with subsurface geology recognized a slight eastern (up-dip) lateral migration that was successful in locating a narrow productive channel between two dry holes. Grantham field (Tannehill Ss) in Dickens County, Texas, was discovered from a dry hole and regional mapping. Geochemical data indicated significant hydrocarbons over the prospective area and was one of the primary tools used during development. However, an unknown local southerly dip created a small offset in the surface expressions of this reservoir which greatly complicated data interpretation. Navigator Field, also in Dickens County, was discovered and developed using an array of exploration tools. The geochemical exploration data consisted of soil gas data and GeoPAC® fluorescence liquid petroleum data which each supplied valuable information about productive reservoirs in a complex geological setting. Recognizing the migration pathways and differences between gas and liquid petroleum migration improved the effectiveness of geochemical data interpretation in these case studies. Knowledge of regional and local dip helped improve the accuracy of reservoir locations from surface expressions. This paper discusses when to look for, how to recognize, and how to interpret the vertical migration information from surface expressions of geochemical data.