Overpressure and hydrocarbon accumulations in Tertiary strata, Gulf Coast of Louisiana
Search and Discovery
By: Philip H. Nelson
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Abstract
Many oil and gas reservoirs in Tertiary strata of southern Louisiana are located close to the interface between a sand-rich, normally pressured sequence and an underlying sand-poor, overpressured sequence. This association, recognized for many years by Gulf Coast explorationists, is revisited here because of its relevance to an assessment of undiscovered oil and gas potential in the Gulf Coast of Louisiana. The transition from normally pressured to highly overpressured sediments is documented by converting mud weights to pressure, plotting all pressure data from an individual field as a function of depth, and selecting a top and base of the pressure transition zone. Vertical extents of pressure transition zones in 34 fields across southern onshore Louisiana range from 300 to 9000 ft and are greatest in younger strata and in the larger fields. Display of pressure transition zones on geologic cross sections illustrates the relative independence of the depth of the pressure transition zone and geologic age. Comparison of the depth distribution of pressure transition zones with production intervals confirms previous findings that production intervals generally overlap the pressure transition zone in depth and that the median production depth lies above the base of the pressure transition zone in most fields. However, in 11 of 55 fields with deep drilling, substantial amounts of oil and gas have been produced from depths deeper than 2000 ft below the base of the pressure transition zone. Mud-weight data in 7 fields show that "local" pressure gradients range from 0.91 to 1.26 psi/ft below the base of the pressure transition zone. Pressure gradients are higher and computed effective stress gradients are negative in younger strata in coastal areas, indicating that a greater potential for fluid and sediment movement exists there than in older Tertiary strata.
| Publication type | Article |
|---|---|
| Publication Subtype | Journal Article |
| Title | Overpressure and hydrocarbon accumulations in Tertiary strata, Gulf Coast of Louisiana |
| Series title | Search and Discovery |
| Volume | August 2012 |
| Year Published | 2012 |
| Language | English |
| Publisher | American Association of Petroleum Geologists |
| Publisher location | Tulsa, OK |
| Contributing office(s) | Central Energy Resources Science Center |
| Description | 33 p.; Article 41000; col. ill.; maps (col.) |
| Larger Work Type | Article |
| Larger Work Subtype | Journal Article |
| Larger Work Title | Search and Discovery |
| Country | United States |
| State | Louisiana |
| Other Geospatial | Gulf Coast |
| Google Analytic Metrics | Metrics page |