Dr. James N. Kellogg
Andean Geophysical Laboratory,
Dept. of Geological Sciences,
University of South Carolina,
Columbia, SC 29208
(803)777-4501 /
Fax: (803)777-9133

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Abstract:

The PGP results show a high correlation between the residual gravity field and the basement geometry in the Subandean basins of Peru. There is also a high correlation between the residual gravity field in northern Peru and the depths to the reservoir rocks of the Albian-age Cushabatay Formation. Locally, gravity observations help constrain interpretations of salt intrusions and Andean Foothills structures. The PGP interpretations of the magnetic field constrain the depths and geometry of basement structures, including Precambrian and Paleozoic terrane boundaries. Locally, these basement structures control the locations of prospective Andean-age hydrocarbon traps, especially in the Ucayali and Madre de Dios basin.

Project Summary:

The discovery of large quantities of gas and condensate in the Ucayali Basin, coupled with government promotion through the national oil company, PERUPETRO S.A., generated renewed exploration efforts in the Andean Foothills Belt of Peru. The Andean Foothills and flanking basins still remain frontier hydrocarbon exploration areas. Only 6 percent of the estimated reserves of oil have been produced. Gravity and magnetic studies can play an important role in the initial as well as ongoing assessment of basin structure. Potential field data can provide the regional setting for basins and constrain models of deep basin geometry, especially in areas of basement-involved deformation. For example, the gravity and magnetic fields may be used to predict the depth to economic basement within the Andean Foothills and marginal structures, a critical problem for interpretation of the structural evolution and prospectivity of the mountain range and foreland basins.

This project is a continuation project of the Colombian Geophysical Project (CGP) completed in November 1995. The CGP was sponsored by 12 companies and used a range of advanced software for geographic information systems (ARC/Info), structural interpretations (Midland Valley), and gravity and magnetic interpretation (Northwest Geophysical and PG&W) which were also available for the Peru Geophysical Project.

The gravity and seismic interpretations are based on open file data from the Andean Geophysical Laboratory and proprietary data held by Sponsoring companies and PERUPETRO. The gravity coverage includes 17,000 land stations and over 19,000 km of aerial gravity flight lines, and the aeromagnetic surveys cover 370,000 square kilometers of Peru's foreland basins. A Letter of Intent to collaborate in the Project was received from PERUPETRO on October 10, 1995. Final agreement on the terms of the Agreement of Participation between PERUPETRO and the USC Foundation was reached on July 15, 1997.

The Huallaga gravity data and geology maps were received just before the Progress Meeting in August 1997. The Ene Basin aeromagnetic and gravity data, Madre de Dios aeromagnetic and aerogravity data, and Santiago Basin gravity data were received after the Progress Meeting. The PGP Progress Report (August 1997) presented Sponsors with preliminary aeromagnetic and gravity maps, 3D Euler deconvolution of magnetic field maps, 20 observed gravity-formation depth correlation charts, seismic styles for 16 blocks, and a flexural interpretation of the regional gravity field. The PGP Final Meeting was held in Lima, Peru in January 1999.

Gravity point data were compiled and merged for the study area. The primary sources for the PGP gravity coverage are: IPC, Murphy, Perupetro, Mobil, and in-house data in the AGL database. The dataset was greatly enhanced by the inclusion of the new Murphy data for Ucayali Basin, Eurocan survey of Ene Basin, and Mobil data for Huallaga and Madre de Dios basins. The observed correlation between gravity residuals and depth to Early Cretaceous rocks enabled us to use the Gravity Residual map to constrain the Depth to Cushabatay Formation map where no seismic or well data was available.

Regional profiles are interpreted to constrain two-dimensional models of the Foothills and flanking basins. We attempt to conserve rock volume in the "balanced models" with seismic interpretations, time sections, depth sections, 2-dimensional gravity and magnetic models, and retrodeformed sections. Complete Bouguer, regional, total field aeromagnetic, Euler deconvolution, structure map - Cushabatay Formation, and prospective areas maps were produced.

Project Strategy:

The key to the PGP strategy is the AGL's multidisciplinary interative geological and geophysical approach to produce integrated structural solutions:
  • Data: The input data includes-Gravity, geology, well, seismic, and magnetic data. They are assigned a level of reliability or interpretability.
  • GIS Database: First, all of the data is digitized into the Geographic Information System database using ARC/Info software for consistency and ease of map production.
  • 2-D Geological Profiles:Two-dimensional geological profiles are developed using surface geology, well data, and seismic profiles. If seismic profiles are used, the first interpretation is a time section. Well velocity surveys are then used to convert the time section to a depth section.
  • Gravity / Magnetic Model:The regional gravity anomaly was calculated using flexural modeling to determine the depth to base of crust. A 2-D gravity / magnetic interpretation is forward modeled directly from the 2-D geological model using GM-SYS software. Densities are derived from laboratory measurements of field samples and well surveys. Magnetic susceptibilities are derived in part from laboratory measurements of field samples, but are based primarily on published average values for basement rocks. All unmetamorphosed sedimentary rocks are assigned 0.0 magnetic susceptibility.
  • Profiles: If required to volume balance the profiles, the geological and gravity / magnetic models are iteratively modified. The final profiles represent an integrated balanced geological - geophysical interpretation.
  • Top Cushabatay Fm. Depth: A top Cushabatay Fm. depth map is constructed from surface geology, well data, seismic profiles, and 2-D geological profiles. Where no other data was available, gravity residual anomalies were used to constrain the top Cushabatay Fm. depth contours.
  • Gravity and Magnetic Interpretation: An Euler Deconvolution Map of the Aeromagnetic Total Field was produced using GRIDEPTH software. A Gravity and Magnetic Interpretation map is then produced based on the Euler map and the 2-D magnetic models. The Tertiary and Cretaceous rocks are non-magnetic for the most part. Therefore, the depths to the bodies producing the magnetic anomalies are always assumed to be equal to or greater than the depths to the top of the Cushabatay Fm.
  • Recommendations:Finally, th PGP makes recommendations for further gravity, seismic, magnetic, geologic, geochemical, and remote sensing data acquisition based on the Gravity and Magnetic Interpretation map.

Project Products:

The final coverage of the PGP database includes:

Magnetic Data Coverage:
Survey Area Length (km) Grid (km x km) Area (sq km) Flight Elevation (m)
1 to 8 40 maps digitized at scale 250,000m 700-1,200
9 16,500 6x6 and 6x18 94,000 1,200
10 3,473 6x8 15,000 1,200
11 5,530 2x5 9,000 2,200
12 -- 4.5x8 40,000 4,000
Other Data Coverage:
  • Seismic Reflection Lines
    • Interpreted over 2,500 km
  • Electrical Logs
    • 27 oil wells log (Sonic, Velocity, and Density)
  • Geology
    • Geology of 185 oil wells
    • Local field geology
    • Regional field geology
Gravity Data Coverage:
Survey # Stations Length (km) Comments
IPC 9,471 -- Along rivers
DMA/AGL 2,275 -- Along rivers and roads
Mobil-Huallaga 3,381 1,547 Along seismic lines
Murphy Sample: 1 sec 2,775 Aerosurvey (6x8km grid)
Mobil-MDD Sample: 1 sec 16,500 Aerosurvey (6x8km grid)
Santiago 1,860 186 Along seismic lines

The PGP Products are contained in five volumes:

  • Technical Project Report (with data CDs) (Vol. 1)
  • Map Atlas (Vol. 2)
  • Profile and Seismic Atlas (Vol. 3a)
  • Cont. Profile and Seismic Atlas (Vol. 3b)
  • Gravity and Magnetic Interpretation Atlas (Vol. 4)

Summary of Basement Structures:

Three structural styles have been identified n the study area: i) Paleozoic normal faults reactivated as inverse faults in the Quechua orogeny, mainly in the foreland. ii) Thrust faults with basement involved due to intra-basement igneous bodies controlling faults, mainly in the Ucayali. iii) Thin-skinned thrust faults related in some cases to salt diapirism, with at least three levels of detachment, in the Devonian, late Permian and Jurassic. (Huallaga and Santiago basins).

Three zones of magnetic anomalies have been identified: i) Those related to the Guiana shield (Northern part), showing high frequency anomalies, and deep magnetic basement; ii) those related to the Amazon basin (Central part), showing low frequency anomalies; and iii) those related to the Brazilian shield (Southern part), showing high frequency anomalies, and shallow magnetic basement.

Neither reduction to the pole nor reduction to the equator solved the problem of low magnetic latitude. Tests in the study area showed that pole reduction prior to the Euler Deconvolution was not necessary.

The magnetic field appears to have much better correlation with basement and early Paleozoic structures and contacts. In areas where magnetic anomalies follow Andean structures, the structures in the basement influence the Andean deformation.

Gravity anomalies identified include: a) regional anomalies caused by basin boundaries and intra-basement igneous bodies; b) local anomalies related to Andean structures.

There is good linear correlation between residual gravity values and the depths of Cretaceous sequences. The best correlation is found at the top of the Cushabatay Fm. (Aptian-Albian).

In the Ucayli Basin, intra-basement igneous intrusions control its structural style (reverse faults with basement involved).

Gravity and magnetic anomalies have delineated important basement structures including the Purus high, the Inuya Jurua high, the Pisco-Jurua fault, the Contaya arch, the Pisqui-Coninca high, and the Iquitos arch.

Two types of salt rocks were recognized interpreting low-gravity anomalies in the Huallaga Basin.

Future Studies:

The AGL is currently working on logistics for a PGP-Phase 2. A Phase 2 will update the project with new high resolution gravity and magnetic surveys of the Maranon, Santiago, Huallaga, Ucayali, and Ene basins. The new survey data covers the key areas of Maranon basin oil production and large gas reserves in the Ucayali basin.

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Last updated July 10, 2007 by Obi Egbue oegbue@geol.sc.edu
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