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Abstract

The Gulf of Mexico, one of the world’s largest petroleum producing basins, has a complex rift/drift history which began in the Late Triassic with the breakup of Pangaea and ended in the Cretaceous when the Yucatan block reached its present location. Most Gulf of Mexico workers agree that Yucatan separated from North America and rotated into position roughly between 170 and 140 mya; however, its exact path is unclear due to the lack of unequivocal spreading indicators. As a result, the distribution of crustal types, specifically the boundary between oceanic and hyperextended crust is, a source of debate. Understanding the distribution of crustal types provides important insights into basin shape and its influence on sedimentation, basinwide variations in heat flow, and early source rock distribution.

For this study, long offset 2D PSTM seismic was used to map crustal types where the seismic image was not adversely influenced by the overlying Sigsbee salt canopy. Stretched continental crust, a necking zone, and oceanic crust were consistently identified in the eastern Gulf. The tilt derivatives of isostatic gravity and RTP magnetic data were calculated and the resulting maps were compared to the seismically derived distribution of crustal types. In many cases, the potential fields maps showed good agreement with the seismically-interpreted crustal types and were used to infer the distribution of crustal types where seismic was unable to produce an adequate image of the deep structure. Gravity inversion was also incorporated into the crustal mapping.

In addition to seismic and potential fields data, approximately fifty 1D basin models for onshore and offshore Gulf of Mexico exploration wells were constructed. The 1D models were calibrated using burial history, corrected well BHT temperatures, and oils data. In addition, the calibration process involved determining an isostatic equilibrium “thickness” of the total section. The total section is defined as the sum of the water/sediment column + the upper crust (UC) + the lower crust (LC) + the mantle lid (ML). A best-fit isostatic equilibrium thickness of ~88 km and a LC thickness were derived by first determining the LC and ML thicknesses at wells on oceanic crust where the upper crust thickness is 0. A 1km variation in the isostatic thickness was accepted and attributed to well-to-seismic depth conversion error and/or variations in bio-stratigraphic age interpretations. Once the isostatic equilibrium thickness was determined, the upper crust and mantle lid were calculated for each well by adjusting the mantle lid and upper crust thickness to obtain a best fit to each well’s corrected bottom-hole temperatures and solving for a total isostatic thickness of 87-88 km. A total crustal thickness was then calculated.

The total crustal thicknesses from 1D-modeled wells on oceanic and stretched continental crust positively correlated with seismically and potential fields-derived crustal type interpretations. The 1D models identified a discreet area of variable but relatively thin total crust (9-24 km) in the central Gulf. This area positively correlates with an area where the ocean-continental boundary has been interpreted by some authors. Alternatively, the OCB may be farther basin-ward and this area may actually be an area of very thin hyper-extended crust.

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