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Petroleum System Modeling and Structural Trap Development of Deepwater Subsalt Plays of the North-Central Gulf of Mexico

Keith I. Mahon and Van S. Mount
Anadarko Petroleum Corporation, The Woodlands, TX, USA

Basin and Petroleum System modeling of deepwater subsalt plays in the Gulf of Mexico requires an understanding of how halokinetic processes impact the structural and stratigraphic evolution of the region. A 64 km sub-regional transect that crosses the K2 oil field in the north-central Gulf of Mexico has been restored from the present-day geometry back to the middle-Jurassic using Halliburton’s LithoTect® software. The restoration process uses a combination of vertical shear and flexural slip deformation mechanisms along with a decompaction model for each restoration step. The sequential structural restorations are imported into Schlumberger’s PetroMod® 2D with TecLink® to forward model the petroleum system.

The restoration steps show that the salt canopy was originally a thin nappe emanating from a salt feeder system to the north in the Late Miocene. The canopy continued to inflate throughout the Pliocene as it continued to advance south, burying successively younger stratigraphy. Presently, the canopy is no longer expanding and the rate of southward advance has slowed. Due to increase in Pleistocene aggradation and cessation of canopy expansion, mini-basin development into the allocthonous salt canopy has begun.

Petroleum system modeling results illustrate a complex, dynamic, and temporally variable hydrocarbon migration pathways into the K2 field, a major structural trap that developed in the Late Miocene. This trap is charged laterally by migration along a fault plane that intersects carrier beds and vertically via invasion percolation below the structure. Although K2’s Middle Miocene reservoirs are hot enough today to eliminate the risk of ongoing biodegradation, oil charge during the Pliocene would be at an elevated risk of biodegradation. The models also illustrate the start of thermogenic gas migration from deeply buried source rocks on the north end of the transect. If the late generated gas migrates and mixes with reservoired oil, the liquid’s asphaltene onset pressure may increase to reservoir pressure, or higher, resulting in flocculation and precipitation of asphaltene-rich tars.

Although a 3D model would be preferred, the amount of time and effort required to build a fully restored 3D model is often not practical. This calibrated 2D model is combined with models along intersecting 2D transects. The restoration and modeling results are tied at the transect intersections. The results are used to evaluate and test the petroleum system risks for the plays and prospects modeled.

AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012