--> Fluid Property Prediction in the Perdido Fold Belt - Gulf of Mexico, USA. Integration of Structural Geology, Organic Geochemistry and Basin Modeling, by Klaus Leischner, Gino Birbiglia, Andy Bishop, Aly Brandenburg, and Eric Tegelaar; #90062 (2007)

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Fluid Property Prediction in the Perdido Fold Belt - Gulf of Mexico, USA. Integration of Structural Geology, Organic Geochemistry and Basin Modeling

Klaus Leischner1, Gino Birbiglia2, Andy Bishop1, Aly Brandenburg3, and Eric Tegelaar4
1 Shell Oil Company, 200 North Dairy Ashford, Houston, Texas, 77079
2 Sarawak Shell Bhd., 98100 Lutong, Locked Bag No.1, Sarawak, Malaysia
3 Petroleum Development Oman, P.O. Box 38, Mina Al Fahal, 116, Sultanate of Oman
4 Shell Oil Company, PO Box 61933, New Orleans, Louisiana, 70161

The Perdido Fold belt in the deepwater section of the United States’ Gulf of Mexico, Alaminos Canyon protraction area, is characterized by hydrocarbon accumulations with highly variable fluid compositions and properties. Several factors control the distribution of fluids, like the contribution from different source rocks, highly variable maturities of different source rock levels, and biodegradation. Additionally, the complex structural evolution and the development of severe overpressures have a significant impact on the fluid fill of individual structures. In order to understand the current fluid distribution of the discovered fields, and more importantly, to be able to predict the fluid properties in un-drilled structures, it is essential to unravel the interplay of all contributing factors.

This presentation discusses a work flow which integrates detailed geochemistry, to identify fluid families, with a basin modeling effort to understand the spatial and temporal development of the petroleum systems, in conjunction with a more rigorous assessment of the structural evolution, e.g. salt emplacement and uplift.

Geochemistry

A detailed geochemical study, including bulk fluid properties, molecular geochemistry and detailed gas geochemistry has been conducted. Intriguing lateral and vertical variations are observed in geochemical properties.

The results of this study indicate that the oils in the main field of the area are dominated by charge from a rather mature carbonate source rock, whereas the origin of the solution gas and volatile fraction is less clear. To the east a stronger control on the fluid composition by a marine clastic source rock is observed. The highly variable composition of the fluids at a small scale is controlled by the contribution of different proportions of these different hydrocarbon source components.

Additionally, the rather low quality oil at shallower levels results from the initially low API gravity of the oil expelled by the carbonate source rocks in combination with biodegradation due to the relatively cool temperature history.

Basin Modeling

A detailed 3D basin modeling study has been conducted to better understand the relationship between the formation of the individual structures, their temperature history and the observed fluids. Although an accurate prediction of the fluid distribution is not possible, uncertainty analysis on source rock parameters (distribution, richness and organic matter types) highlights important trends, which can be used to gain insight into the fluid distribution in un-drilled areas.

Critical to this assessment is the integration of the temperature and pressure evolution of the traps with a reasonable understanding of the main PVT properties of the fluids. The in-house basin modeling tools allow modeling the PVT behavior within individual traps, which then can be compared to the observed fluids at a large scale. Because the basin modeling tools do not allow modeling at the required resolution for the complex geometries at individual trap levels, some generalizations must be made.

It is very clear that, between the major structures, the deep grabens likely contain deep carbonate source rocks that are very mature (>1.4%Vre). Modeling indicates they expelled most of their hydrocarbons pre-30Ma, when structures were not formed yet or as AFTA indicates, still had to undergo a major re-structuration. Underneath the individual traps the timing of the expulsion of hydrocarbons is overall later, and expulsion from the carbonate source can be assumed to be more favorable particularly to the west. To the east clearly the shallower source rock, dominated by a marine clastic facies, controls the properties of the trapped fluids.

All of the modeled reservoirs have highly variable temperature histories, which are a function of salt thickness, timing of salt emplacement, depth of burial and distance to salt. Early in their evolution most structures were rather shallow, hence an early oil charge will be more biodegraded, and often is later overprinted by fresher, more mature charge. Shallower intervals are even today at reservoir temperatures below ca. 55oC, and show strong indications of biodegradation. To gain a better view on the extent of biodegradation, an attempt has been made to model biodegradation with Shell’s in-house software.

The formation of late overpressure has lead to the breaching of several traps, which adds a further complexity to the understanding of the petroleum system in the area.

In conclusion, only the integration of the different subsurface disciplines allows characterizing this rather complex petroleum system and ultimately forms an essential foundation to carry out a consistent risking of the fluid properties away from un-drilled areas.

 

AAPG Search and Discovery Article #90062©2006 AAPG Hedberg Research Conference, Veracruz, Mexico