THE ROLE OF THE LOS BAJOS FAULT SYSTEM IN OIL MIGRATION AND ENTRAPMENT

By Steve Geetan

· The Los Bajos Fault system extends from the Offshore Soldado Main Field area to the onshore Morne Diablo area.

· Historically it is considered a right lateral strike slip fault of 7 miles horizontal displacement, basement related.

· This study considers that the Los Bajos System is a complex linkage of a strike-slip fault into an old thrust fabric. Strain would vary along the strike of the fault being governed by the pre-existing fabric anisotropy. Further, in its onshore expression, the Los Bajos is a detached system more important as a seal than a migration pathway.

The following considerations are relevant:

- A basement related feature should show on seismic. Line TD91-307 is strike perpendicular.to the fault and can be easily interpreted using well, outcrop, and seismic continuity as a lateral ramp feature.

-Surface geology work does not identify the existence of the Los Bajos Fault in the vicinity of TD91-307. Kugler surface geology map is incorrect in this area identifying Morne L'Enfer sediments as Lower Cruse. The Morne L'Enfer sediments are overturned in their field expression, oil impregnated and stands out as high relief areas. This outcrop pattern is interpreted as a lateral ramp fold with oversteepened limbs.

The Los Bajos Fault in the vicinity of TD91-307 is a detached feature and suggests that no deep crustal shear is present along a NW-SE trend. The crustal shear is probably oriented E-W (the strike of the LBF in Main Field) with the land expression of the Los Bajos being a cover response after it was shunted into the thrust related shear zone. This system represents an excellent seal based on oil-field juxtapositions on either side of the fault zone.

- Recent work published by Trinmar documents that the Los Bajos fault does not continue north past Main Field and does- not cut the Warm Springs Fault.

-The migration/entrapment system for the north and south walls of the fault are different. The north wall is more highly deformed and records a thrusted fabric. Migration from source to trap would be intimately associated with movement along the thrust faults.
The south wall may contain its own migration entrapment system more intimately associated with strike-slip and extensional faults.

-In its land expression, the Los Bajos fault system is a more effective seal than a migration pathway. Oil generated in the north wall would migrate along the ambient stress gradient governed by the thrust fabric. Oil can become "ponded" into lateral structures near the Los Bajos and along strike parallel structures. The strike parallel structures are sensitive to fault breakthrough since it appears that these faults are not as good a sealer as the Los Bajos. The strike parallel features therefore require structural closure to retain their charge, whereas the Los Bajos justifies fault dependent closure.

Is there enough justification for a Geophysical Study of the SE section of Los Bajos ?

from a local consultant:

In the attached maps (not included due to copyright), Dr. Pindell shows Los Bajos as a possible Early Jurassic fault (note it is sinistral here).

The Los Bajos we know today may be a Late Miocene - Pliocene re-activation (that is dextral).

Most Regional Workers ( Including Holcombe, Case and Mann) indicate that Los Bajos may be continuous over the Gulf of Paria and inter-related with the El Pilar / Moron / Bonoco Fault system.

Since the 1997 Earthquake in Venezuela there is significant (shallow micro-earthquakes) seismicity along the NW trend of Los Bajos based on the attached Seismic Map (1997-2004). The seismicity could be along El Soldado Wrench as well as Los Bajos. I have asked the Geophysical Experts to tell us more. It is unclear if that seismicity has always been there or began or was aggravated subsequent to 1997.

Other workers suggest there is a tear in the S. American Plate as it subducts the Caribbean Plate in the NW Gulf of Paria region. Reference "Neotectonics"

I have mapped what appears to be a triple- multiple junction with Los Bajos ( at least the SE end through Trinidad) as a potential boundary fault. I have asked regional tectonic workers with more experience to look into the possibility of this.

If the fault is at least Early Jurrasic ( as the Pindell Map suggests )- that implies it may be basement related and thus a candidate for being transform in an area with many possible microplates (Holcombe and Mann)

I believe there is enough data to suggest a seismic threat if it re-activates and the Atlantic LNG facility is upon it with 4 trains as well as Trinmar's Tank Farms and the Point Fortin Junior Secondary School. The political ramifications should be secondary to potential loss of life in Point Fortin even if the fault does not move for the next 100 years. The 1997 Venezuela earthquake should be warning enough. 80 people died.

Thus, I have recommended that Petrotrin initiate a more throrough geo-physical study into the faults and its inter-relationships with El Pilar, Warm Springs and the Guapo Fault before we speculate on how significant the seismic threat is, what to do and what the political ramifications are. I believe there are enough local and forgein tectonic workers doing studies in the area to determine if this is really a triple-multiple junction and if Los Bajos is a more dangerous fault than previously believed.

Anyone with more data that can assist us in clarifying the matter or can assist in such a study, please have your say so that we may advise Petrotrin accordingly...as is our civil right and moral duty to do so.

I am forwarding an e-mail which I received recently. The first attachment is a map which I sent to this gentleman together with my interpretation of it. I am not entirely pleased that the map has been circulated to 12 unnamed geologists without my being acknowledged as the source and with an interpretation quite different from mine. I am unable to contact the 12 geologists directly since I do not know who they are but they are likely to be members of GSTT. Would it be possible to remind all members that there are seismicity maps for the entire eastern Caribbean (including the Gulf of Paria) on our website at http://www.uwiseismic.com. These data can be used by anyone subject to the disclaimer at http://www.uwiseismic.com/disclaimer.html

For anyone who is interested the Los Bajos Fault is already being intensively seismically monitored thanks in great part to funds provided by the Atlantic LNG company. We can say with a great deal of confidence that the land portion of the fault is seismically inactive - there have been no earthquakes on this fault since the Seismic Research Unit began operations in 1952. It is possible, I suppose, that the fault continues as a physical feature across the Gulf of Paria but there is no evidence whatsoever to suggest that any part of it is seismically active unless it suddenly turns sharply westward halfway across the Gulf.

With Regards

John

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