Northern Gulf of Mexico deep prospects sought
Andrew Mehlhop
Anadarko Canada Corp.
PART I: This is part one of a two-part series on salt tectonic style classification as an aid to exploration.
Exploration for new discoveries in the northern Gulf of Mexico is now domi-nantly focused on deep targets between 25,000 and 30,000+ ft in both the shallow water shelf and within the vast deepwater realm. Current structural models subdivide the northern Gulf of Mexico into nine distinct tectono-stratigraphic provinces. The distribution of these provinces is depen-dent upon the following first-order controls:
- Depth and structure of the basement
- Original thickness of Jurassic mother salt
- Thickness and distribution of Tertiary clastic depositional sequences.
Each province exhibits a unique salt inflation/deflation history that differs from adjacent provinces. As a result, each province has a unique a set of predictable structural styles, trap-types, and deep chrono-stratigraphic objectives. Therefore, drilling targets beyond 25,000 ft will vary from one province to the next. Here are criteria for high-grading areas in the Gulf of Mexico based on timing of canopy emplacement, salt system type, and salt/sediment balance.
Sequence stratigraphy and salt tectonics
The following fundamental principles of regional salt/sediment interaction were applied:
- The regional distribution of salt is a direct expression of the sequence stratigraphy of the basin
- Vertical salt movement (diapirs, sub-vertical ridges, roots, base of salt ramps) occurs on a sub-regional scale during sequences of active deposition
- Lateral salt movement (canopy emplacement, base of salt flats) occurs on a sub-regional scale during sequences of non- deposition (maximum flooding surfaces or condensed sections).
The careful mapping of deep salt evacuation surfaces (welds) is a key technique for unraveling the timing of regional salt canopy emplacement. Because each salt province exhibits a unique salt inflation/deflation history, salt canopy emplacement records the sub-regional variations in the sequence stratigraphy of the clastic load. In general, regional salt canopies are older in the north and younger in the south due to the progradational nature of sedimentation in the Gulf of Mexico. In addition, lateral salt distribution varies from east to west due to regional avulsion patterns of the deltas.
Within individual salt provinces, most of the major laterally extensive salt canopies were formed at nearly the same time. For example, along the outer shelf Flextrend province, the shallowest sub-horizontal salt canopies have numerous well penetrations from early sub-salt exploration on the outer shelf. In the majority of these sub-salt wells along the Southern Addition areas, the major lateral component of each canopy ties to the 3.72 million year ago (mya) Pliocene nannofossil Reticulofenestra pseudoumbilica. In wells outboard of these Pliocene salt canopies, the same 3.72 mya event is noted by biostratigraphic data in wells as a condensed section or maximum flooding surface with high faunal abundance.
The northern Gulf of Mexico is divided into nine tectono-stratigraphic salt provinces, modified after Hall, 2001.
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Most of these laterally extensive canopy complexes do not exist across the entire Gulf of Mexico; rather, they are tied to sub-regional maximum flooding events that are local to one or more provinces. In the South Auger province directly south of the Flextrend province, the same 3.72 mya event is not condensed, but a site of active clastic deposition due to sediment bypass to the north. In other provinces, regional canopy development did not occur at the 3.72 mya event due to continual clastic deposition or a lack of mobile salt relative to sediment. The regional implication is that these maximum flooding events are local in areal extent and are actually responses to local avulsion patterns in the deltas. In some cases, they can be correlated to global sea-level curves, but in areas of active deposition, the same event is not condensed, but is a fairway of abundant clastic deposition. Therefore, the areal distribution of regional salt canopy emplacement is directly related to the regional avulsion patterns of the major deltaic "staging areas" on the shelf. This model can be applied for Miocene and older sequences to reduce the risk for deep exploration where updip deltaic avulsion patterns can be related to the lateral distribution of deep weld surfaces.
Salt system type and sediment-salt balance
Regional salt provinces are described as single-tiered or multi-tiered salt systems. In a single-tiered salt system, the allochthonous salt diapirs and canopies are directly attached to the autochthonous mother salt. There is a strong relationship between deep structures and basement elements. Single-tiered salt systems are lower risk areas for deep exploration because the strong vertical linkage of allochthonous salt to the mother salt tends to create large downward-flexing structures.
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In a multi-tiered salt province, deep drilling targets lie upon a deep weld surface that was emplaced as an early canopy structurally above the mother salt. In multi-tiered salt systems, deep targets are not generally related to basement structure, but are structurally controlled by the nature of salt withdrawal structures that form in classic stepped counter regional, salt stock canopy, and roho salt evacuation systems.
As clastic sedimentation distributes a load upon the original autochthonous mother salt, the salt becomes mobilized and rises vertically into sub-vertical diapirs and linked salt ridges. During periods of regional avulsion of the clastic sediments out of the plane of section, the vertical diapirs begin to move laterally into a coalescing canopy complex. These early canopy complexes are eventually preserved only as deep weld surfaces with vertical diapirs and shallower canopies rooted to them.
The total balance of salt and sediments within sub-regional areas of the Gulf of Mexico is one of the most important criteria in defining as well as high-grading individual salt provinces. In general, the maturity of an area is related to the total sediment-to-salt ratio. An immature or salt-dominated area is described as one in which the total salt volume is much greater than the total sediment.
Laterally extensive salt canopy development occurs as a linked system in a prograding sequence stratigraphic framework. The salt/sediment balance is a criterion for high-grading salt systems.
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Salt-dominated provinces are generally areas at highest risk for exploration. When there is too much salt in the system, minibasins are dominantly upward flexing on all sides and have not yet "touched down" to basal weld surfaces. Upward flexing minibasins are sometimes near circular in shape and lack closure along all upturned basin edges. Additionally, in areas of abundant salt relative to sediment, source rocks may be just entering the oil window due to lack of burial and the "wicking away" of heat by the salt.
Hydrocarbon migration is impeded by the lack of weld surfaces and complex faulting. Structural styles are limited and field sizes are generally small in areal extent.
An even balance between total salt volume and sediment input generally leads to a broader range of structural styles and better hydrocarbon maturity and migration. This type of system is partially welded. Minibasins are commonly asymmetrical with a steep landward side and a downlapping basinward side. Subsalt traps remain high risk due to upward flexing closures on the landward sides of minibasins; however, more abundant fault traps exist, which lower the overall risk within the region. Classic structural styles related to salt stock canopy, stepped counter regional, and roho evacuation systems exist in an evenly balanced salt province. Medium to some large field sizes are expected in areas of faulted sigmoid structures caused by partial inversion of these asymmetrical minibasins.
Mature, sediment-dominated provinces exist where the total salt volume is much less than the total sediment input. This is usually caused by the near complete evacuation of deep salt. Minibasins are mature with large, downward flexing closures and turtle structures. Abundant faulting enhances hydrocarbon migration and trapping leaving a maximum range of structural styles with some small, medium, large, and super-giant fields. Sediment-dominated areas are generally lowest risk for deep exploration.
Finally, the timing of salt movement within each province is critical to the prediction of subsalt and deep stratigraphic objectives.
The salt inflation and deflation history differs for each individual province within the northern Gulf of Mexico. Through the regional mapping of deep salt welds and their relationship to the regional knowledge of the updip staging areas of the deltas, a generalized sediment/salt history can be concluded for each tectono-stratigraphic salt province.
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In conclusion, each salt province in the Northern Gulf of Mexico has a unique set of structural and stratigraphic characteristics. Some of these provinces are prolific areas for large hydrocarbon accumulations, while other provinces have very few discoveries. Basic observations such as salt system type, salt-sediment ratio, and timing of salt movement explain why some salt provinces are better than others for deep hydrocarbon exploration. The areas of highest risk are salt-dominated, multi-tiered areas with little indication of an updip sandy deltaic staging area. Lowest risk provinces are sediment-dominated, single-tiered systems with an active updip sandy delta.
Part two of this article will discuss each of the nine provinces in the context of this criteria system with focus on the chief risks for deep exploration. This simple methodology is an effective quick-look technique for reducing the risk of prospecting within the Gulf of Mexico and can be useful in other complex salt basins around the world.
