Diagenetic traps are formed by the creation of secondary porosity in a non-reservoir rock by replacement, solution or fracturing with the tight unaltered rock forming the seal for the trap (Rittenhouse, 1972).
Figure 1.
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An example of a diagenetic trap formed by replacement is the Deep River field in Michigan, in which dolomitization of a preexisting limestone deposit has resulted in the formation of secondary intercrystalline porosity (Figure 1 ).
The development of solution porosity is commonly associated with carbonate rocks (Figure 2 ), but may involve sandstones as well.
Fracturing can cause secondary porosity in any brittle rock — whether carbonate, sandstone, shale, igneous or metamorphic rock (Kostura and Ravenscroft, 1977). The Spraberry trend in west Texas forms a series of diagenetic traps (with oil reserves of about one billion barrels) within a producing fairway about 240 kilometers long and 80 kilometers wide (Wilkinson 1953). A structure map contoured on the productive Spraberry formation, a 300-meter-thick section of tight Middle Permian shales, siltstones, limestones, and fine-grained sandstones shows that in the southern Midland basin, the areas of oil production have little relationship to structure ( Figure 3 ). Production comes from areas of fracturing throughout the otherwise tight Spraberry formation.
The depositional and diagenetic stratigraphic traps just considered occur in normal comformable sequences, although they may also occur at unconformities.
Figure 2.
Figure 3.
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Another major group of stratigraphic traps is represented by traps for which an unconformity is essential (Table 1 ) (Levorsen, 1934).
Table 1.(no table exist)
Significantly large percentages of the known global petroleum reserves are trapped adjacent to major unconformities. In addition to being held in pure stratigraphic traps, many of these reserves are held in structural and combination traps as well. Unconformity-related traps can be subdivided into those which occur above the unconformity and those beneath (Figure 1 , Schematic of traps located above and below an unconformity).
Traps which occur above an unconformity will be discussed first.
Figure 1.
Shallow-marine or fluvial sands may onlap a planar unconformity. A stratigraphic trap can occur where such sands are overlain by shales and are underlain by impermeable rock which provides a seat seal. Onlapping updip pinch-out sands such as these could occur as sheets (Figure 2a , Schematic of onlapping pinch-out sands-- occurring as a sheet deposit) , or as discrete paleogeomorphic traps ( Figure 2b , Schematic of onlapping pinch-out sands--occurring as a discrete paleogeomorphic sand).
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A good example of an onlap stratigraphic trap is provided by the Cut Bank field of Montana, with recoverable reserves of over 200 million barrels of oil (MacKenzie, 1972). Here the Cretaceous Cut Bank sand unconformably onlaps Jurassic shales and is itself onlapped by younger shales (Blixt, 1941;
Shelton, 1967). Figure 3 , (Southwest-northeast E-log correlation section A-Z, Cut Bank sandstone, Montana) is a cross section through this field.
Figure 3.
One type of paleogeomorphic trap is represented by channels which cut into the unconformity;
another occurs where sands are restricted within strike valleys cut into alternating hard and soft strata (Figure 4 , Schematic of channel and strike valley sands above an unconformity) (Harms, 1966; Martin, 1966; and McCubbin, 1969). It is important to note that closure is necessary along the strike of such traps, not just updip as shown in Figure 2a . In Figure 5 (Schematic of sandstone pinch-out intersecting with a structural nose), closure is provided by the intersection of a sandstone pinch-out with a structural nose.
Figure 2.
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Figure 4.
Figure 5.
The second group of traps associated with unconformities is truncation traps which occur beneath the unconformities (Figure 6 , Schematic of traps below unconformity).
Figure 6.
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Again, it is generally overlying shales which provide a seal (and often the source as well) for such traps. As with onlap, pinch-out, and paleogeomorphic traps, closure is needed in both directions along the strike ( Figure 7 , Schematic of trap below unconformity, featuring closure provided by the intersection of a dipping structural nose and a flat unconformity). This may be structural or stratigraphic but for many truncation traps, it may be provided by the irregular topography of the unconformity itself, such as a buried hill providing closure for a subcropping sandstone formation (Figure 8 , Schematic of trap below unconformity, featuring closure provided by buried hill).
Figure 8.
Many truncation traps have had their reservoir quality enhanced by secondary solution porosity due to weathering. Secondary solution porosity induced by weathering is most common in limestones, but also occurs in sandstones and even basement rock. Examples in limestones are found in Kansas, and in the Auk field of the North Sea (Brennand and van Veen, 1975).
Figure 7.
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Development of subunconformity solution porosity in sandstones has occurred in the Brent Sand of the North Sea (Bowen, 1972), and in the Sarir group of Libya (Selley 1982). Basement rock weathering is found in the Augila field of Libya (Williams 1968, 1972).
One of the best known truncation traps in the world is the East Texas field (Halbouty, 1972; Halbouty and Halbouty, 1982) which contained over 5 billion barrels of recoverable oil. The trap is caused by the truncation of the Cretaceous Woodbine sand by the overlying impermeable Austin chalk ( Figure 9 , Generalized west-east cross-section, East Texas basin). It has a length of some 60-70 kilometers and a width of nearly ten kilometers. Figure 10 (Structural contours on top of Woodbine sand, East Texas field) illustrates the structural closure at the northern and southern ends of the field.
Figure 9.
Figure 10.
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Exercise 1.
On the left is a listing of stratigraphic trap types (a-i).
Match each with the appropriate stratigraphic trap categories (1-4) from the right column.
(There may be some types which occur in more than one category.)
TRAP TYPE TRAP CATEGORIES
a) Barrier bar trap _____ Within Normal b) Truncation trap _____ Conformable Sequences
c) Reef trap _____ 1) Depositional
d) Fracture-related trap _____ 2) Diagenetic
e) Seconary-solution trap _____ Related to Uncomformities f) Onlap trap _____ 3) Above unconformity g) Replacement trap _____ 4) Below unconformity h) Channel trap _____
i) Strike-valley trap _____
Solution 1:
a) Barrier bar trap 1
b) Truncation trap 4
c) Reef trap 1
d) Fracture-related trap 2 e) Secondary-solution trap 2 & 4 f) Onlap trap 1 & 3 g) Replacement trap 2 h) Channel trap 1 & 3 I) Strike-valley trap 3 Exercise 2.
Construct a cross section through an unconformity showing some of the different types of stratigraphic traps which can occur.
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Exercise 3.
Your company is developing the concession on the maps shown inFigure 1 ,Figure 2 andFigure 3.
A seismic reflector is contoured on the map; it conforms to the paleoslope of the sediments. Well A found oil in a barrier bar sand above the reflector at 2100m. Well B found oil in a channel sand below the reflector at 2300m, but did not encounter the barrier bar sand above the reflector.
Indicate on the map the most likely extent of the two fields. Then construct a dip-cross section along line S-T, and a strike-cross section along line U-V. Pick the best well location for reservoir development.
Figure 1.
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Figure 2.
Figure 3.
Figure 3.
The approximate extent of the two traps is shown inFigure 4 ,Figure 5 and Figure 6 , along with the next location.
This combines the opportunities of testing the southeastward extension of the barrier bar sand, and the southwestward extention of the channel sand.
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