Stratigraphic Traps

Under hydrostatic conditions, hydrocarbons may accumulate in hydrostatic traps, which include structural traps, stratigraphic traps and combination traps (Section 5.1). 

Under hydrostatic conditions, the hydrocarbons will become trapped in the reservoir rock when buoyancy-induced lateral upward hydrocarbon migration in the carrier-reservoir rock is stopped by a capillary pressure boimdary. Hydrostatic trapping positions include structural traps, stratigraphic traps and combination traps. The maximum height of a hydrocarbon column that can be contained in a hydrostatic trap is determined by the sealing capacity and geometry of the rocks, or rocks and faults, that form the trap. 

Traps (stratigraphic and structural) must exist so that accumulations can form. 

Stratigraphic traps where impermeable strata seals the reservoir... 

Tar sand deposits are widely distributed throughout the world (Fig. 2) (5,6) and the various deposits have been described as belonging to two types stratigraphic traps and stmctural traps (Table 2 Fig. 3) (7). However, there are the inevitable gradations and combinations of these two types of deposits, and thus a broad pattern of deposit entrapment is beheved to exist. In general terms, the entrapment character of the very large tar sand deposits involves a combination of both stratigraphic and stmctural traps. 

Fig. 3. Types of traps for tar sand deposits, where 0 represents a stratigraphic trap, x, an intermediate between stratigraphic and stmctural/stratigraphic traps H, a stmctural/stratigraphic trap , an intermediate between stmctural/stratigraphic and stmctural traps and A, a stmctural trap.

Stratigraphic traps are formed by depositional and sedimentary factors. In such traps the depositional process and the follow-on cementing process, which changes the sediment bed into a rock, create porosity and permeability alterations in geometric forms that provide traps. 

Combination tra/ s—sedimentary trap features that result from both stratigraphic and structural mechanisms. There can be many combinations for stratigraphic and structural traps. An example of such a trap would be a reef feature overlaying a porous and permeable sandstone, but in which the sequence has been faulted (see Figure 2-54). Without the fault, which has provided an impregnable barrier, the hydrocarbons would have migrated further up dip within the sandstone. 

A new-field wildcat is a well located on a structural feature or other type of trap which has not previously produced oil or gas. In regions where local geological conditions have little or no control over accumulations, these wells are generally at least two miles from the nearest productive area. Distance, however, is not the determining factor. Of greater importance is the degree of risk assumed by the operator, and his intention to test a structure or stratigraphic condition not previously proved productive. 

The C02 can be stored in supercritical conditions, rising by buoyancy and can be physically held in a structural or stratigraphic trap, the same way as the natural accumulation of hydrocarbons occurs. The advantage of the capacity of containment system has been demonstrated by the retention of oil for millions of years. If the site is in production, it is used to increase the recovery of oil or gas (EOR recovery - enhanced oil, gas-enhanced recovery - EGR). These operations, EOR/EGR, provide an economic benefit that can offset the costs of the capture, transport and storage of C02. 

Vroblesky, C. D., Robertson, J. F., and Rhodes, L. C., 1995, Stratigraphic Trapping of Spilled Jet Fuel Beneath the Water Table GroundWater Monitoring Review, Spring, pp. 177-183. 

Fig. I. Types of natural gas reservoirs and entrapments (a) anticlinal trap (b) coral reef trap (c) stratigraphic trap (d) fault trap and (e) unconformity...

Some estimates indicate that about 65.8% of ultimately recoverable natural gas in the United Slates will be found in structural traps the remaining 34,2% in stratigraphic traps. 

Stratigraphic Traps. Petroleum geologists also seek another kind of trap, the stratigraphic trap, which results when a porous layer is pinched ... 

Reef-Type Traps. These have accounted for some of the most important production in recent years. The reef is generally considered a type of stratigraphic trap. The reef was formed under the right combination of conditions by the remains of millions of small underwater animals. Building their limestone residences on top of those built by their ancestors, the tiny... 

Fig. 5. Example of a stratigraphic trap. This unconformity represents the condition where upward movement of oil has been halted by the impermeable cap rock laid clown across tine cutoff (possibly by water or wind erosion) surfaces of the lower beds. This type of reservoir is found in the great East Texas field...

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