Structural and stratigraphic traps

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. 

Oil and natural gas reservoirs are found in sedimentary rocks which are either porous sandstone or porous limestone. The oil is held within these permeable rocks by structural or stratigraphic traps and by capillary forces (1). The oil is produced from the reservoir by making it flow into a production well, initially under its own pressure (primary production) and later by injection of water (secondary production) or other displacing fluids (tertiary production) into the reservoir via injec-... 

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. 

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 structural 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 believed to exist. In general terms, the entrapment character of the very large tar sand deposits involves a combination of both stratigraphic and structural traps. 

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

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

Hydrodynamic conditions affect the sealing capacity of a rock or fault, and consequently influence the holding capacity for hydrocarbons of hydrostatic structural, stratigraphic and combination traps. In addition, hydrodynamic conditions may create additional regions of minimum potential energy for separate phase hydrocarbons, i.e. purely hydrodsmamic trapping positions. 

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. 

For this study a section of Zybza field was selected which had sufficient number of wells for steam injection, for oil recovery and for conduct of different temperature surveys. The section was located on a large stratigraphic trap in the central part of the field. A structure contour map of the section was drawn up, and all the different investigations preliminary to block-cyclic steaming were completed before the start-up of actual steam treatment. 

The attempt to locate on the Saharan Platform and interparticular in the Triassic Province traps of the non-structural type is of particular interest for prospecting in view of the exhaustion of the inventory of unexplored structural traps. The stratigraphic traps are more characteristic of the Paleozoic sedimentary complex because of the presence of wedging zones and angular unconformities in the respective basins. Lithological traps are developed essentially in the sandy-argillaceous formations of the Triassic and result from the facies variations characteristic of these sediments. 

The Tambaredjo field prodnces from the Paleocene age Saramacca formation. These coastal plain sandstones are locally identified as the T-unit and were deposited on top of a Cretaceous unconformity. The sands pinch out updip, south of the field, providing stratigraphic traps for the oil. The pilot area, located in the Tambaredjo field, has imdergone very little structural deformation and minimal diagenesis since the deposition of the Paleocene T-reservoirs consequently the sediments are relatively unconsolidated. This type of sedimentary processes of coastal plain deposition resnlted in reservoir compartmentalization, which is demonstrated by the pressure distribution along the area. 

The Chateaurenard field is located in the Paris Basin, approximately 100 km south ea of Paris (fig. 1). It is situated on a fauited structurai monocline which dips gentiy 2 to the north west. Structurai faults divide the Chateaurenard field into three compartments Saint Firmin, Chuelles and Courtenay. In the Courtenay compartment, the Griselies sands form a structural stratigraphic trap at a depth of 600 m. 

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. 

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