Unconformity trap

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...

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...

In the upper structural level the presence of lithological traps at the base above the unconformity is more likely. This is observed in the northern part of the Oued el-Mya... 

Depression where there is a south-facing wedge of productive sandy horizons in certain parts of the Triassic and Permo-Triassic (Fig. 2.4). Along this wedge zone of reservoir rocks we may observe lithological traps in the basal rocks of the Mesozoic complex. It is even possible to locate stratigraphic traps resulting from the closure of reservoir rocks by internal unconformities. 

The reservoirs in the Flamra Quartzites and the El-Atchane sandstones which had been eroded along the Hercynian unconformity are wedging out around this uplift. In the Triassic sediments around the northern edge of the uplift a lithological trap might be developed. The reservoirs lie at a depth of 3 000-4 200 m and the quality of the Triassic reservoirs improves towards the north. 

The identification of such dia- or catagenetic barriers is of great importance in the search for traps as the accumulation of hydrocarbons may have taken place below a more pronounced barrier located above the unconformity. 

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. 

Unassociated with unconformities (channels, bars, and reefs) Diagenetic traps (due to solution or cementation)... 

FIGURE 8 Block diagram to illustrate the diflierent types of stratigraphic traps. (A) Fossil reef limestone. (B) Coastal barrier-bar sand. (C) Sandstone infilled channel. (D) Onlap of sand above unconformity. (E) Truncation of limestone beneath unconformity. 

At its simplest, an onlap trap may be a blanket sand that pinches out up-dip (Fig. 8D). It is sealed by impermeable rocks beneath and by an onlapping shale (generally the source rock, as well as the cap). Many unconformities are old land surfaces, however, and sands may be deposited in old topographic lows. Alternating hard and soft sediments may have been weathered and eroded to form scarps, dip slopes, and strike valleys. Fluvial or shallow marine sands may have been deposited along the old valleys and sealed by marine muds. Stratigraphic traps of this type are known as the Mississippian Pennsylvanian imconformity of Oklahoma. Alternatively, the unconformity may have been a planar land surface that was locally incised by alluvial valleys. These may have been sand filled and drowned by... 

In addition, the U-Pb date of zircons in the gabbro of the Cotton Plateau (668 1 Ma) implies that Rodinia broke up somewhat later and that the rift valley between Laurentia and the East Antarctic craton began to trap sediment later than the previous interpretation of the evidence had indicated. The reassignment of the bulk of the Goldie Formation to the Starshot Formation of the Byrd Group also means that the contact between the inboard part of the Goldie Formation and the over-lying Shackleton Limestone is now presumed to be an unconformity. 

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. 

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