Traps Formed By Stratigraphy
speaking, there are four main types of unconformities:
The last two mentioned are the only ones to concern us, and are illustrated below.
Unconformities and disconformity
The contact between sedimentary layers and intrusive salt, gypsum, and shale domes is very similar to an angular unconformity, but the process is caused by compression and the traps are considered to be structural rather than stratigraphic.
can be classified:
according to the amount of missing geological time as:
there is no change in dip trend between the upper and lower strata
of a paraunconformity or a disconformity, it may go completely
unnoticed except for changes in microfossils. However, a disconformity
may be detected by the following features:
- local erosion, which can result in a local high or local low at the unconformity surface. When deposition resumes, this dipping surface is filled by the overlying sediments. The erosional surface is sometimes apparent on logs.
Disconformities and angular unconformities are relatively easy to spot on a dipmeter log; they are the so-called black patterns, or major changes in dip angle or direction. The dip of bedding planes above an angular unconformity differs from that of the bedding planes below. The underlying strata have been tilted during the period of non-deposition. Like faults, angular unconformities are characterized by a change of trend of dip (either dip angle or dip azimuth or both). In addition, erosion may cut a pre-existing structure and produce an irregular topography, characterized by varying dip angles and directions on the old surface.
There may be no significant curve shape anomaly at an unconformity, for example where one shale bed lies unconformably on another. There may be a resistivity or apparent porosity change due to differences in silt content or shale compaction. If the unconformity is at the top of a sandstone, the erosional surface may create a very sharp break at the top of a regressive sequence or at the top of a high energy sequence, but may go unnoticed at the top of a transgressive sequence.
It is easy to mistake a fault for an angular unconformity and vice versa, based on dip information only. In general, dip is steeper below the unconformity surface than above it, although more recent tilting may have altered this relationship. Weathering, local erosion, and change in rock quality may occur at an angular unconformity as well as at a disconformity, and gouge or brecchia may occur at a fault. Both cases produce erratic dips at the boundary.
Unconformities and disconformities do not always form traps, but if porous rock lies below and shale above the unconformity, the regional picture may provide the trapping mechanism, especially in the case of angular unconformities. The most familiar unconformity sand trap in the United States is the East Texas field; it has produced over 3.1 billion barrels of oil since its discovery.
A similar unconformity in Canada, with far less reserves, is formed by the Mississippian unconformity, capped by Jurassic shales. Similar traps, not too far away, occur where the Jurassic sandstones pinch out under the Lower Cretaceous shales. A typical unconformity trap is illustrated below.
Porosity Permeability Pinchouts
The large Pembina field in Alberta is a good example of this type of trap. These traps extend for many miles along a fairly narrow belt at the updip limit of the sand. Although sand pinchouts are stratigraphic traps, folding and faulting may be important in controlling production.
Frequently porous limestone or dolomite grades updip into a non-porous rock. These are called porosity permeability pinchouts and may be of local or regional importance. The Carthage gas field in east Texas is an excellent example of a permeability pinchout. The producing limestone grades updip into an impermeable limestone that is barren. Later arching of the sediments formed the Carthage pool that covers nearly 250,000 acres.
A porosity permeability trap is formed when the thin edge of the porosity is updip from the thicker part of the zone.
Obviously very complex combinations of deltas, cut by channels, and faced by offshore marine bars can exist. An example is shown below. The same comment is true in river channel cut and fill situations where meandering streams can provide a complex depositional pattern, unfortunately seen only as isolated one dimensional views by the logs run in the well bore.
The reef core grows upward and usually outward as the sea level rises. Detrital reef material falls on the ocean side, forming the fore reef. The back reef is formed on the lagoon or quiet side by deposition of limestone and lime mud.
If sea level rises too fast, the reef may drown and die. If water level drops it may begin to grow again, forming very complex structures. Some examples are shown below.
Reefs are usually easily identified by draping dips, often extending several hundred to a few thousand feet above the reef. Dips in the reef core and fore reef are erratic, and those in the back reef may be visible or nonexistent.
Drape and sag
These traps look like folds in a cross section or on the dipmeter patterns. They were not formed by tectonic activity, but rather by the sedimentary process itself. Dips underneath the reef or bar will be regional, in contrast to the anticline. Drape is important in identifying sedimentary structures from dipmeter data, and is often overlooked as a trapping mechanism in the beds lying above the target formation.
Drape is illustrated schematically for both the reef and the sand bar case. Channel fill can also cause drape, again due to differential compaction of surrounding shale. Bedding inside the channel may be complex, but is usually regional under the channel. However, the mass of a reef or channel sand may compact the rock under the body, causing apparent sag below the base of the zone.
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