Petroleum Traps Formed By Stratigraphy
An appreciation of sedimentary structures helps greatly in trying to analyze dipmeter patterns. Some dip patterns can point to several possibilities, so it is nice to know what is possible, what is probable, and what is impossible (bed boundaries that cross each other, for example.


1. Unconformities
An unconformity is a hiatus in the normal geological sequence caused by a break in the process of deposition, by erosion, or by structural deformation. It results in a missing amount of sediments corresponding to a missing geological time as compared to the normal sequence. It is made of two different series of strata separated by a surface of unconformity.

Strictly speaking, there are four main types of unconformities:
   1. nonconformity, in which sediments overlie igneous rocks.
   2. paraunconformity, in which strata are parallel on both sides of the unconformity, but some of the rock sequence is missing, due to lack of deposition (not due to erosion).
   3. disconformity, in which strata are also parallel on both sides, but there is an erosional surface as well as the missing section. Lack of deposition may also have occurred.
   4. angular unconformity, in which the strata above and below are not parallel. Erosion has almost always taken place.

 

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.

Unconformities can be classified:
   1. according to lateral extent as:
      - regional, occurs across a large area or possibly the entire basin
      - local, occurs over a small area

   2. according to the amount of missing geological time as:
      - major, where a long time sequence is missing
      - minor, where a short time span is missing

Since 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:
      - weathered zone, reflecting the effects of the work of ground water, such as solution vugs or caverns, brecchia, jumbled rock, or cross bedding occurring immediately above or below the the disconformity surface.

      - 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.


Typical unconformity trap

2. Porosity Permeability Pinchouts
As a general rule, shallow water sandstone beds are likely to thin, and deeper water beds such as limestones are likely to thicken, away from the shoreline. If a rock layer continues to thin in a certain direction, it may finally pinchout or lense-out altogether. The beds above and below it will then become contiguous. This can happen to blanket sands, beaches, bars, and delta fronts and are called sand pinchouts. River channel fill and point bars in meandering streams and rivers thin toward their edges; this effect is also called a pinchout. The thin edge of any sand body can be described as a pinchout.

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.


Sand pinchout (top) and Sand channel (bottom)

A porosity permeability trap is formed when the thin edge of the porosity is updip from the thicker part of the zone.


Permeability pinchout trap (shaly sand shown, similar traps are also formed in carbonates)

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.


Complex stratigraphic traps

3. Reef Traps
Reefs are productive in many parts of the world. Many types exist, such as atolls, table reefs, pinnacle reefs, barrier reefs, fringing reefs, biostromes, and bioherms. They occur as small dome like features that may be reflected in the overlying sediments by drape. Drape is described in the next section of this Chapter. Some reef trends extend for hundreds of miles, such as the Leduc reef trend in Alberta. The size of a reef ranges from a few acres to several square miles. Seismic exploration is the best way to find reefs.

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.


Reef trap

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.


Complex lithology of a Devonian reef

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.


4. Drape Traps
Differential compaction causes drape over reefs and sand bodies and this can form traps. A sandstone or carbonate layer above the bar or reef can be bent in such a way as to have closure, that is, the ability to contain and trap hydrocarbons. The bending is caused by the fact that the reef or sand body does not compress to the same degree as the shales to either side of it. Therefore a topographic high can be propagated upward through the section for quite some distance.

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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