Rock Facies: Origin, Depositional Environment
There are only four basic kinds of stratigraphic traps: unconformities, porosity permeability pinchouts, reefs, and drape structures. However, within the permeability pinchout category, there are many different types. Knowing which type is crucial to understanding how to explore for, and develop, these reservoirs.
The methods used to identify stratigraphic traps from logs involve curve shape analysis for grain size and environment, analysis of dipmeter data for definition of bedding, and conventional log analysis calculations for porosity and lithology. In addition, the use of formation microscanner images to assess detailed stratigraphy is becoming more common.
The end result of the analysis is a description of the rock facies and a three dimensional view of the sedimentary structure. This will include the type of structure, thickness, reservoir quality, and if possible, its shape and probable extent.
As with any log analysis technique, calibration and control by using core and sample descriptions is very beneficial. In addition, well to well correlation and mapping can be used to help confirm stratigraphic interpretation made from dipmeter and curve shape analysis.
A description of a rock by its detailed type, origin, and depositional environment is usually called a facies description. It can be derived by observation of the rocks, or inferred from analysis and interpretation of well log data. To determine facies from well logs requires calibration to known rocks (cores, samples, or outcrops). Understanding the rock facies is the only way to reconstruct the paleogeography of a rock sequence, which in turn provides clues as to a potential reservoir's quality and lateral extent.
Facies description based on well logs is often called electrofacies analysis, because electrical logs are used. However, radioactive and acoustic data is also incorporated, so this Handbook does not stress the term electrofacies, as it is slightly misleading.
rock type can be derived from:
If the world was perfect, all three sources of data would be available and would agree with each other. The data sources do not always agree, so the analyst must learn to compare, contrast, and possibly discard some data.
The origin of a rock can be inferred from its present depositional environment and a reconstruction of paleogeography. Both of these can, at least sometimes, be inferred from log data, especially from dipmeter data, which tells us about depositional energy and direction of transport, in conjunction with other log curves, which suggest the grain size of the rock. Log analysts usually concentrate on depositional environment and bedding patterns, along with dip direction and angle, and provide this information to geologists who make subsurface maps representing the analysis.
who do the whole job need to have special skills in open hole
log analysis and should not rely entirely on the curve shapes
of the raw logs. For example, the curve shapes on SP and gamma
ray logs may be easy to interpret in a conventional shaly sand
sequence, but could be very misleading in a complex sequence of
anhydritic, dolomitic, shaly sands bounded by carbonates.
Radioactive sandstones and carbonates, silty sands (so-called
gas shales), and evaporite sequences require a clear
understanding of all log responses, not just the correlation
of Depositional Environments
Most detrital sediments are continental or transitional, and most chemical sediments are marine.Continental and transitional sediments:
1. glacial - formed by glacial action, eg. gravel bars, drumlins
2. eolian - formed by wind action, eg. sand dunes
3. alluvial - formed by flooding or when fast moving water dumps sediment into slow moving water, eg. deltas, sand bars, beaches
4. fluvial - formed by a river, eg. point bars, channels
5. lacustrine - formed in a lake, eg. mudstones, marls, chert
6. paludal or carbonaceous - formed in a marsh or swamp, eg. peat, coal
The first four describe detrital sediments and the last two chemical sediments.
but the last may be biological sediments and all can
be chemical sediments. However, detrital material
can occur in nearly all of them, including evaporites.
There are four basic kinds of stratigraphic traps: unconformities, porosity or permeability pinchouts, reefs, and drape structures. River channels, beaches, bars, and deltas are sedimentary structures, usually associated with porosity pinchout traps. Drape structures over these may form additional traps.
Nearly one-third of the important oil fields of the United States are stratigraphic traps and many were discovered by random drilling rather than by scientific exploration methods. This indicates that strat traps are fairly common in the subsurface and make up a tremendous potential oil and gas resource. Today, 3-D seismic and sequence stratigraphy have evolved to the point where start traps can be defined quite accurately and even very small targets are drilled on purpose instead of by accident.
The analysis of sedimentary structures from logs, augmented by core, sample, and seismic data, is somewhat complex. There are, however, only a few major types of sedimentation patterns. Most of these patterns can be represented by a set of models which serve as a basis for interpretation and comparison by log analysts. The methods used involve curve shape analysis for grain size and environment, analysis of dipmeter data for definition of bedding, and conventional log analysis calculations for porosity and lithology, followed by geological mapping.
difficulties in identifying sedimentary structures, and hence
their associated facies descriptions, include the following:
2. there may be no unique solution even if all possible data were available.
3. interpretation is based on the preponderance of evidence, no single item will conclusively prove a hypothesis.
4. absence of a feature is common, so such absences do not help the analysis.
These points should be seriously considered when presenting results of a geological analysis of log data.
Sedimentary structures can be subdivided into predepositional, syndepositional, and postdepositional sedimentary features, which aid in describing the sequence of events which created the structure.
Predepositional sedimentary structures are those observed on the underside of a bed. These include erosional features, scour marks, flute marks, ripple marks, mud cracks, worm burrowings, grooves, and channel cutting. Of these, only channel cutting may sometimes be recognized on the dipmeter by the log analyst, although the smaller events may be seen on Formation Microscanner images.
Postdepositional sedimentary structures are those observed on the top side of a bed. These include load casts, quicksand structures, and movement by slump or creep. Drape due to differential compaction, and its counterpart, sag, can be measured by the dipmeter and can be diagnostic of certain types of sedimentary structures.
sedimentary structures are those occurring within the bed and
take the form of cross bedding or current bedding. We are usually
interested in the magnitude of current bedding angles, their characteristics
such as whether the current beds are planar, wedge shaped, or
festoon type, and their variations versus depth. These factors
provide clues to the depositional mechanism, which in turn define
the significance of the structure as a potential source of hydrocarbons.
Stratigraphy and Genetic Units
In this way, the context of the structure in its surroundings is used to help define the structure, as shown below.
genetic increment of strata (GIS) is made up of a series of depositional sequences, each being
a further series of conformable strata or beds, as shown below. The bedding planes within the depositional sequence are
useful to us, because their dip angle and direction can tell us
something about the arrangement and possible extent of the beds.
The structure of these genetically related beds determines whether
or not a stratigraphic trap is formed.
genetic sequence of strata (GSS) is a group or series of GIS's,
laid down with reasonable continuity, ie., there are no major
unconformities or major changes in depositional environment. Thus,
a GIS may be repeated several times in vertical succession. A
GSS corresponds roughly to a formation and a GIS to a unit or
member of the formation.
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