Rock Facies: Origin, Depositional Environment
The sedimentary rock sequence can vary considerably in thickness, texture, grain size, and lithology from place to place. These differences create traps that will hold hydrocarbons which are called stratigraphic traps. Superimposed regional or local structure may also play a role in stratigraphic traps.

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.

The rock type can be derived from:
   1. observation of samples
   2. observation of cores
   3. lithology analysis of an adequate log suite

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.

Geologists 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 curves.
 

Classification of Depositional Environments
The simplest breakdown of depositional environments is:
   1. continental
   2. coastal or transitional
   3. marine

Depositional environments

Most detrital sediments are continental or transitional, and most chemical sediments are marine.

The first four describe detrital sediments and the last two chemical sediments.

Marine sedimentary rocks:
   1. shelf margin - formed at the edge of the continental shelf
   2. inner shelf - formed near shore
   3. outer shelf - formed farther from shore
   4. atoll/pinnacle reefs - formed by biological skeletons in shallow water
   5. lagoonal/back reef - formed in the quiet shallow water protected by a reef
   6. basinal - formed in deep water
   7. evaporitic - formed by evaporation of sea water

All 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.
 

Sedimentary Structures
The term sedimentary structures refers to stratigraphic features in the subsurface, created by erosion and deposition of sediments, as opposed to tectonic structures created by tension, compression, uplift, and subsidence.

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.

The difficulties in identifying sedimentary structures, and hence their associated facies descriptions, include the following:
   1. interpretation is based on multiple lines of evidence (eg logs, cores, inferred geometry, fossils, mineralogy) obtained concurrently or in no special order.

   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.

Syndepositional 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.
 

Sequence Stratigraphy and Genetic Units
Sequence stratigraphy is a phrase used to indicate a method for describing the depositional environment of a sequence of rocks. The terms stratigraphic unit, genetic unit, or genetic increment of strata (GIS) are used to describe the presence of a sedimentary structure. A genetic unit encompasses the structure and its surroundings, usually the interval between an upper and a lower marker bed or lower erosional surface, with the sedimentary structure sandwiched in between. The marker beds are usually shales. The more massive shale beds are called maximum flooding surfaces and more minor shales are called local flooding surfaces. The name suggests that the breaks between successive genetic units are caused by inundation which stops this particular depositional cycle.

In this way, the context of the structure in its surroundings is used to help define the structure, as shown below.

Sequence stratigraphy and genetic units

A 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.

Bedding planes define genetic units

A 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.