Sedimentary Facies

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Sedimentary Facies Definition

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The concept of sedimentary facies depicts that the sedimentary facies are actually the bodies of sediment recognizably distinct from adjacent sediment accumulated in a different depositional environment. As conditions alter with time, so various depositional sites may also their shapes and characteristics.

Usually, facies are differentiated by the aspect of the rock or sediment that is being studied. Therefore, facies based on petrological characters such as mineralogy and grain size are referred to as lithofacies, whereas facies based on fossil content are known as biofacies.

Types of Sedimentary Facies

Sedimentary facies indicate a depositional environment, each facies being a distinct type of sediment for that area or environment. Thus, there are various ways of describing or designating sedimentary facies. Taking into account the principal physical (or lithological) characteristics, one is able to identify lithofacies. The biological (or more appropriately, paleontological) characteristics—the fossils—describe biofacies. It is quite usual to speak of alluvial facies, bar facies, or reef facies, considering the environment as a criterion.

Facies under coal, clay, shale, and sandstone may be repeated several times and are known as cyclothems. Rhythmic or cyclic sedimentation has been documented in various rocks in different parts of the world and may occur in various ways; however, re-assessment of many successions originally defined as cyclic displays that this phenomenon is not as common or as consistent as had been thought of.

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Sedimentary Facies Analysis

Talking about the Sedimentary facies and reservoir characteristics and analysis, we will undertake the 8 sandstones. This sandstone is a unit in the Western Sulige field which is one of the most prolific gas‐generating intervals in the Ordos Basin.

Purpose of Sedimentary Facies Analysis

Sedimentology and reservoir characteristics analysis are basically undertaken in order to interrogate the reservoir's petrological, petrophysical, diagenetic properties, and production features. Three types of sandstone and four types of lithofacies were identified. The two frequently seen lithofacies are labelled lithofacies C and D.

Lithofacies C is made up of fine‐ to medium‐grained litharenite (Sandstone I); lithofacies D composed of coarse‐grained sublitharenite (Sandstone II) and coarse‐grained/gravelly litharenite (Sandstone III).

Results of Sedimentary Facies Analysis

In comparison, Lithofacies C, Lithofacies D was accumulated as a series of overlaying sand sheets in a greater energy fluvial system. The firmness of Sandstone I is an outcome of an event of intense compaction of the rock substructure giving rise to early dissolution of grains and deformation of ductile rock splinters. The mechanisms that resulted in the low permeability of Sandstone II and Sandstone III are more complicated. Moreover, intense compaction, the development and emplacement of quartz and clay cements plays a critical factor. Besides these,

  • Evolution of Diagenesis and porosity assessment exhibits that the petrophysical properties of these sandstones were better at the gas charging time.

  • The permeability and porosity values of Sandstones II and III are same at the surface, but considerably different at formation pressure. This causes differences in gas production rates.

  • Sandstone II possesses greater gas production rates and cumulative gas production than Sandstone III.

  • The presence or absence of Sandstone II monitors and curbs the dispersion of sweet spots within less productive tight gas reservoirs.

Fun Facts

  • The suite of structures actually develops facies.

  • Sedimentary facies are either terrigenous, emancipating from the deposits of particles weathered from older rocks and carried to the depositional site; biogenic, depicting depositions of whole or broken shells and other hard parts of organisms; or chemical, depicting inorganic precipitation of substance from a solution.

  • Each facies consists of a three-dimensional configuration and overtime shifts its position.

  • A facies (Latin terms used for appearance or aspect) is a body of rock (i.e. a series of beds) or sediment marked by a specific combination of compositional, biological and physical structures that differentiate it from bodies of rock/sediment.

  • A sedimentary facies contains a set of properties that makes it distinctive, which the geologist defines.

  • In general, facies are described based on a suite of characteristics in rocks/sediment.

FAQ (Frequently Asked Questions)

Q1. How are Different Sedimentary Facies Classified?

Answer: Facies are groupings of rock types based on identical characteristics. We use these groupings for the purpose of generalizing individual properties into useful, genetically associated categories. Some examples include:

Facies Based on Grain Size:

  • Coarse-grained sandstone having 1-5% pebbles (indicates high flow speeds)

  • Fine-grained, well-sorted sandstone (indicates low flow speeds with either consistent flow speed or only one size sediment source)

  • Mudstone (indicates standing water)

Facies Based on Grain Composition:

  • Coarse-grained sandstone having quartz (50%), lithic fragments (25%), and feldspar (25%)

  • Coarse-grained sandstone having contents including quartz (80%), mica (10%), and feldspar (10%)

  • Coarse-grained sandstone having 99% quartz and trace gold flakes

Facies Based on Sedimentary Structures:

  • Fine-grained sandstone having current ripple cross lamination

  • Fine-grained sandstone containing upper planar lamination

  • Fine-grained sandstone deficient in cross-stratification, but with plentiful burrows

Q2. How are Facies Important?

Answer:  Nowadays it is recognized that facies relations and distribution depend upon interlinked controls. The most significant include sedimentary processes, sediment supply, biological activity, sea-level changes, weather, water chemistry, tectonic activity (earth movements), and volcanic activity. Of these, the climate and tectonic activity are predominant since they may ultimately regulate the other factors.

In industries that deploy earth resources such as fossil fuels, facies (or sedimentary basin) assessment is critical in research. It may result in forecasting where coal, petroleum, natural gas, or other sedimentary substances may be found. Besides that, evaluation of rock specimens, this type of sedimentary facies analysis may also rely largely upon the electrical magnetic, geophysical properties, and radioactive properties of the rocks.