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Genesis, Transport and Deposition of Clastic Sediments – An Overview

What are clastic sediments?

Clastic sediments are formed by loose particles of various sizes, which can evolve into sedimentary rocks such as sandstones, shales and conglomerates. They are essentially made up of sand, mud and gravel, formed from the weathering and erosion of pre-existing rocks.

  • Weathering comprises physical, chemical and biological processes that alter, disintegrate, or dissolve rocks. Physical weathering occurs when a rock is fragmented through purely mechanical processes, which do not significantly alter its chemical or mineralogical composition. Chemical weathering, in turn, involves chemical and mineralogical changes of the pre-existing rocks, which may result in hydration, oxidation, reduction or dissolution of their mineral components, and formation of new mineral phases stable under surface conditions.
  • Erosion is responsible for moving materials derived from weathering, to be carried by the action of water, gravity, wind or ice, and deposited in various marine or continental environments.
Figure 1: Natural arch produced by wind erosion on rocks subjected to a differential weathering in Jebel Kharaz, Jordan. – Source: link

It is possible to characterize the sediments that compose a clastic sedimentary rock with the help of a magnifying glass on a hand sample, or through the analysis of a thin section with a polarized microscope. Composition, textural, structural and fabric aspects can be described through the use of Petroledge® software. The program automatically generates textural and compositional classifications, recognizes the tectonic provenience of the sediments, and also interprets diagenetic environments, aspects which will be further addressed in incoming texts.

After erosion, the particles are available for transportation. Water, wind, ice and gravity are the main agents for sediment transport. Gravity can act alone or associated to other agents, such as water, thus constituting the main sediment transport agent in nature. In fact, rivers account for approximately 95% of the sediment flow to the oceans.

The sedimentary particles remain in motion while the flow energy (usually proportional to its velocity) is sufficient to sustain them. This can be represented for the different particle sizes by the Hjulströms Diagram.

Hjulströms (1939) diagram – Erosion, transport and deposition intervals according to flow velocity and particle size. Source: link

Types of sedimentary transport

Sediment transport can be classified according to its competency (related to the transported grain size), its capacity (related to the amount of sediment that the agent can transport) and the load (amount of sediment that the agent effectively carries). A river capable of carrying particles larger than sand sized is a highly competent agent. Also, the greater the volume flow of the river, the greater the load in motion and, therefore, the greater its capacity.

The force that moves the particles during transportation is provided by fluids, and depends mainly on their velocity and viscosity. The flow of fluids can be separated into 2 types: laminar flow and turbulent flow. In laminar flows, the particles immersed in the fluid move parallel to each other, in the direction of transportation. In turbulent flow, however, the particles immersed in the fluid move in all directions, but with a displacement parallel to the transportation direction. This type of flow has a much greater erosion power than laminar flow. As velocity increases, the flow tends to become turbulent.

A dimensionless parameter called Reynolds Number (Re), named after Osborne Reynolds, who studied fluid dynamics during the nineteenth century, indicates the intervals at which a flow of fluid is laminar or turbulent. The Reynolds number is proportional to the velocity of the flow (V) and to the depth of the channel or diameter of a pipe (L), as well as to the ratio between the density (d) and the viscosity of the fluid (u), according to the equation:

For values of Re < 500, the flow is laminar, whereas for values of Re> 2000, it is turbulent. Laminar flows are common in high viscosity or low velocity fluids. Wind flow is, therefore, essentially turbulent due to the low viscosity of air. Water flows can be laminar when their velocity is very low. However, significant volumes of sediment are usually transported by turbulent water flows.

Representation of flow types in a pipe.

In addition to the effects of fluid viscosity and inertial forces, gravity also influences the way a fluid transfers waves or moves sediment dunes. The Froude Number (Fr), which can be considered the ratio between the average velocity of the flow and the velocity of a wave contained therein, is also a useful dimensionless value for sedimentology studies, expressed as:

where U represents the average velocity of the flow, L the depth of water and g the acceleration due to gravity.

When Fr value is less than 1, the flow is considered subcritical or quiet and a wave can move upstream (against flow). If the value is greater than 1, the waves cannot propagate upstream and the flow is considered supercritical or fast. In addition to being used to define the critical velocity from which a flow is considered subcritical or supercritical at a given depth, the Froude Number is also related to different flow regimes, which are related to characteristic bedforms.

Two flow regimes are recognized: lower and higher. The lower flow regime comprises a subcritical flow in which ripples, dunes and plane-parallel type stratifications are formed. The upper flow regime, in turn, comprises a supercritical flow, in which antidunes and plane-parallel stratifications are stable. This can be understood from a bedform stability diagram (Figure 4). The use of this diagram allows obtaining velocity estimates or identifying changes in velocity or type of flow that resulted in deposition of sediments according to their depositional structures.

Figure 4: Bedform stability diagram for particles under a given water flow (depths between 25 and 40 centimeters and temperature of 10 ° C) (Nichols, 2009).

Fluid-flow dynamics

Water

Under the action of water flowing over a moving background condition, the solid particles that make up the substrate tend to come into motion. In a simplified way, coarse particles such as sand and gravel will be transported if the energy of the stream overcomes the weight of the particles. In the case of finer particles, such as silt and clay, the cohesive force is the main force of resistance that flow must overcome. The forces acting on a particle submerged in a water stream are: the submerged weight of the particle, the lift force that causes the rise of the particle, and the drag force, which drives the particle in the direction of flow. From the relation between these three forces, we can classify the particles transportation according to four different modalities:

  • Suspension: When the lift force balances that of gravity, the particles move floating in the liquid mass.
  • Sliding: occurs when particles slide on the bed.
  • Rolling: occurs with coarser particles, which shapes allow them to rotate.
  • Saltation: occurs through the interaction between the lift and drag forces, a situation in which particles move through jumps (such as a suspension transport in which the particle sometimes comes into direct contact with the bottom).
Figure 5: Particles transported by rolling, saltation and suspension (adapted from Nichols, 2009).Sliding, rolling and saltation are types of transport by dragging. Dragging occurs where the weight of a particle exceeds the lift force.

Ice

The viscosity of ice is so high that it is not directly recognized as a fluid. However, ice may transport large amounts of sediment. The ice flow is very slow and laminar, and carries suspended sediments of all sizes.

Air

Air is a fluid of very low density and viscosity. The principles involved in wind transport are similar to those present in water transport; however, the low density and viscosity of air reflect at different thresholds for particle transportation. Usually, air is capable of transporting in suspension only particles below the fine sand size, while coarser sediments are transported by rolling and saltation. Transport occurs at relatively high speeds, in a turbulent way. Like water, wind transport results in the deposition of strata that occur in ripple size up to dunes many meters high.

Gravitational flows

The action of gravity can transport sediments in subaerial or underwater conditions. In gravitational flows, it is common that 20 to 70 percent of the sediment is transported in suspension.

Through gravity, grain flows can occur, in which part of the grains are suspended by contact interactions between the grains. It is common for grain flows to occur on the sliding face of wind dunes. Highly concentrated sediment and water mixtures can generate mud flows or debris flows. In this case, coarse particles are supported by a mud matrix, which has cohesive strength. This characteristic makes the flow less predictable, with non-Newtonian characteristics.

Liquefied or fluidization flows are a type of gravitational flow in which the grains are held in suspension by the upward movement of the interstitial fluid that escapes with the gravitational settlement of the grains, or by forced ejection of the fluid filling the pores. This occurs in shallow-buried sediments, which can behave as fluids after a sudden shock, which causes an instantaneous loss of contact between the grains thereafter suspended in the fluid.

Turbidity currents are a type of density current that occurs at the bottom of a sea or lake, consisting of turbid mixtures of variable density of sediments temporarily suspended in water. They are less dense than the debris flows, with a relatively high Reynolds number. The result of the deposition of sediments transported by this type of current are the turbidites. Turbidity currents slow down over time and as they move away from their source, resulting in deposition. Since the coarser suspended materials are the first to be deposited, the turbidites have a coarsening upward character.

Turbidity currents from low to medium density ideally form a sequence known as the Bouma Sequence, consisting of 5 divisions. This sequence is represented by poorly selected and massive sands at base (a), overlain by laminated sands of upper flow regime (b), sands with cross lamination (c), laminated silt (d) and hemipelagic mud at the top (e).

High-density turbidity currents, in which the density is greater than 1.1g cm-3, form thick deposits of coarse texture and massive structure at their base, as a result of the interaction between particles. The top of the deposit is characterized by fluid escape structures and is more similar to the Bouma Sequence, representing deposition from lower density flows.

Figure 6: Turbidites are deposited from sediments usually mobilized by underwater avalanches.

Deposition of clastic sediments

Sedimentary particles are deposited when the transportation agent loses competence to carry them or when the force that causes the movement is cancelled. Loss of competence for water or air transportation may be related to decreased flow velocity. In the glaciers, the deposition occurs with the stagnation or retention of the glacier, which may occur due to increase in the melting rate, or decrease of snow accumulation rate. Deposition rates in the different environments are very variable, and usually present values from 3 millimeters per year, in abyssal seabeds, to tens of meters per year in deltaic regions, in which the sediment supply is very high.

The deposition of sediments transported by gravity, water, wind or ice can occur along several transportation cycles. Deposition may be temporary (when the sediment moves repeatedly) or permanent (when the deposited sediments remain immobilized and are buried).

Sedimentation may be episodic or continuous. Episodic sedimentation is characterized by processes of great magnitude separated by long periods of non-deposition. It occurs when an instantaneous depositional process is triggered. This makes estimating the deposition rate quite complex, so that an annual estimate cannot be used. Continuous sedimentation is associated with virtually constant depositional conditions over long periods of time. In the abyssal sea floor, the continuous sedimentation results in the slow accumulation of fine particles. However, in this environment, turbidites deposits may accumulate during events of episodic deposition. It is common, therefore, that events of episodic deposition represent an important part of deep marine sequences.

The superposition of depositional events results in the vertical stacking of layers that record the processes involved in sediment deposition. These sequences can be seen in outcrops or cores, and are of extreme importance for the characterization of depositional environments.

Figure 7: Marginal marine sequence of siltstones and limestones of the Virgin Formation, southwestern Utah, USA. Source: link

The description of sedimentary layers recorded in cores and outcrops can be performed efficiently through Strataledge®, a software that allows their complete and systematic description, comprising the types of rock, minerals and other constituents, textures, structures, thickness and types of contact between beds. With this tool, all the data is acquired in a digital and organized way. In addition, the system allows an easy integration with petrographic data, geophysical logs, photos and other media.

References

  1. Nichols, Gary. Sedimentology and stratigraphy. John Wiley & Sons, 2009.

Author

  • Elias Cembrani da Rocha – Endeeper

Digital Petrography – Fundamental Tool for Understanding Carbonate Reservoirs of Campos Basin

Learn why petrographic characterization is a fundamental tool for understanding Carbonate Reservoirs of the Campos Basin.

The Challenge

Campos basin is the most prolific Brazilian basin. Hydrocarbons are sourced mainly from lacustrine rift section, which also contains important carbonate reservoir rocks. Diagenetic processes strongly influenced the porosity and permeability of these lacustrine carbonates. Understanding the controls and patterns of diagenesis is fundamental for the construction of geologically realistic and effective models for the exploration and production of these reservoirs.

Petroledge Petrography Carbonate

The Solution: Systematic Petrography using Petroledge®

A systematic petrographic study of the rift carbonate reservoirs and associated lithologies was developed in central Campos Basin with use of the Petroledge® software. The petrographic characterization, which comprised all major aspects of depositional structures, textures, primary composition and diagenesis, helped to define the depositional and post-depositional conditions of the succession, as well as the main controls on the reservoirs quality. The Petroledge® system has unique features, designed to facilitate and support petrographic description, as well as automated classifications and multi-format reporting, ensuring efficient and rapid data analysis. Systematic acquisition and processing of petrographic data and information provided by the Petroledge® software allows an optimized use of petrographic information for understanding of the distribution of porosity and permeability.

Geological Context

The origin of the Campos Basin is linked to the initial stage of separation of the African and South American continental blocks in the Early Cretaceous. The initial phase of basin evolution was characterized by rift half-grabens, where fluvial and lacustrine sediments were deposited. The vertical succession analyzed in this study interval is composed of a siliciclastic and volcanoclastic basal section, covered by a complex succession of ooidal stevensite arenites, bioclastic grainstones and rudstones (which includes the reservoirs), and mudrocks.

Results

The integration of the results of the petrography with seismic, stratigraphic and sedimentological information allowed to conclude that:

  • The analyzed rocks are composed of extrabasinal sediments (siliciclastic and volcanoclastic grains and siliciclastic mud) and mainly intrabasinal carbonate and stevensite constituents.
  • The main carbonate rocks correspond to ostracod grainstones and bivalve rudstones, commonly known as “coquinas”, which correspond to the main reservoirs.
  • There is widespread mixing of the bivalve bioclasts with stevensite ooids and peloids. As the precipitation of stevensite occurs only at highly alkaline conditions (pH> 10, high concentration of Mg and Si), which would be intolerable by the bivalves, such mixing would be possible only through re-sedimentation. The distribution of the seismic facies corresponding to the bioclastic deposits and their massive structure indicate that this re-sedimentation took place from different shallow water environments to deep lacustrine settings, though gravitational flows.
  • The mixing of bioclastic and stevensitic constituents has important implications for the quality of the rift reservoirs. Hybrid deposits with significant mixing are commonly strongly cemented, while rudstones with minor or no mixing with stevensitic grains show better preservation of interparticle porosity. These best reservoirs would correspond either to bioclastic deposits in their in situ shallow sites, or to re-sedimented deposits that were not mixed with stevensite sediments.

The systematic petrography of the bioclastic carbonate reservoirs of Campos Basin allowed by the Petroledge® software was essential for the understanding of depositional and post-depositional conditions of the rift succession, as well as of the main controls on the quality of the reservoirs.

Author

  • Sabrina Danni Altenhofen – Endeeper

Systematic Petrography Supports Petrofacies Definition and Porosity Distribution Understanding in Pre-Salt Reservoirs

Learn how systematic petrography guided by software helps geologists to define reservoir petrofacies and understand porosity distribution in Pre-Salt Reservoirs.

The Challenge

Understanding the controls and distribution patterns of the quality of complex and heterogeneous lithic pre-salt reservoirs of Sergipe-Alagoas Basin, northeastern Brazil, is of key importance for the optimization of their production.

The Solution

Quantitative petrographic analyses of 135 thin sections performed with the Petroledge® software allowed the definition of reservoir petrofacies according to the main textural and structural attributes, essential primary composition, and main diagenetic processes affecting the types of and distribution of porosity and permeability in the reservoirs.

Background

Pre-salt sandstones and conglomerates of the Sergipe-Alagoas Basin represent rare examples of lithic oil reservoirs rich in ductile low-grade metamorphic rock fragments, such as phyllite, schist, and slate, showing a complex diagenetic evolution. The reservoirs are very heterogeneous, with intercalation of partially cemented porous areas and tight areas intensely cemented by dolomite. The main diagenetic processes affecting the analyzed samples were generated before compaction, in shallow burial conditions, under the influence of ascending thermobaric and alkaline depositional fluids.

Systematic Petrography using Petroledge®

Petroledge® software allows performing detailed petrographic descriptions and interpretations in a systematic workflow, and storing and processing petrographic information within a relational database. An extensive knowledge base works integrated with analytical tools for providing several automatic classification, provenance and diagenesis interpretation.

Results

The systematic petrographic characterization of the pre-salt lithic reservoirs made possible by use of the Petroledge® software, revealed the following:
• Predominance of siliciclastic rocks, medium- to coarse-grained sandstones and conglomerates, massive or with irregular lamination.
• The most abundant detrital constituents are low-grade metamorphic rock fragments (essentially phyllite and schist) and granitic/gneissic plutonic rock fragments.
• Some samples containing ooids, peloids, microbial and recrystallized carbonate intraclasts were classified as hybrid arenites.
• Dolomite is the main diagenetic mineral, mainly filling intergranular pores as blocky and macrocrystalline cement, and replacing grains, locally as discrete blocky crystals.
• Dolomite cementation played an essential role on porosity reduction, where most pores were filled, or preservation, where partial cementation supported the framework, limiting compaction.
• Preserved primary intergranular porosity is much more abundant in the siliciclastic rocks than in the hybrid rocks. Intragranular porosity, mainly from dissolution of feldspars is very abundant in the siliciclastic rocks.
• Mechanical compaction is observed mostly by the deformation of mica grains, metamorphic fragments and mud intraclasts, locally promoting the formation of pseudomatrix.

Systematic Reservoir Characterization and Evaluation

In this study, the influence of diagenesis, depositional texture and primary composition on the quality of the reservoirs was evaluated through the definition of reservoir petrofacies. Dolomite cementation was recognized as the main diagenetic process controlling porosity distribution in the reservoirs. The reservoir petrofacies were separated into four petrofacies associations, according to total porosity, intergranular porosity and cementation: good quality, medium quality, low quality/cemented, and low quality/compacted. Systematic acquisition and processing of petrographic data and information provided by the Petroledge® software supported a better understanding of the distribution of porosity and permeability in the complex lithic pre-salt reservoirs of Sergipe-Alagoas Basin.

Reservoir Petrography - Petroledge
Reservoir Petrography – Petroledge

Author

  • Sabrina Danni Altenhofen – Endeeper

Petrography of the Cenomanian-Turonian transgression in the Potiguar Basin: Petroledge Success Case

This article about petrography and Petroledge was written by Ana Bárbara Sampaio da Costa (Terra & Mar, Geophysics and Geology Solutions). Endeeper gratefully acknowledges Terra & Mar, Geophysics and Geology Solutions.

The Cenomanian-Turonian passage is globally known as the largest marine transgression during the 250 Ma.

In the Potiguar Basin, northeastern Brazil, this passage occurs within the stratigraphic interval constituted by the Açu and Jandaíra formations.

A detailed petrographic analysis of this interval was executed by the description of 190 thin sections using the Petroledge® system for petrography.

The study using petrography was performed through the detailed characterization and quantification of the primary and diagenetic constituents, and pore types.

The reconstruction of the primary detrital composition supported the interpretation of the clastic provenance. The analysis of the diagenetic processes and constituents allowed identifying their impacts on the porosity, establishing paragenetic sequences, and inferring paleoenvironmental information. The identification and quantification of the different pore types made possible to reveal the relationships among the diagenetic processes and the modification of pore space.

The effective integration of petrographic data and information, made possible by use of the Petroledge® system, revealed the following:

  • The predominantly siliciclastic units Açu-3 and Açu-4, from the initial phase of development of the marine transgression, provided from uplifted blocks of the plutonic basement, being affected essentially by diagenetic processes indicative of continental eodiagenesis under dry climate, including mechanical clay infiltration.
  • The late phase of eustatic development (upper Açu-4 unit) is represented by the deposition of hybrid sediments, constituted by extrabasinal grains derived from the uplifted basement, carbonate and non-carbonate intrabasinal grains. Their diagenetic alterations are indicative of eodiagenetic conditions transitional between marine and meteoric.
  • The deposition of the Jandaíra Fm. Characterizes the maximum of the transgression, with establishment of a carbonate platform, which sediments were affected by marine eodiagenetic processes, including intense calcite cementation.

The use of the Petroledge® software was fundamental for assuring the quality and coherence of the obtained data, allowing their effective integration with stratigraphic and sedimentologic information.

Digital Petrography by Petroledge - Illustrative
Digital Petrography by Petroledge – Illustrative

Petrography by Petroledge: Campos Basin success case

This post presents an example of project that uses Petroledge for understanding the Campos Basin rift reservoirs using petrography.

 

Digital Petrography by Petroledge and Stageledge
Digital Petrography by Petroledge and Stageledge

An integrated, seismic-stratigraphic-sedimentological-petrographic project, developed by Brazil’s Rio Grande do Sul Federal University for BG Group, shed new light on the depositional and diagenetic controls on the origin, geometry, distribution, quality and heterogeneities of Campos Basin rift reservoirs and associated lithologies.

The use of the PETROLEDGE® system was vital for the systematic acquisition, storage and processing of petrographic data from the complex and unconventional rocks that constitute the pre-salt, rift section of the basin. “Our understanding of the dominantly intrabasinal nature of the rift sediments and their conspicuous re-deposition by gravitational processes was enhanced by the detailed, yet flexible petrographic descriptions allowed by PETROLEDGE®”, says Dr. Karin Goldberg, head of UFRGS project. Campos rift sediments are essentially constituted by complex mixtures of carbonate bioclasts, siliciclastic and volcaniclastic particles, and stevensite (Mg-smectite) ooids and peloids.

Endeeper PETROLEDGE® system is being used globally by a series of universities and exploration companies, which are taking advantage of the systematic and efficient acquisition and processing of petrographic information generated by the system.