Siliciclastic reservoir geology

Siliciclastic (commonly referred to as clastic) rocks are composed of terrigenous material formed by the weathering of pre-existing rocks, whereas carbonate rocks are composed principally of sediment formed from seawater by organic activity. This difference effects hydrocarbon recovery and therefore is important to understand.

Classification and properties

Clastic sediments are composed of grains and clay minerals, and siliciclastic sediments are first classified according to grain type. As shown in Fig. 1a, the three basic grain types are:

• Quartz
• Feldspar
• Rock fragments

and the end members are:

• Quartz sandstone
• Arkosic sandstone
• Lithic sandstone

Second, siliciclastics are described in terms of grain size (Fig. 1b). Grain-size classes include:

• Gravels (boulder size to 2 mm in diameter)
• Sands (2 to 0.0625 mm)
• Mud, which includes silts (0.0625 to 0.004 mm) and clay (< 0.004 mm)

Mixtures are described with a modifying term for a less-abundant size, such as clayey sandstone, sandy siltstone, or muddy sandstone (Fig. 1c). Mudstone, composed of clay and silt, is not to be confused with carbonate mudstone. In this classification, mud and clay are terms used to indicate size, not mineralogy.

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  Fig. 1 – Classification of siliciclastic rocks by (a) composition, (b) grain size, and (c) texture. The ϕ scale is defined as ϕ = -log₂(size).

The porosity and permeability of unconsolidated siliciclastic sediments were measured by Beard and Weyl. Porosity varies from 45% for well-sorted sands to 25% for very poorly sorted sands and does not vary with changes in grain size for well-sorted media. Permeability ranges from 400 darcies in well-sorted, coarse-grained sands to 0.1 darcies (100 md) in very poorly sorted, fine-grained sands. Permeability varies with grain size and sorting because it is controlled by pore-size distribution. Most modern sands are reservoir-quality rock. Modern claystones and mudstones, which are composed primarily of clay minerals, have little permeability and are not reservoir quality.

The type, amount, and habit of clay minerals in siliciclastic rocks are important characteristics (see Fig. 2). Clay minerals are sheet-structure silicates that have a profound impact on the petrophysical and production properties of sandstones. They can be deposited as muddy sediment or formed during burial by diagenetic processes (sometimes referred to as authigenic clay). Common clay minerals are:

• Kaolinite [Al₂Si₂O₅(OH)₄]
• Illite [KAl₃Si₃O₁₀(OH)₂]
• Chlorite [(Al, Mg, Fe)Si₄O₁₀(OH)₂]
• Smectite or montmorillonite [(Al, Mg)Si₄O₁₀(OH)₂]

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  Fig. 2 – Diagrammatic illustration of the basic, different types of clay habit in siliciclastic rocks. Figure courtesy of Sneider Exploration Inc.

The mineralogy of the clay minerals has a great effect on pore size and petrophysical properties. For example, kaolinite-cemented sandstones are more permeable than are illite-cemented sandstones because kaolinite tends to form boolets that reduce pore size and porosity, whereas illite tends to form thin threads that reduce pore size with little effect on porosity. Clay minerals are also known to hinder inflow into the wellbore. Smectite, for example, tends to swell, reducing permeability when in contact with fresh water. Kaolinite is known to get dislodged by high-velocity flow and plug pore throats near the wellbore, reducing permeability. The iron in chlorite is commonly released during acid treatments, plugging perforations.

Depositional environments

The following discussion is taken primarily from Galloway and Hobday. Grain type, size, and sorting, as well as other characteristics of siliciclastic reservoirs are most commonly controlled by the depositional environment. Many siliciclastic reservoirs are geologically young, and the sediment has undergone only moderate compaction and cementation. Therefore, diagenesis is not a major factor, and petrophysical properties can be predicted on the basis of sedimentology.

Siliciclastic sediments are transported and deposited by wind and flowing water. On land, clastics are deposited by wind and flowing water. In the marine environment, they are transported by tidal, wave, ocean, and density currents. See Fig. 3 for a visual representation of different land, transitional, and marine environments.

Land-based environments are grouped into the following systems:

• Alluvial-fan
• Fluvial
• Eolian systems

Transitional environments (between land and marine) include:

• Delta systems
• Barrier bars

Marine environments are grouped into

• Shelf
• Slope
• Basinal systems

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  Fig. 3 – Range of sandstone depositional systems that typically host hydrocarbon resources.

Alluvial fans are conical, lobate, or arcuate accumulations of predominately coarse-grained clastics extending from a mountain front or escarpment across an adjacent lowland. Some fans terminate directly in lakes or ocean basins as fan deltas, which generally show some degree of distal modification by currents or waves. Most sediment is deposited by stream and debris flow. Stream flow is commonly confined to one or two channels but may spread across the fan as sheet-flow. Debris flows result when clay and water provide a low-viscosity medium of high yield strength capable of transporting larger particles under gravity. Wave and tidal currents modify the distal terminations of fans that build into lakes or the ocean, improving sorting and reservoir quality.

Eolian deposits are typically fine- to medium-grained, well-sorted, quartzose sand with pronounced crossbedding. The sand is transported and deposited by wind currents, which are the most effective agents for sorting clastic particles. Hot, arid regions are the most favored locales for eolian accumulation. Eolian environments can be divided into the following facies:

• Dune
• Interdune facies

Dunes are large bed forms that come in an array of forms. Barchans, barchanoid ridges, and transverse dunes form in response to essentially unidirectional winds. Longitudinal dunes arise from varying wind directions. Draas comprise large stellate rosettes with a high central peak and radiating arms and form in response to intense, multidirectional wind systems. The interdune environment is generally a broad, featureless plain covered by lag gravels resulting from deflation (erosion). Deposition in the interdune area results from rainfall in desert highlands, promoting ephemeral streams that deposit sediment in streambeds and small alluvial fans. Flooding may produce interdune braided-stream deposits. Ponding of water between dunes can create lakes that can precipitate evaporite minerals if the groundwater is sufficiently saline.

Fluvial systems are a collection of stream channels and their floodplains. The channels are sinuous (meandering), with the degree of sinuosity increasing seaward. Braided streams are the result of sand-rich channels. Channel deposits are composed of:

• Sand bars
• Lag deposits

The point bar, a major feature of a high-sinuosity channel, forms by lateral accretion of sediment in the lower-energy, leeward side of a meander. Deposition normally occurs during the ebbing phase of a flood. The highest energy, found in the channel proper, erodes the channel bank, causing the channel to shift constantly; lag deposits are characteristic of the channel. Abandoned channels are commonly clay filled.

Floodplain deposits are deposited as:

• Levees
• Crevasse splays
• Flood-basin sediments

Levee deposits are fine sand, silt, and clay deposited along the margins of the channels, when decelerating water rich in suspended sediment spills over the banks during flood stage. Crevasse splays are formed when local breaches in the levees funnel floodwater into near-channel parts of the flood plain. These sediments tend to be highly heterogeneous, composed of:

• Sand of variable size
• Plant debris
• Mud clasts

Flood-basin deposits are broad, clay-rich sediments that have been reworked by:

• Burrowing animals
• Plant growth
• Pedogenic (soil-forming) processes

Delta systems form when a river transporting sediment enters a standing body of water, commonly an ocean or a lake, and consist of both fluvial and marine sediments. The depositional architecture of a delta system is characteristically progradational and may fill a small basin. The combination of fluvial and marine processes creates a unique facies assemblage and reservoir architecture. Deltaic sediments are deposited as:

• Channel fills
• Channel-mouth sands
• Crevasse splays
• Delta-margin sand sheets

Together, these facies compose a delta lobe, which is a fundamental building block of a delta system. Delta systems are divided according to major energy type into:

• Fluvial-dominated deltas
• Wave-dominated deltas
• Tidal-dominated deltas

Each system has a unique depositional architecture.

Shore-zone systems, excluding deltas, compose a narrow transitional environment that extends from wave base (≈50 ft of water) to the seaward edge of the alluvial coastal plain. They include the following facies:

• Shoreface
• Beach
• Barrier
• Lagoon
• Tidal-flat

These systems are supplied principally by onshore transport of river-derived and shelf sediments. Sands are concentrated in barrier-island complexes and tidal sand bodies, with finer sediment landward. Accretion of beach ridges seaward can form a sheetlike sand body referred to as a strandplain sand. The "shoreface facies" refers to that part of the shore zone that is below the zone of wave swash. It is commonly divided into lower-, middle-, and upper-shoreface deposits partly on the basis of water depth and associated energy levels, the highest energy level being the surf zone (upper shoreface). Beach facies includes wave swash and dune zone, all deposited above mean tide. The barrier is formed by aggradation or by progradation of shoreface sands seaward. The lagoon facies, located behind the barrier, is generally composed of clay and fine sand. The barrier may be breached during storms, allowing tidal currents to transport coarser sediment from the ocean into the lagoon, forming tidal deltas.

Shelf systems are broad, deepwater platforms covered by terrigenous sediment. Sediment distribution is controlled by ocean currents, including:

• Tidal, wave
• Storm surge
• Density

Facies are defined by bed form and include:

• Sand ribbon
• Wave
• Ridge
• Storm
• Mud

Slope and basin systems are found in the relatively deep water beyond the shelf break. Deposition is characterized by the dominance of sediment transport by gravity and density flow, although pelagic settling also occurs. The upper slope is typically a zone of sand remobilization and bypass, with characteristic erosion and channel cutting; the lower slope and basin floor are sites of deposition. Regionally, grain size is the coarsest in the upper slope and decreases in the basin-floor direction. Slope and basin systems are typically distinguished from other systems by their fining-upward-graded bedding, which results from grain settling from a suspended sediment load. Submarine fans are typical slope and basin-floor deposits. Fed from point sources, such as river mouths or submarine canyons, they receive the bulk of their sediments from turbidity currents, a density current produced by sediment-rich water. The upper-fan environment is characterized by feeder channels or canyons that serve as sediment conduits, and sediments are typically coarse gravels. The midfan is characterized by a series of bifurcating, distributary, or braided channels that accumulate massive and pebbly sands showing lenticular bedding, and the lower fan is a smooth, gently sloping surface that received slowly deposited, suspended sediment punctuated by pulses of fine-grained to silt-sized sand. The resulting graded beds are thin, laterally persistent, and monotonously repetitive, commonly through a considerable thickness.

Diagenetic environments

Sandstones are less susceptible to diagenetic change than carbonates. Common diagenetic processes in sandstones are:

• Quartz overgrowth cement
• Carbonate (calcite and dolomite) cement
• Compaction
• Grain dissolution and associated formation of clay minerals
• Alteration of sedimentary clay minerals

Many of these products can be related to the burial history.

Pore space is reduced by:

• Mechanical and chemical compaction, resulting in more closely spaced grains and smaller pores
• Quartz overgrowths, which are commonly sourced from chemical dissolution of quartz grains during burial

Carbonate cements are formed by:

• Dissolution and precipitation of indigenous carbonate shell material
• Importation of carbonate from a more distant source

Iron-rich, pore-filling dolomite is not uncommon.

Feldspar minerals found in rock fragments are commonly unstable in the burial environment and are susceptible to dissolution, forming grain molds similar to those in carbonate rocks. Clay minerals (commonly chlorite) are deposited in the intergrain spaces associated with this dissolution process. Chlorite linings of pore space are thought to inhibit burial cementation and compaction and preserve porosity at depth. Clay minerals are altered during burial diagenesis, and authigenic (diagenetic) clay minerals are formed. Clay-mineral diagenesis causes large increases in surface area and microporosity that, in turn, have large effects on reservoir performance and log analysis.

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See also

Reservoir geology

Carbonate reservoir geology

Geology in reservoir models

Geostatistical reservoir modeling

Rock types

PEH:Reservoir_Geology