Defenitions

Types of sedimentary rocks

A- Terrigenous Sedimentary rocks:

derived from the weathering of pre-existing rocks. They have a clastic (broken or fragmental) texture consisting of:

1- Clasts or Grains (larger pieces, such as sand or gravel).

2-Matrix (mud or fine-grained sediment surrounding the clasts).

3- Cement (the glue that holds it all together), such as: calcite - fizzes in HCl acid iron oxide - reddish brown color silica - neither fizzes nor is reddish brown.

Terrigenous sedimentary rocks are classified according to their texture (grain size):

Gravel: Grain size greater than 2 mm. If rounded clasts: conglomerate. If angular clasts: breccia

Sand: Grain size 1/16 to 2 mm

Sandstone is most commonly made of quartz and feldspar, but it may contain grains of nearly any mineral composition, and also grains of fine-grained rock types (such as basalt). If dominated by quartz grains called quartz arenite. If dominated by feldspar grains called litharenite.

Silt: Grain size 1/256 to 1/16 mm (gritty) Siltstone 4-Clay: Grain size less than 1/256 mm (smooth)

-Shale (if fissile)

-Claystone (if massive)

B- Chemical/biochemical Sedimentary rocks:

This group includes the evaporites, the carbonates (limestones and dolostone), and the siliceous

rocks.

These rocks form within the depositional basin from chemical components dissolved in the seawater. These chemicals may be removed from seawater and made into rocks by chemical processes, or with the assistance of biological processes (such as shell growth).

Evaporites - The evaporites form from the evaporation of water (usually seawater). Rock salt - composed of halite (NaCl).

Rock gypsum - composed of gypsum (CaSO4.2H20), anhydrite (CaSO4)

Carbonates - The carbonate sedimentary rocks are formed through both chemical and biochemical processes. They include the limestones and dolostones.

Two minerals are dominant in carbonate rocks:

-Calcite (CaCO3) -Dolomite (CaMg(CO3)2)

Siliceous rocks - The siliceous rocks are those which are dominated by silica (SiO2). They commonly form from silica-secreting organisms such as diatoms, radiolarians, or some types of sponges.

-Diatomite - looks like chalk, but does not fizz in acid. Made of microscopic planktonic organisms called diatoms.

-Chert - Massive and hard, microcrystalline quartz. May be dark or light in color. Often replaces limestone. Does not fizz in acid.

C- Organic Sedimentary Rocks (Coals):

This group consists of rocks composed of organic matter (mainly plant fragments). Because of this, they lack minerals (which must be inorganic, be definition). These are the coals. There are several types of coal.

Carbonate rocks can be classified according to the texture and grain size of the rock, known as Dunham's classification.

Porosity (PHI)

Porosity is defined as the ratio of the void space in a rock to the bulk volume of that rock multiplied by 100 to express in percent. It is also referred to as the storage capacity of underground formations. Porosity can be classified according to the mode of origin as :

Original (primary) – developed during deposition of the sediment Induced (secondary) – developed by some geologic process subsequent to the deposition of the rock.

Original porosity is typified by the intergranular porosity of sandstones, carbonates, and the interparticle and oolitic porosity of some limestones. Induced porosity is typified by fracture development as found in some limestones or by vugs or solution cavities commonly found in limestones or by dissolution of feldspar in a sandstone. Rocks having original porosity are more uniform in their characteristics than those rocks in which a large part of the porosity is induced.

Porosity depends on grain packing, not grain size
Rocks with different grain sizes can have the same percentage porosity but different permeability.

Carbonate Primary porosity

Interparticle porosity: Each grain is separated, giving a similar pore space arrangement as sandstone.

Intragranular porosity: Pore space is created inside the individual grains which are interconnected.

Intercrystalline porosity: Produced by spaces between carbonate crystals.

Mouldic porosity: Pores created by the dissolution of shells, etc.

Carbonate Secondary porosity

Fracture (Fissure) porosity: Pore spacing created by the cracking of the rock fabric.

Channel porosity: Similar to fracture porosity but larger.

Vuggy porosity: Created by the dissolution of fragments, but unconnected.

Permeability (k)

Permeability is not measured; it is calculated. The steady-state equation for calculating permeability (using an integrated form of Darcy’s law).

Formations that transmit fluids readily, such as sandstones, are described as permeable and tend to have many large, well-connected pores. Impermeable formations, such as shales and siltstones, tend to have smaller, fewer, or no interconnected pores.

Absolute permeability is the measurement of the permeability conducted when a single fluid, or phase, is present in the rock.

Effective permeability is the ability to flow or transmit a particular fluid through a rock when other immiscible fluids are present in the reservoir (for example, effective permeability of gas in a gas- water reservoir). The relative saturations of the fluids as well as the nature of the reservoir affect the effective permeability.

Relative permeability

Geology is the ratio of effective permeability of a particular fluid at a particular saturation to absolute permeability of that fluid at total saturation. If a single fluid is present in a rock, its relative permeability is 1.0. Calculation of relative permeability allows for comparison of the different abilities of fluids to flow in the presence of each other, since the presence of more than one fluid generally inhibits flow.

Wireline Techniques

Commonly are classified by the kind of energy that is input or received including electric, seismic or acoustic, nuclear, magnetic, gravity, or optical. Logging tools are also classified according to whether they are for use in open holes or cased holes. Data from several methods are often combined to evaluate a single geologic or engineering characteristic.

Borehole calipers

The Caliper Log is a continuous profile of the borehole wall showing variations in borehole diameter. Caliper logs are required to assist in the quantitative interpretation of many other logs that are sensitive to borehole diameter and wall roughness (rugosity). Compensated logs such as density and neutron are corrected for these factors.

The caliper shows where deviations occur from the nominal drill bit diameter. The deflections are towards smaller radius where mud cake has accumulated in porous formations and the oversize excursions where caving has taken place.

Shales and coals are lithologies that tend to cave.The absence of mud cake adjacent to a porous bed may indicate a tight sand or possible overpressure.

The objectives of wire-line logging

To obtain a permanent, continuous record of the properties of rocks penetrated by the well bore.

Wire line logs are then combined with drilling data, mud logs, measurements while drilling and core information in order to choose correct testing and completion intervals and to properly evaluate the production potential of the well.
Well logs are then used for many years by exploration and production staff both in the development of reservoirs and in prospecting for new drilling locations.

In a vertical well, the logging objectives of an open hole are generally:

1. Determining lithology and porosity. Determining saturation.

2. Determining the oil-water and gas-oil contact. Determining pay zone intervals.

3. Identifying barriers in the reservoir. Determining pressure and permeability profiles.

genesis of Reservoir Rocks