Porosity and types in geology based

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1. Porosity and types BY: THOMAS CHINNAPPAN .A , M.SC.APPLIED GEOLOGY, PERIYAR UNIVERSITY, SALEM.

2. Porosity : Defination: The ratio of the pore volume in a rock to the bulk volume of that rock. Denoted by φ. Express in Percent. Mathematical Form: φ = Vp/Vb

3. Types of porosity : There are two types of porosity 1) Primary porosity 2) Secondry porosity

4. 1) Primary porosity : The porosity of the rock that formed at the time of its deposition. Two basic stages Predeposition and depositional stage Predeposition stage : begins when individual sedimentary particles form and includes intragranular porosity such as is seen in forams, pellets, ooids, and other nonskeletal grains.

5. Depositional stage : time involved in final deposition burial of a sediment or a growing organic framework total volume of carbonate porosity observed in carbonate rocks and sediments

6. Secondary porosity : developed at any time after final deposition time interval may be divided into stages porosity-modifying processes occurring in shallow surficial diagenetic environments stages: eogenet, telogenetic, and mesogenetic

7. Further Types of porosity : They are further subdivided into : 1) Total porosity 2) Effective porosity 3) Microporosity 4) Mesoporosity 5) Macroporosity

8. Total porosity : The ratio of the entire pore space in a rock to its bulk volume. Effective porosity : The measure of the void space that is filled by recoverable oil and gas φ = Vol. of interconnected pores + Vol. of deadend Total or bulk vol. of reservoir rock In very pure sandstone Total porosity is equal to Effective porosity

9. Macroporosity : Macroporosity In solids , the term 'macroporosity' refers to pores greater than 50 nm in diameter.

10. Microporosity : the term 'microporosity' refers to pores smaller than 2 nm in diameter. Mesoporosity : Mesoporosity In solids , the term 'mesoporosity' refers to pores greater than 2 nm and less than 50 nm in diameter.

11. Fabric selectivity : solid depositional and diagenetic constituents of a sediment or rock are defined as its fabric. solid constituents consist of :primary grains, such as ooids and bioclast later-formed diagenetic constituents calcite, dolomite, and sulfate cements recrystallization or replacement components, such as dolomite and sulfate crystals

12. FABRIC SELECTIVE : Interpartical Intrapartical Fenestral Shelter Framework Intercrystal Moldic NOT FABRIC SELECTIVE : Fracture Channel Vug Cevaron Basic porosity types

13. Interpartical porosity : Porosity between particles Fabric selective Intrapartical porosity : Porosity within individual particles or grains

14. Fenestral : Pores larger than grain-supported interstices (interparticle) Growth framework : Porosity created by in-place growth of a carbonate rock framework

15. Shelter : Porosity created by the sheltering effect of large sedimentary particles Porosity formed by selective removal of an individual constituent of the rock Modlic porosity :

16. Not fabic selective Fracture porosity : Porosity formed by fracturing Channel porosity : Markedly elongate pores

17. Vuggy porosity : Pores larger than 1/16 mm in diameter and somewhat equant in shape Cavern : Very large channel or vug

18. Fabric selective or not Breccia : Interparticle porosity in breccia Boring : Porosity created by boring organism Burrow : Porosity created by organism burrowing Shrinkage : Porosity produced by sediment shrinkage

19. Determination of Porosity : 1) Direct method 2) Water evaporation method There are two types :

20. Direct method : Determining the bulk volume of the porous sample, and then determining the volume of the skeletal material with no pores . pore volume = total volume − material volume. Water evaporation method : pore volume = weight of saturated sample − weight of dried sample)/density of water

21. Interparticle porosity : Mud-free carbonate sediments, like their siliciclastic counterparts dominated by intergranular porosity at the time of deposition. sediments exhibit porosities from 40- 50% & near the upper limit of 48% Primary porosity

22. wide variability of particle shape seen in carbonates excess porosity over the 27-30% expected in spherical particles maximum packing and commonly observed in siliciclastic sediments

23. This shape variation seems to be a function of their biological origin the common presence of intraparticle porosity that may occupy a significant percentage of the bulk volume of the sediment

24. Intrapartical porosity : one of the fundamental differences between carbonate and siliciclastic porosity originate in a variety of ways. living chambers of various organisms such as Foraminifera, gastropods, Rudists, and brachiopods

25. ultrastructure of some abiotic grains, such as ooids, and composite grains such as pelloids, which consist of packed, needle-shaped crystals

26. lead to significant intraparticle porosity activity of microboring algae and fungi may significantly increase the intraparticle porosity of carbonate grains ultrastructure of the tests and skeletons of organisms

27. Depositional porosity of mud- bearing sediments : Carbonate sediments containing mud range in porosity from 44 to over 75% Grain-supported muddy sediments such as packstones show the lowest porosity range

28. deep marine oozes can have porosities of up to 80% high porosities seen in the mud-supported shelf sediments are surely the effect of shape and fabric

29. perhaps the effect of oriented sheaths of water molecules responding to the strongly polar aragonite crystals high porosities reported for deep marine oozes high intragranular porosity found in the dominant organic components

30. Framework and fenestral porosity activity of reef-building organisms, can be depositional porosity type in the reef environment. Framebuilders, such as scleractinian corals, can construct an open reef framework volumes of pore space during the development of the reef

31. construct an open framework reef, coralline algae, and in the past stromatoporoids, and sponges have tended to erect a more closed framework structure with significantly less framework porosity

32. associated with supratidal, algal- related, mud-dominated sediments can be locally important opening communication between the larger fenestral pores through the intercrystalline porosity developed in the matrix dolomite.

33. Secondary porosity Formation by dissolution : Dissolution of limestones and sediments change in the chemistry of the pore fluid change in salinity, temperature, or partial pressure of CO2. development of a meteoric water system in a shallow shelf sequence

34. hydrocarbon maturation or shale dewatering may provide aggressive fluids meteoric waters early in the burial history of a carbonate sequence

35. Associated with dolomitization : Intercrystalline porosity associated with dolomites form a reservoir type in a no.of setting ranging from supratidal/sabkha to normal marine sequences

36. role of dolomitization in porosity development and destruction close relationship between percentage of dolomite and porosity

37. Associated with breccias : Brecciation of carbonate rock sequence no.of situations including: evaporite solution collapse, limestone solutioncollapse faulting, and soil formation

38. Limestone breccias, particularly those associated with evaporite or limestone solution collapse, Reservoir for hydrocarbons or a host for mineralization.

39. Associated with fractures : Fracturing is particularly effective and common in carbonate reservoirs brittle nature of carbonates relative to the more ductile fine-grained siliciclastics Associated with Faulting, folding, differential compaction, salt dome movement, and hydraulic fracturing within overpressured zone

40. Fractures in carbonates are commonly filled with a variety of mineral species including, calcite, dolomite, anhydrite, galena, sphalerite, celestite, strontianite, and fluorite

41. Summary Carbonate sediments and rocks generally have a much more complex pore system than do siliciclastics because of the wide variety of grain shapes common in carbonates, the presence of intragranular, framework, and fenestral porosity in carbonates, and the potential for the development of moldic and highly irregular dissolution-related porosity in carbonates.