Diagenesis of Siliciclastics PDF

12 Diagenesis of Siliciclastics University of Guyana – GEM 3114 Sedimentary Rocks I Andy Chater Ph.D.,P.Geo. 30 November and X December 2024 1 Carbonate Oil Reservoirs Are in Most of the World’s Largest Petroleum Producing...

12 Diagenesis of Siliciclastics University of Guyana – GEM 3114 Sedimentary Rocks I Andy Chater Ph.D.,P.Geo. 30 November and X December 2024 1 Carbonate Oil Reservoirs Are in Most of the World’s Largest Petroleum Producing Areas Reminder This is a 2005 Slide Compilation 2 However, There Are a Lot More Siliciclastic Reservoirs in the World Holding More Oil Reminder This is a 2005 Slide Compilation 3 Sandstones (Arenites) Are Also Formally Named by Composition, Using a Ternary Diagram - in practice, any grain that is not either quartz or feldspar is called a lithic (which can be a rock grain or a non quartz or feldspar mineral) 100% Reminder Slide 100% 100% 4  Using % Matrix (< 0.03mm) the Gradation Between Sandstones and Mudstones is Clearly Defined and Called Wackes, (Being Characteristic of Greywackes Generally) Reminder Slide This is called the Dott Classification Note: There are other classifications 5  Commonly Observed Sandstone Diagenetic Reactions In Relation to Burial Temperature and Organic Solvents Which Influence pH and Alkalinity (as a Buffering Capacity of pH) 6 Compaction and Cementation Are Both Processes of Diagenetic “Lithification” Reminder Slide Very Different 7  Note: Differential Compaction of Permeable Sands and Impermeable Clays and Silts is Commonly Responsible for Over Pressures as Well as Cementation and Clay Changes 8 Fine Sand Representatives of Common Grains Found in Sandstones Deformed Representatives of Same Common Grains Deformed and Cemented Same Common Grains 1/16 mm = 62.5 microns 200 microns = 1/5 mm 9  Most Sandstones Are Quartz Arenites or Arkosic Quartz Arenites Usually With a Few Lithic Clasts As Well – All Represented Here Fine Sand 10  Simple Burial Compaction Leads to Selective Grain Deformation, More Grain Contact, Brittle Fracturing, Porosity Loss Fine Sand 11 As Burial Proceeds, More Grain Deformation and Now Chemical Precipitates From Pore Water and New Formation Water inflows Fine Sand 12 The Chemical Changes to a Rock in Diagenesis are Driven by Changes in Pore Fluid Chemistry, By Temperature and By Pressure Given enough time, changes in pore fluid chemistry must result in changes to “unstable” minerals (in that particular pore fluid for those particular minerals) to restore equilibrium These changes apply to the grains present as well as to the cements that may have been deposited During progressive burial temperature and pressure will change and so must the chemistry of everything in contact with the pore water as well as the pore water’s chemistry being changed by interaction with the pre-existing cements and the now already changed grains from their original point of deposition In progressive burial, this process is a continuum of changes to which the pore fluids and the grains (or what is left of them) must constantly re-adjust When there is no pore fluid left then changes are accomplished by metamorphism 13 Example of Sands and Silts on a Passive Margin Where Meteoric Groundwater is Flowing From the Land into Saline Groundwater Offshore Delta of the Ganges River Land Sea 14  Meteoric Water With a Low pH Flows through the Continental and Near Shore Sands (green & orange) Where it Meets Saline Water With a Higher pH Sea Land High Permeability 15 Subsurface Mixing of Saline and Meteoric Waters Can Precipitate Silica Cements – Mixing Zone and Its Precipitation Location Can Change Over Time Low pH Mixing = SiO₂ ppt Higher pH 16  There is More Silica is Solution in Seawaters (at Higher pH) Than in Freshwaters (at Lower pH) – But Low pH Meteoric Waters Initially Carry More Silica Than They Do When Neutralized pH – First Mixing Precipitates Silica as Opal, Then “Syntaxial” Quartz 17 Opal-CT Growing With Chalcedony in Chert (Opal-CT is Amorphous Cristobalite and Tridymite) 18  Very Low Temperature Microbial Opal-CT in Very Small Spheroids, As Grain Coatings 0 100 nm 19 Opal-CT Can Only Be Differentiated From Opal-A (Siliceous Ooze) By XRD - thin early opal sometimes known as micron silica 20  Example of “Micron” Silica as a Quartz Grain Coating – Being Either Opal-CT, Cryptocrystalline Quartz or Micro Quartz 21  Besides Opal-CT, Many Detrital Quartz Grains, Especially the Smaller Grains, Have Thin Coatings of Illite-Smectite Stained With Iron Oxides and Hydroxides – This Can Inhibit Syntaxial Quartz Overgrowth Cement to Some Degree PPL XPL 22 “Syntaxial” Quartz Overgrowth Cement Means That the Quartz Cement Grew in Optical (Crystallographic) Continuity With Detrital Grain’s Own Crystal Orientation 23 Initial Quartz Cements Tend to Elongate Grains Then Progressive Layers Ultimately Mimic a Quartz Crystal 24 Syntaxial Quartz Overgrowths In Lithic Sandstone (Shown at Arrows), in PPL Brown rims are clay and iron oxides Remnant Porosity 0 300 Micron (0.3mm) 25  This Sandstone Shows Abundant Evidence of Syntactic Quartz Overgrowths (Original Grain Boundaries Shown as Q) + Later Calcite Cement (C) Remaining Porosity XPL 26 Syntaxial Quartz Overgrowth Cement Can Reduce the Porosity to Almost Zero (Becomes a Quartzite) – Black in XPL is Quartz in Extinction (Left Image) While the Right Image Shows Pre-Cement Grains XPL 300 Micron (0.3mm) Sketch 27 Euhedral Quartz Grain Mimicking a Quartz Crystal With Its Syntaxial Overgrowths, as Seen in this SEM Image 0 50 Micron (0.05mm) 28  SEM Image of Euhedral Quartz Growths With Pyramidal Terminations – Later Phase of Kaolinite (Small Books) and Pyrite (Spherical Framboids) 0 50 Micron (0.05mm) 29  Sometimes, Quartz Grains Are Eroded From a Previous Sandstone Which Had Been Uplifted and Weathered – Some Grains Retain This History 30  Where Does All The Silica Come From to Make Sandstones’ Cements? Cement in sandstones depends on the quartz framework grain space and how long they are in the “silica supply window” Sandstones in Rift Basins (most are arkosic) and in collision basins (subduction and mountain making) tend to have little silica cement Quartz arenites of Intracratonic basins (on stable areas), foreland basins (e.g. east of Andes and Cordillera Mountains) and in passive-margin basins have the most quartz cement In rapidly subsiding basins (e.g. Gulf of Mexico, North Sea), most quartz cement is precipitated by cooling formation waters coming from burial depths of several kilometers where temperatures are 60-100°C Pressure solution of touching grains, the conversion of smectite to illite, feldspar alteration/dissolution and some carbonate replacements of quartz cements supply most of the silica ce3m 1 ent As Temperature Decreases Silica Is Less Soluble 32 Stress on Grain Boundaries Makes Silica 33 Silica Also Comes From the Destructive Diagenesis of Feldspars in Arkosic or Lithic Arenites There are 2 primary feldspars, K-feldspar which is KAlSi₃O₈ and Plagioclase which is a mixture of two end members: NaAlSi₃O₈ (albite) and CaAl₂Si₂O₈ (anorthite) These feldspars can be altered (by hydrolysis) to clay minerals + K, Na and Ca ions + SiO₂ in solution – they are significant sources of silica and calcite cements in siliciclastics as well as producing clay minerals – simplified, these are the hydrolysis reactions: K-feldspar can alter to kaolinite: Al₂Si₂O₅(OH)₄+SiO₂+K⁺+H⁺ Or K-feldspar can alter to illite: KAl₂Si₄O₁₀(OH)₂+SiO₂+H⁺ Albite plagioclase (NaAlSi₃O₈) will alter to kaolinite: Al₂Si₂O₅(OH)₄+SiO₂+Na⁺+H⁺ Anorthite plagioclase (CaAl₂Si₂O₈) will alter to kaolinite: Al₂Si₂O₅(OH)₄+SiO₂+Ca²⁺+H⁺ Potassium feldspar (K-feldspar) always contains minor amounts of plagioclase and plagioclase always contains a little K-felds34par Clay Filled Pits Were, Almost Certainly, K-Feldspars Before Diagenesis – Roraima Sandstone, Guyana 0 1 cm 35 All the Clay Has Been Washed Away by 2Ga years of Formation Fluid Flow Leaving Significant Porosity 0 1 cm 36  Composition Ranges for Plagioclase and K-Feldspar Commonly occurring feldspars – they are less stable than albite or anorthite 37  Unaltered Microcline K-Feldspar (cross- hatch texture) with Quartz (grey colours) XPL 38  Unaltered Plagioclase in Thin Section Showing Characteristic Style of Twinning XPL 39 Illite Forming from K-Feldspar in Diagenesis (Notice How Small New Illite Crystals Are) 40  In Certain Conditions, some K-Feldspar is “Albitized” (KAlSi₃O₈+Na⁺ goes to NaAlSi₃O₈+K⁺) (center grain below) While There is New Porosity in Other K-Feldspar Grains (black areas in this image) (Dissolved K-Feldspar) (Ca-rich zeolite mineral as a cement) 41  Another “Albitized” K-Felspar Grain – Remaining K-Feldspar is Partially Dissolved 42 Carbon Dioxide from Maturing Black Shales Accelerates Dissolution of Feldspars and This Occurs Prior to the Generation of Oil 43  CO₂ Influx Also Propels the Formation of SiO₂ and Ca-rich Cements and, With an Mg Source, These Can be Dolomite Cements 44  This is a Model from the Magnus Oil Field in the North Sea Demonstrating a pH reduction With a CO₂ Influx Being Buffered by K-Feldspar Dissolution 45  Much meteoric water Phase flush drives the change of K-spar to kaolinite Diagram for Isothermal changes in the availability (activity) of K⁺/H⁺ ions versus availability (activity) of SiO₂ (Worden and Burley 2003 ) 46  Much meteoric water flush drives the change of K-spar to kaolinite At Higher K⁺ Concentrations K-Feldspar Will Go More Directly to Illite, Which is a Potassium – rich Clay 47 What you start with decides what you get for deep burial diagenesis 48 The Rate at Which Burial Occurs Also Decides How Much Enhanced Porosity Will be Created – If Burial Includes Fracturing, Porosity and Permeability are Significantly Improved 49  In General, Arenites With More Lithic Fragments Tend to Lose Porosity More Than Those Without Lithics – Lithics Are Less Stable 50  Smectite Clays (Swelling Clays - Includes Montmorillonite) Are Mainly Derived from Volcanic Ash or Volcanic Lithoclasts 51 Smectite Clays Come from Dry Weathering Detritus, They Covert to Kaolinite on the Way to Proximal Sedimentation But Being the Smallest Clay They are Washed Out to Sea and Are Deposited in Deep Water 52 Once They Are Buried, Kaolinite and Smectite Covert to Illite and/or to Chlorite Depths depend on geothermal gradient 53  It is Not Just Temperature Which Controls the Smectite/Illite Transition But Adequate Time Is Essential – It begins With Mixed Layer Clays 54 This Transformation Requires Diffusive Chemical Changes Which Happen at the Molecular Level 55  Clay Minerals Commonly Coat Clastic Grains in Sandstones - Reduces Permeability, Retains Water and Retards Movement of Fines (XS Clays) Significant Permeability Reduction High Irreducible Water Saturation Migration of Fines Problem Secondary Electron Micrograph Carter Sandstone North Blowhorn Creek Oil Unit ~ 10 m (Photograph by R.L. Kugler) 56 Black Warrior Basin, Alabama, USA Authigenic (grown in situ) lIlite is Fibrous and It Seriously Impedes the Permeability of Sandstone Reservoirs When It Grows Scanning Electron Micrograph Significant Permeability Reduction Negligible Porosity Illite Reduction High Irreducible Water Saturation Migration of Fines Problem Fibrous Authigenic Illite, Jurassic Norphlet Sandstone (Photograph by R.L. Kugler) 57 Hatters Pond Field, Alabama, USA Smectite Plus Kaolinite Can Also Form Chlorite as Well as Illite When Deeply Buried But Chlorite Does Not Impede Permeability So Much - happens frequently and also by deposition from pore water - this image is of chlorite (which is similar to a clay mineral) Secondary Chlorite As Seen By An Electron Micrograph Occurs in Many Deeply Buried Sandstones Occurs as Thin Coats on Detrital Grain Surfaces Jurassic Norphlet Sandstone ~ 10 m (Photograph by R.L. Kug 58 ler) Offshore Alabama, USA  Effects of Clay Grain Coatings on Reservoir Quality (Permeability) are Not Very Predictable - some affect porosity most while some affect permeability most, without a clearcut corresponding relationship - this diagram shoes how illite has more affect than chlorite Authigenic Illite Authigenic Chlorite 100 1000 Permeability (md) 100 10 10 1 1 0.1 0.1 0.01 0.01 2 6 10 14 2 6 10 14 18 Porosity (%) (modified from Kugler and McHugh, 519990) Growth in Pore Throats Always Restricts Flow - small sized pores showing some degree of cement growth - pore throats are the connections through which pore space can give up or receive liquids, and so control permeability Pore Throat Pores Provide the Volume to Contain Hydrocarbon Fluids Pore Throats Restrict Fluid Flow Scanning Electron Micrograph Norphlet Formation, Offshore Alabama, USA 60 Wiry to Filamentous Illite Acts as a Filter to Slow Fluid Flow – Notice How Small the Crystals Are 61 The Smectite to Illite Transition Mostly Happens at the Onset of Oil Generation Temperatures – But Sources Are Usually Deeper Than Reservoirs 62 Transition is Quick and Shallower in K-Feldspar Rich Sands (supply of K⁺) Versus K-Feldspar-Poor Sands - this happens to shales too but is not K-Feldspar dependent 63 Actual Transition Depths Vary From Basin to Basin - this is an example from the North Sea 64  These are US Basins Showing Reduction in Smectite Content (Proxy for Authigenic Illite) - note very different geothermal gradients 65 This is From The Forties Field, The First and Largest Oil and Gas Field in the North Sea - shows chlorite appears at depth as kaolinite diminishes (due to many lithic clasts for Mg and Fe source) 66  In the End, There are Enormous Variations of Porosity and Permeability, All Dependent on the Local Course of Diagenesis – This is a Compilation From Many Marine Facies Examples Worldwide All From Passive Margins – Orange are Siliciclastics, Blue Are Carbonates and Grey Are Turbidites – Trends in Red and Black 67 This Was Compiled for the Same Set of Samples (Micallef et al, 2020) 68 Quite Separately From the Subsurface Creation of Cements, Sandstones Can Be Impregnated With Authigenic (grown in place) Glauconite at Sediment Surfaces, Rather Like the Phosphorite Replacements of Fecal Pellets and Carbonates Glauconite Impregnated Sandstones Are Called “Greensands” 69 Wikipedia Image of Green Glauconitic Sandstone Glauconite is an Iron Potassium Phyllosilicate (mica group) Mineral Its formular can be simplified to (K,Na)(Fe³⁺,Al,Mg)₂(Si,Al)₄O₁₀(OH)₂ 70  Aptian Greensand Disconformably Underlies Cenomanian Chalk 71  The Chalk Immediately Overlying Greensand is a Bioturbated Hardground 72  Nearby, the Chalk Immediately Overlying the Greensand Contains Glauconite – Related to Bioturbated Hardground Glauconite is Forming in a Sedimentary “Hiatus” 73 These Hard Grounds Have Regional Extent 74  The Name “Glauconite” Comes From the Greek Word “Glaukos” Meaning Blue-Green Glauconite is universally found only in marine rocks formed on shallow continental shelves Glauconite forms only when there is a period of very slow deposition or when there is a complete sedimentary “hiatus” Glauconite can be found in sands, clays, impure limestones and in chalk – it needs a supply of K⁺ and Fe³⁺ to form Glauconite is “authigenic” – it forms by replacement of benthic fecal pellets, peloids and clays, and also by direct precipitation from seawater Glauconite generally appears as a granular mineral, making sand-sized glauconite “grains” - it can be detrit75al The Gradual Trend to Compositional Maturity Provides lots of K⁺ and Fe⁺⁺ (and Mg⁺⁺) 55% Feldspar 10-20% Feldspar 76 Example of Glauconite Sandstone (Greensand) Used as a Building Stone – It Weathers to a Rust Colour Due to Its High Iron Content 77 Glauconite is Sometimes Associated With Phosphorite – P-Rich Glauconites Make Good Fertilizers – Example of Glauconite Fertilizer Information Below For Sales The major chemical description is ((K,Na)(Fe+3, Al, Mg)2(Si,Al)4O10(OH)2) General chemical information: Iron (Fe) 12-19% Potassium (K) 5-7 % Silicon (Si) 25.0% Oxygen (O) 45% Magnesium (Mg) 2-3 % Aluminum (Al) 1.9 % Sodium (Na) 0.27% Hydrogen (H) 0.47% Over 30 other trace minerals and many micronutrients. 78  Most Popular General Explanation for Glauconite Formation in Siliciclastic and Carbonate Sediments – This and Next Slide, Fernandez-Landero (2021) 79 80 The Process Begins With Smectite – With Organic Involvement – Chemistry Decides What You Get 81  Weathering Can Change the Pre-Existing Rock Very Significantly Before it is Eroded and Transported to a Basin of Deposition - Weathering Makes Clays from Solid Rock Polar Reminder Slide Equatorial Clays (clay) (clay) Note 82 Similar Explanation – Lopez-Quiros et al (2019) 83 Another Similar Explanation – Huggett (2013) 84  (a) and (b) are glauconite grains in PPL in a limestone’s carbonate cement; (e) is an oolitic limestone with Glauconite grains; (f) its brown grains are hematized glauconite (iron is Fe³⁺) 85  C = Oxidised Glauconite With Phosphate Cement, D = Shrinkage Cracks in Glauconite, E = Glauconite Core to Ooid , F = Same Ooid Showing Chemistry (where blue = Silica, green = phosphorus, rose = iron) 86  Being Formed Close to the Sediment-Water Interface, Soft Glauconite-Rich Sediment Can be “Ripped-Up” and Become Detrital Grains (black is quartz in extinction, brown is mud clasts - note carbonate cement) XPL 87 Glauconite-Rich Sandstone From Wikipedia Where the Glauconite Grains Were Probably Detrital 88  Presence of Glauconite in Sandstones Reduces Their Porosity and Permeability and Significantly Adds to Irreducible Water Also, Glauconite is a Soft Mineral (hardness 2) Which Compacts Easily Making it Become a Matrix Component of the Rock Instead of Staying a Framework Grain 89  In Soft Sediment, Glauconite is a Framework Grain – As in Sands Below 90 Glauconite (green-brown) Has Become a Matrix in this Sandstone 91  This Diagram Illustrates How One Part of a Sandstone Body May Have Reduced Porosity and Permeability While Other Parts of it Are Unaffected USGS Interpretation 92 93 Reminder Slide Once They Are Buried, Kaolinite and Smectite Covert to Illite and/or to Chlorite Depths depend on geothermal gradient 94 Transition is Quick and Shallower in K-Feldspar Rich Sands (supply of K⁺) Versus K-Feldspar-Poor Sands - this happens to SHALES too but is not K-Feldspar dependent Smectite to Illite Transition Can Create Overpressures in Seal Rocks Causing the Seal to be Degraded Reminder Slide Generally, transition from smectite to illite happens to all shales similarly 95 Shale With Some Quartz and Calcite Grains (cc = calcite) “Floating” in a Sea of Partially Weakly Parallel-Oriented Illite Grains 96 Good Image of Parallel-Oriented Illite Grains 97 General Expectations For Shale Diagenesis 98 Expectations For Organic Matter Diagenesis - see next slide for details 99 100 Illite Becomes More Uniformly Crystalline 101 As Temperature Increases Past Maturity Some Organic Matter Become a Proto-Graphite (G) 102 In These Overmature Marcellus Shales with 3-7% TOC at R₀ = 4+, They Become Weak Conductors At ~250+°C This core mostly has resistivities of < 0.1 ohm-meter 103  Mineralogy of the Core in Previous Slide - Quartz+Illite+Calcite+Albite+Pyrite+Chlorite 104  Only Bitumen Left Plus Proto-Graphite Connecting Through Minute Pore Spaces and Grain Boundaries Enough to be Conductive 105 Where the 150m Core of Marcellus Black Shale in Previous Slides Came From (in summary) If Cements Arrive Before Compaction the End Result is Different From a Reverse Timing 106