Reading Sedimentary Rocks And Sedimentary Basins

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SedimentaryRocks andSedimentaryBasinsReading Stanley, S.M., 2015, Sedimentary Environments,– Ch. 5. Earth Systems History On EcampusWhat is a Sedimentary Basin?Sedimentary Rocks–– Intro Origin of sedimentary rocks– Clastic Rocks– Carbonate Sedimentary Rocks Interpreting Sedimentary Rocks– Environment of deposition Implications for the Petroleum SystemWhere are the Sedimentary Basins?A thick accumulation of sedimentNecessary conditions:1. A depression (subsidence)2. Sediment SupplyWorld Map of Sedimentary BasinsA B Watts6

Our Peculiar Planet:Liquid Water and Plate TectonicsThe Rock CycleHydrologic /Rockcycle-pg.pdfSEDIMENT Unconsolidated products of Weathering & Erosion– Loose sand, gravel, silt, mud, etc.– Transported by rivers, wind, glaciers, currents, etc. Sedimentary Rock:– Consolidated sediment– Lithified sedimentDetrital Material Transported by a River3 Basic Types of SedimentaryRocks Detrital ( Clastic)– Made of Rock Fragments Biochemical– Formed by Organisms Chemical– Precipitated from Chemical SolutionFormation of a SedimentaryRocks1. Weathering– mechanical & chemical2. Transport– by river, wind, glacier, ocean, etc.3. Deposition– in a point bar, moraine, beach, ocean basin,etc4. Lithification– loose sediment turns to solid rock8

Processes during Transport 1. Sorting– Grain size is related to energy of transport– Boulders high energy environment– Mud low energyFacies: Rock unitcharacteristic of adepositionalenvironmentSignificance for Petroleum System? Shales: Fine grain, clay rick?, very low energyenviron., low permeability, high organic content? Source rocks, Seals, Permeability barriersProcesses during Transport: 2. RoundingSand Grains Sandstones: Coarse grain, quartz-rich, high energyenv., low organic content, high porosity? Reservoirs, Migration pathwaysabrasion is progressive– angular grains near source– rounded grains long transportPetroleum SystemImplicationsWell roundedWell sortedQuartz sandstoneHigh porosity andPermeabilityGoodReservoir!Bimodal roundingPoorly sortedLithic sandstoneLow porosity andPermeabilityPoorReservoir!Many depositionalenvironments with particularcharacteristics:

Compaction:LithificationShale Porosity vs.Depth Curve Compaction Decrease in pore space Due to increasing lithostatic pressure with burial Cementation Pores filled Quartz or calcite cementsΦ 41.73 e –z/8197 ftΦ 41.73 e –z/2498 m Recrystallization New mineral growth Beginning of metamorphism Can kill porositySchmoker and Halley, 1982LithificationPetroleum System Implications Lithification of ShaleDetermines the nature of the reservoirCritical for economic viabilityHighly variableHard to predict ahead of the drillRockFragmentDewatering of shale Overpressure? Expulsion of hydrocarbons?feldsparCementqtzSEM-CL image- Frontier Fm. -Lithic SandstoneReed, UT Austin

Classification of Clastic Sediments Based on Particle size– Gravel (more than 2 mm) Pebbles (small) (Driveway gravel) Cobbles (medium) (Plum to melon size) Boulders (large) (Melon to bus size)Detrital RockNamesConglomerate– Sand (2 mm - 1/16 mm)– Silt (1/16 - 1/256 mm)– Clay ( 1/256 mm)Biochemical Sedimentary Rocks Carbonate Rocks– Limestone calcite (CaCO3 )– Dolostone dolomite (Ca,Mg (CO3 )2)ShaleCARBONATE SEDIMENTARYROCKS Made of Calcium Carbonate or Mg -CaCarbonate Precipitated from seawater– Mostly by organisms (biochemical) algae, shells, corals Coal Organic-rich shales have large biochemicalcomponent– In some cases chemically (inorganic)– Do not confuse with organic Carbon-rich rocksChalk- Microscopic algae

ChalkFields in the North Seaproduce from fracturedchalkWhite Cliffs of Dover, UKSedimentary Environments Type and geometry of sedimentaryrocks Key to paleogegraphy Prediction of distribution of sourcerocks and reservoirs Use the modern to interpret the ancientCarbonate BankBahamasCoral ReefCoral Reef CarbonatesSedimentary Environments ofDeposition

Non-Marine Environments GlacialDesertsAlluvial FansLakesSwampsBraided StreamsMeandering StreamsHow do we recognize them? ModernDesertsRock TypesSedimentary StructuresFossilsGeometryJurassic Dunes (160 million years old)Utah Very good sorting Great reservoirs Rarely preservedAlluvial FansTypes of RiverMountainBraidedMeanderingDelta Conglomerates Poorly sorted Not so great!

Fluvial Environments (Rivers)Braided StreamMeandering StreamMeandering StreamChannel and FloodplainDevelopment of MeandersTimeKimberley, AustraliaSandstoneNational Geographic PhotoChannel SandstoneMudstoneSandstoneConglomerate

Well log ResponseVerticalsequence ofrocks left bya meanderingriverPeoria Field, COWiki.apg.orgLog signature of Sedimentary RocksfineRiver Point BarcoarseDelta-marinecoarseBarrier IslandfineTransgressive sandAtkinson et al., 1988Take home Points Sedimentary Environments produce recognizablerock sequences Useful for predicting the lateral rock changes in thesubsurface Control reservoir properties Non-Marine Environments Deserts Rivers Electrical logs produce patterns for differentenvironments are distinctive52

Sedimentary Rocks and Sedimentary Basins Reading Stanley, S.M., 2015, Sedimentary Environments, - Ch. 5. Earth Systems History On Ecampus Sedimentary Rocks Intro Origin of sedimentary rocks - Clastic Rocks - Carbonate Sedimentary Rocks Interpreting Sedimentary Rocks - Environment of deposition

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