Reading Sedimentary • Stanley, S.M., 2015, Sedimentary Environments, – Ch. 5. Earth Systems History Rocks and • On Ecampus Sedimentary Basins What is a Sedimentary Basin? Sedimentary Rocks – A thick accumulation of sediment – Necessary conditions: •Intro 1. A depression (subsidence) • Origin of sedimentary rocks 2. Sediment Supply – Clastic Rocks – Carbonate Sedimentary Rocks • Interpreting Sedimentary Rocks – Environment of deposition • Implications for the Petroleum System World Map of Sedimentary Basins Where are the Sedimentary Basins? 6 A B Watts Our Peculiar Planet: The Rock Cycle Liquid Water and Plate Tectonics Hydrologic Cycle Tectonics http://www.dnr.sc.gov/geology/images/Rockcycle-pg.pdf 8 SEDIMENT 3 Basic Types of Sedimentary Rocks • Unconsolidated products of Weathering & Erosion • Detrital ( = Clastic) – Loose sand, gravel, silt, mud, etc. – Made of Rock Fragments – Transported by rivers, wind, glaciers, currents, etc. • Biochemical – Formed by Organisms • Sedimentary Rock: – Consolidated sediment • Chemical – Lithified sediment – Precipitated from Chemical Solution Detrital Material Transported by a River Formation of a Sedimentary Rocks 1. Weathering – mechanical & chemical 2. Transport – by river, wind, glacier, ocean, etc. 3. Deposition – in a point bar, moraine, beach, ocean basin, etc 4. Lithification – loose sediment turns to solid rock Processes during Transport • 1. Sorting – Grain size is related to energy of transport – Boulders high energy environment – Mud low energy Facies: Rock unit characteristic of a depositional environment Processes during Transport: 2. Rounding Significance for Petroleum System? Sand Grains • Shales: Fine grain, clay rick?, very low energy environ., low permeability, high organic content? → Source rocks, Seals, Permeability barriers • Sandstones: Coarse grain, quartz-rich, high energy env., low organic content, high porosity? → Reservoirs, Migration pathways abrasion is progressive – angular grains near source – rounded grains long transport Petroleum System Implications Well rounded Well sorted Good Quartz sandstone Reservoir! High porosity and Permeability Bimodal rounding Poorly sorted Poor Reservoir! Lithic sandstone Many depositional environments with particular Low porosity and characteristics: Permeability Lithification Compaction: Shale Porosity vs. • Compaction Depth Curve • Decrease in pore space • Due to increasing lithostatic pressure with burial • Cementation • Pores filled Φ =41.73 e –z/8197 ft • Quartz or calcite cements Φ =41.73 e –z/2498 m • Recrystallization • New mineral growth • Beginning of metamorphism • Can kill porosity Schmoker and Halley, 1982 Lithification Petroleum System Implications • Determines the nature of the reservoir • Critical for economic viability • Highly variable • Hard to predict ahead of the drill Rock Lithification of Shale Fragment Dewatering of shale • Overpressure? feldspar • Expulsion of hydrocarbons? Cement qtz SEM-CL image- Frontier Fm. -Lithic Sandstone Reed, UT Austin Classification of Clastic Sediments Detrital Rock • Based on Particle size Names – Gravel (more than 2 mm) • Pebbles (small) (Driveway gravel) • Cobbles (medium) (Plum to melon size) Conglomerate • Boulders (large) (Melon to bus size) –Sand(2 mm - 1/16 mm) – Silt (1/16 - 1/256 mm) Shale –Clay(< 1/256 mm) CARBONATE SEDIMENTARY Biochemical Sedimentary Rocks ROCKS • Carbonate Rocks •Madeof Calcium Carbonate or Mg -Ca Carbonate – Limestone calcite (CaCO3 ) • Precipitated from seawater – Dolostone dolomite (Ca,Mg (CO ) ) 3 2 – Mostly by organisms (biochemical) • algae, shells, corals •Coal – In some cases chemically (inorganic) • Organic-rich shales have large biochemical component – Do not confuse with organic Carbon-rich rocks Chalk- Microscopic algae Chalk Sedimentary Environments Fields in the North Sea produce from fractured chalk • Type and geometry of sedimentary White Cliffs of Dover, UK rocks • Key to paleogegraphy • Prediction of distribution of source rocks and reservoirs • Use the modern to interpret the ancient Carbonate Bank- Bahamas Coral Reef Coral Reef Carbonates Sedimentary Environments of Deposition Non-Marine Environments How do we recognize them? • Glacial • Rock Types • Deserts • Sedimentary Structures • Alluvial Fans • Lakes • Fossils •Swamps •Geometry • Braided Streams • Meandering Streams Deserts Modern Jurassic Dunes (160 million years old) Utah •Very good sorting •Great reservoirs •Rarely preserved Alluvial Fans Types of River • Conglomerates Mountain • Poorly sorted • Not so great! Delta Braided Meandering Meandering Stream Fluvial Environments (Rivers) Braided Stream Meandering Stream Channel and Floodplain Development of Meanders Time Kimberley, Australia National Geographic Photo Channel Sandstone Sandstone Mudstone Sandstone Conglomerate Vertical Well log Response sequence of rocks left by a meandering river Peoria Field, CO Wiki.apg.org Log signature of Sedimentary Rocks fine River Point Bar coarse Delta-marine coarse Barrier Island fine Transgressive sand Atkinson et al., 1988 52 Take home Points • Sedimentary Environments produce recognizable rock sequences • Useful for predicting the lateral rock changes in the subsurface • Control reservoir properties • Non-Marine Environments • Deserts • Rivers • Electrical logs produce patterns for different environments are distinctive .