12-11 Essentials

12-11 Essentials - Essential Tools Sediments to Sedimentary...

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“The present is Key to the past Sediments to Rocks Hydrocarbon Traps Porosity and Permeability The what and why of Well Logs • Log Correlation Basics Essential Tools Sediments to Sedimentary Rocks
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‘The Present is the Key to the Past • Basic Geological Assumption: Earth’s processes (physical, chemical, biological laws) we see working today -- worked the same way in the past • i.e. Force of Gravity, Tidal Forces, etc. – AND . . . regular cycles and unusual events (i.e. floods, hurricanes, etc.) are preserved in the rock record
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The Present is the Key to the Past • Geologists study modern environments in order to provide a framework for recognizing those environments in the rock record. Cutaway section in modern sand dune These rocks were once a sand dune Present Past
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The Present is the Key to the Past • Geologists use interpretations about the environment of deposition to explore for oil and gas in stratigraphic traps • Interpretation gives clues for: – Drilling location for first well – Trend of the reservoir – Where to drill subsequent wells
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The present is the key to the past Map and Cross Section of gas production from stream channel in Canadian Co., Oklahoma
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Sediments to Sedimentary Rock • Detrital Sediments - from weathering of older rocks – Gravel - sand - mud become: – Conglomerate - sandstone - shale Braided stream flowing today Recognize braided stream conglomerate in Cretaceous-age rock outcrop Braided stream conglomerate in Triassic-age rocks produces Oil in Prudhoe Bay, AK
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Sediments to Sedimentary Rock • Carbonate Sediments – CaCO 3 mud = Micrite • inorganic processes • Marine biochemical processes -- Fossiliferous limestone - contain fossils – Carbonate reefs (or bioherms) – Dolostone/Dolomite - Ca,Mg (CO 3 ) 2 • Form when Mg-rich waters circulate through limestone and Mg substitutes for some Ca
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Sediments to Sedimentary Rock Sediments become rock by burial by younger sediments When buried - sediments are subjected to increasing: Temperatures • Average geothermal gradient = 25 o C/km (range 18-55 o C/km) Pressures • Typical lithostatic gradient = 1 psi/ft Hydrostatic pressure = load of interconnected column of water in the pore spaces within the rock Hydrostatic gradient depends on salinity of the water (and Temp) – Fresh water gradient - 0.43 psi/ft – Moderate salinity water = 0.465 psi/ft (88,000 ppm solids) Burial - Diagenesis ( dia = change, genesis = origin)
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Sediments to Sedimentary Rock • Diagenesis includes Lithification (“ make stone” ) • Lithification : Compaction and Cementation Compaction : Weight of overlying material compresses the deeper sediments • Grains press closer together • Open space between grains (pore space) decreases – ex. - buried clays reduced 40% in volume • Water trapped in sediments is driven out • Most significant in fine-grained rocks (siltstone & shale)
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Sediments to Sedimentary Rock • Lithification : Compaction and Cementation Cementation
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This note was uploaded on 10/11/2011 for the course GEOG 3413 taught by Professor Hileman during the Fall '10 term at Oklahoma State.

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12-11 Essentials - Essential Tools Sediments to Sedimentary...

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