Subsurface Geology and Resource Exploration

Transcription

1 LAB$11. Subsurface Geology and Resource Exploration Locating earth resources such as aluminum, copper, gold and gemstones has been an important job for geologists for a long time. This lab deals with the exploration for oil, which is important to Louisiana, the Gulf of Mexico area, and the whole economy of the modern world. Hydrocarbons are the solid, liquid or gaseous material, like crude oil (petroleum) and natural gas, that are refined for use as fuels. When sediments are deposited, they include mineral grains, rock fragments, water with chemical elements in solution as well as organic material. Under the increased pressure and temperature of burial, the organic matter included in the sediments can be transformed into oil and gas through chemical reactions. The migration and accumulation of hydrocarbons and water in a given sedimentary rock depends on several factors: Porosity is the percentage of void space (e.g. fractures or holes) in a rock body. Permeability is a measure of how easily fluids can pass through a rock body. mineral grains pore spaces Grains and pore spaces are easily seen in this microscope photograph. Impermeable rocks that impede the movement of hydrocarbons are known asconfining beds (e.g. shales), and permeable rocks that allow hydrocarbons to migrate arereservoir rocks (e.g. sandstones, limestones). "Oil and water do not mix", because the density of natural gas is less than that of oil which is less than that of water. So, the three will separate out from each other in hydrocarbon traps. shale confining bed sandstone reservoir rock

2 Hydrocarbon traps are the structures, stratigraphic differences or permeability and porosity contrasts (e.g. unconformities, faults and folds) that force the accumulation of hydrocarbons in high enough concentrations to be viable for extraction. The hydrocarbon accumulation is known as apool. Salt domes are large masses of salt (low density) that dome upward and deform the more dense rocks around them, creating structural traps on their margins. These are very common in the Gulf of Mexico and can be located on the seafloor by the hummocky or "bumpy" hill-like features they cause on the seafloor surface. They are also found on land (e.g. Avery Island, Louisiana). How do geologists decide where to drill for oil? Using all the available data, geologists will try to find traps and assess the possibilities of oil being there. Even if they decide oil is there, the possible amount of oil and the total cost of drilling for it and refining it must be compared to the amount of money that will be made by selling the final product. Well log data is the information that comes from previously drilled wells. Geologists have identified the rocks and fossils that come up in the "cuttings" as a well is drilled. Well A Well B Depth (feet) Sea Floor Notice that the geology between the wells is inferred from the well log data.

3 Seismic data can be used with existing well log data to interpret the geology between the wells. A source (like airguns,vibration plates or even explosives) sends sound wave energy down into the eart Receivers (called "geophones") measure the time it takes the wave energy to be reflected back off of different layers. Source Receivers The waves will travel at different speeds according to the different densities of the different layers. The resultingseismic section is like a cross section, but the vertical axis is the two-way travel time, the time it takes the waves to go from source to receiver. Avelocity model must be applied to convert the time data into depth below the surface. Then a geologist can correlate seismic "reflectors to the geology seen in well logs. Seismic section - ready to interpret Processing of seismic data Seismic data acquisition Subsurface geology The seismic reflectors could be depositional contacts, unconformities, faults, contrasts in densities, or other features. A geologist will combine all the available data - well logs, surface data from maps, knowledge of the physical properties of different rocks, etc. to then make reasonable interpretations of what the seismic reflectors represent. With the cross section created on the seismic section, evaluations can be made of the most likely places to find petroleum.

4 You are a geologist with Got Gas? Oil Company (GGOC). You have an opportunity to explore for oil in the Gulf of Mexico, and you have new seismic data to interpret along with well log data. You must tell your boss, Big Earl Spindletop, where the oil is and if it is economical to recover it. In other words, if can you make money on it Use the well logs and seismic lines to do the job Remember with seismic data, a source has produced sound waves that propogate beneath the seafloor of the Gulf. The waves travel at different speeds through different media, and it is the density contrasts between adjacent layers that show up as discontinuities in the seismic data. Medium Seismic Wave Velocity [ft/sec] Water 4950 Sandstone 9750 Limestone 16,170 Avg. Sedimentary Rocks 11,000 Salt 15,000 Granite 16,040 Basalt 18,990 The formula for converting depth to time and vice versa in seismic data is: Z = V x T Where: Z is depth V is wave velocity T is time To convert the seismic data, which are in two-way travel time into depth below the seafloor, you must use an average velocity or velocity model that represents the geology seen in the well logs. Then you can use the well logs, which are in depth below seafloor, to determine which seismic discontinuities are caused by formation contacts, faults etc. Remember, two-way travel time is the amount of time it takes for the waves to travel from leaving the source, bounce off a subsurface discontinuity and travel back to the receivers. You must take this into account when calculating the depths. Well Logs 1. Using the well log cuttings, identify and list the rock types from each formation on the well log data sheet. 2. Determine which seismic wave velocity should be used for two-way travel time conversions.

5 SeismicLine1. 1. Accordingtothewelllogs,howdeeparetheShortyHarrisandEddieBowells? 2. Whatkindofgeologicstructureliesbetweenthetwowells?Identifyitandtraceit outontheseismicdata,andbesuretolabelit. 3. TheShortyHarriswellispumpingoilfromtheSt.LouisFmreservoirrock.Thepoolit istappingisestimatedtohaveamillionbarrelsofoilinit.whatkindoftrapallowed theoiltopoolthere? SeismicLine2 1. Whatdoestheseafloorsurfaceintheareaofyourseismiclinelooklikeaccordingto yourbathymetricmap?sincemediawithrelativelyhighervelocitiesthanwhat surroundsitlooklike brightspots onseismicprofiles,whatcouldthelargebright structureinthisprofilebe? Drawtheoutlineofthestructureonyourprofile,wherethesedimentarybed reflectionsendagainstit,andlabelit. 2. AlthoughtheReverendHortonHeatwellgetsintotheSt.LouisFormationreservoir rock,itisnotproducingoil.usingthewelllogandseismicvelocitiesgiveninthe tableinthepacket,drawthecontactsofthedifferentformationsontheseismic line.startatthewell,andthentracetheseismicreflectorsoutinbothdirectionsas farasseemsreasonable.

6 3. Usingyourcontactandstructureinterpretationsabove,whereshouldyou recommendthat BigEarl drillforoil:option1oroption2?also,markthat locationonyourbathymetricmap. HowdeepwouldthewellhavetobetotaptheSt.LouisFormation? Whatkindoftrapmightbethereaccordingtoyourinterpretations? IftheoilpoolatyourproposedwellisestimatedtobeasbigasthatoftheShorty Harriswell(question4above),andoiliscurrentlysellingfor$25/barrel,howmuch moneycanbemadefromit? Yourengineerstellyouthatitwillcost$2500afoottodrillawell.Howmuchdidit costtodrilltheunsuccessfulreverendhortonheatwell? Howmuchwillyourproposedwellcost? ConsideringthecostofdrillingforyourwellandtheunsuccessfulReverendHorton Heatwell,andassumingtheoilpoolinyourdiscoveryisasbigasthatoftheShorty Harriswell,willGGOCmakemoneyonyourwell?Ifso,howmuch?Ifnot,atleast howmanybarrelsofoilmustbeinthediscoveryinorderforthecompanytomakea profitatcurrentprices?iftheretrulyis1millionbarrels,whatwouldthepriceof eachbarrelhavetobejusttobreakeven?

7

8

9

10