mineral grains pore spaces

Subsurface Geology and Resource Exploration

Locating earth resources such as aluminum, copper, gold and gemstones has been an important job forgeologists for a long time. This lab deals with the exploration for oil, which is important to Louisiana, theGulf 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, thatare refined for use as fuels.

When sediments are deposited, they include mineral grains, rock fragments, water with chemicalelements in solution as well as organic material. Under the increased pressure and temperatureof burial, the organic matter included in the sediments can be transformed into oil and gas throughchemical reactions. The migration and accumulation of hydrocarbons and waterin a givensedimentary 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.

Grains and pore spaces are easily seen in thismicroscope photograph.

Impermeable rocks that impede the movement of hydrocarbons are known as confining beds (e.g.shales), and permeable rocks that allow hydrocarbons to migrate arereservoir rocks (e.g. sandstones,limestones).

sandstonereservoir rock

shaleconfining bed

"Oil and water do not mix",because the density of naturalgas is less than that of oil whichis less than that of water. So,the three will separate out fromeach other in hydrocarbontraps.

LAB$11.

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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 highenough concentrations to be viable for extraction. The hydrocarbon accumulation isknown as apool.

Salt domes are large masses of salt (low density) that dome upward and deform the more denserocks around them, creating structural traps on their margins. These are very common in the Gulfof Mexico and can be located on the seafloor by the hummocky or "bumpy" hill-like features theycause 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 itmust 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 identifiedthe rocks and fossils that come up in the "cuttings" as a well is drilled.

Well A Well B

5001000

1500200025003000

Depth(feet)

Sea Floor

Notice that the geology between the wells is inferred from the well log data.

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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 earth.

Receivers (called "geophones") measure the time it takes the wave energy to be reflected back off ofdifferent layers.

Source Receivers

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 toconvert the time data into depth below the surface. Then a geologist can correlate seismic "reflectors"to the geology seen in well logs.

The waves will travel at different speeds according to the different densities of the different layers.

Processing ofseismic data

Subsurfacegeology

Seismic section -ready to interpret!

Seismic dataacquisition

The seismic reflectors could bedepositional contacts, unconformities,faults, contrasts in densities, or otherfeatures. A geologist will combine allthe available data - well logs, surfacedata from maps, knowledge of thephysical properties of different rocks,etc. to then make reasonableinterpretations of what the seismicreflectors represent. With the crosssection created on the seismicsection, evaluations can be made ofthe most likely places to findpetroleum.

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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.

Seismic'Line'1.''

1. According'to'the'well'logs,'how'deep'are'the'Shorty'Harris'and'Eddie'Bo'wells?''''''

2. What'kind'of'geologic'structure'lies'between'the'two'wells?'Identify'it'and'trace'it'out'on'the'seismic'data,'and'be'sure'to'label'it.''''''

3. The'Shorty'Harris'well'is'pumping'oil'from'the'St.'Louis'Fm'reservoir'rock.'The'pool'it'is'tapping'is'estimated'to'have'a'million'barrels'of'oil'in'it.'What'kind'of'trap'allowed'the'oil'to'pool'there?'

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Seismic'Line'2''

1. What'does'the'seafloor'surface'in'the'area'of'your'seismic'line'look'like'according'to'your'bathymetric'map?'Since'media'with'relatively'higher'velocities'than'what'surrounds'it'look'like'“bright'spots”'on'seismic'profiles,'what'could'the'large'bright'structure'in'this'profile'be?'''''Draw'the'outline'of'the'structure'on'your'profile,'where'the'sedimentary'bed'reflections'end'against'it,'and'label'it.'''''

2. Although'the'Reverend'Horton'Heat'well'gets'into'the'St.'Louis'Formation'reservoir'rock,'it'is'not'producing'oil.'Using'the'well'log'and'seismic'velocities'given'in'the'table'in'the'packet,'draw'the'contacts'of'the'different'formations'on'the'seismic'line.'Start'at'the'well,'and'then'trace'the'seismic'reflectors'out'in'both'directions'as'far'as'seems'reasonable.''''''''

'3. 'Using'your'contact'and'structure'interpretations'above,'where'should'you'

recommend'that'“Big'Earl”'drill'for'oil:'Option'1'or'Option'2?'Also,'mark'that'location'on'your'bathymetric'map.''How'deep'would'the'well'have'to'be'to'tap'the'St.'Louis'Formation?'''What'kind'of'trap'might'be'there'according'to'your'interpretations?'''If'the'oil'pool'at'your'proposed'well'is'estimated'to'be'as'big'as'that'of'the'Shorty'Harris'well'(question'4'above),'and'oil'is'currently'selling'for'$25/barrel,'how'much'money'can'be'made'from'it?'''Your'engineers'tell'you'that'it'will'cost'$2500'a'foot'to'drill'a'well.'How'much'did'it'cost'to'drill'the'unsuccessful'Reverend'Horton'Heat'well?'''How'much'will'your'proposed'well'cost?'''Considering'the'cost'of'drilling'for'your'well'and'the'unsuccessful'Reverend'Horton'Heat'well,'and'assuming'the'oil'pool'in'your'discovery'is'as'big'as'that'of'the'Shorty'Harris'well,'will'GGOC'make'money'on'your'well?'If'so,'how'much?'If'not,'at'least'how'many'barrels'of'oil'must'be'in'the'discovery'in'order'for'the'company'to'make'a'profit'at'current'prices?'If'there'truly'is'1'million'barrels,'what'would'the'price'of'each'barrel'have'to'be'just'to'break'even?'

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