Subsurface Geology and Resource Exploration

8/13/2019 Subsurface Geology and Resource Exploration

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Hydrocarbon traps are the s tructures, s tratigraphic differences or permeability and poros ity 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 mass es 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 hum mocky or "bumpy" hill-like features theycause on the seafloor surface. They are also found on land (e.g. Avery Island, Louis iana).

How do geologists decide where to drill for oil?

Using a ll the available data, geologists will try to find traps and assess the poss ibilities 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

mus t 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

500

1000

1500

2000

2500

3000

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 s ound wave energy down into the ear

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

different layers.

Source Receivers

The resultingseismic section is li ke a cross section, but the vertical axis is the two-way travel time,

the time it takes the waves to go from s ource to receiver. Avelocity model mus t be applied to

convert the time data into depth below the surface. Then a geologis t can correlate seismic "reflectorsto the geology seen in well logs.

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

Processing of

seismic data

Subsurface

geology

Seismic section -

ready to interpret!

Seismic data

acquisition

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 reasonableinterpretations 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.


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You are a geologist with Got Gas? Oil Company (GGOC). You have anopportunity to explore for oil in the Gulf of Mexico, and you have new seismicdata to interpret along with well log data.

You must tell your boss, Big Earl Spindletop, where the oil is and if it iseconomical to recover it. In other words, if can you make money on it! Use thewell logs and seismic lines to do the job!

Remember with seismic data, a source has produced sound waves thatpropogate beneath the seafloor of the Gulf. The waves travel at different speedsthrough different media, and it is the density contrasts between adjacent layersthat show up as discontinuities in the seismic data.

Medium Seismic Wave Velocity [ft/sec]Water 4950

Sandstone 9750Limestone 16,170

Avg. Sedimentary Rocks 11,000Salt 15,000

Granite 16,040Basalt 18,990

The formula for converting depth to time and vice versa in seismic data is:

Z = V x T Where: Zis depthVis wave velocityTis time

To convert the seismic data, which are in two-way travel time into depth belowthe seafloor, you must use an average velocity or velocity model that representsthe 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 byformation contacts, faults etc.

Remember, two-way travel time is the amount of time it takes for the waves totravel fromleaving the source, bounce off a subsurface discontinuity and travelback to the receivers. You must take this into account when calculating thedepths.

Well Logs

1. Using the well log cuttings, identify and list the rock types from eachformation on the well log data sheet.

2. Determine which seismic wave velocity should be used for two-waytravel time conversions.


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Subsurface Geology and Resource Exploration