Seismic Interpretation basics



Introduction:
Seismic interpretation, whether for hydrocarbon exploration or geotechnical studies, is the decision of the geological significance of seismic data. It is rare that the correctness (or incorrectness) of an interpretation tin last ascertained, because the actual geology is rarely known inwards plenty detail. Instead, the examine of a adept interpretation is consistency alongside all of the available data. In stone oil too gas exploration, emphasis is placed on finding an interpretation that is most favourable for hydrocarbon accumulation. As alongside many scientific investigations, interpretations are almost ever non-unique.

Petroleum Geology – Basic Concepts:

    -Hydrocarbons are formed yesteryear burial too transformation of organic affair through chemic changes that are controlled yesteryear temperature too duration. Rocks that comprise sufficient organic affair for the formation of hyrocarbons are called origin rocks.
    -Hydrocarbons tend to last less dense than formation waters (brines) too hence ascent upwards nether bouyancy forces.
    -The upward migration is stopped if at that topographic point is a permeability barrier. The hydrocarbon trap must receive got closure.


Key elements required for stone oil too gas accumulation:

    -A adept origin rock.
    -Sufficiently long burial of the origin stone for the generation of hydrocarbons.
    -There must last a migration pathway.
    -There must last a hydrocarbon trap alongside a adept reservoir stone (i.e., porous too permeable), a adept seal (permeability barrier) too closure.
    -After accumulation of the hydrocarbons, nil happens to degrade the reservoir.


Seismic exploration is indirect!
Source rocks, score of origin stone maturity too migration pathways are to a greater extent than oft than non not detectable using seismic data. It is sometimes (but non always) possible to distinguish hydrocarbons from formation fluids. Therefore, inwards hydrocarbon exploration programs the ultimate goal of seismic surveys is unremarkably to map potential reservoirs for closure. Potential hydrocarbon traps are after tested yesteryear drilling.


Examples of hydrocarbon traps:

    -Anticline
    -Termination of dipping layers at a fault
    -Termination of dipping layers at an unconformity
    -Facies alter (permeable to impermeable)
    -Porous reef.
    -Bed final result at the side of a tabular array salt dome. Basic Seismic Interpretation Procedure


Common Pitfalls of Seismic Interpretation:

    -Pull-up too pull-down caused yesteryear velocity distortions.
    -Multiple reverberations


Synthetic Seismograms:
Synthetic seismograms are artificial seismic traces used to constitute correlations betwixt local stratigraphy too seismic reflections. To make a synthetic seismogram, a sonic log is needed. Ideally, a density log should likewise last used, exactly these are non ever available.

Procedure:

    -Convert sonic transit times to velocity yesteryear taking the reciprocal (and applying scale factor as appropriate). Initially, the logs are sampled evenly inwards depth (e.g., twenty cm spacing betwixt readings).
    -Use integrated velocity to convert log depths to two-way time.
    Sometimes velocity too density logs are resampled to last spaced as inwards two-way time, instead of evenly spaced inwards depth.
    -Calculate the reflection coefficient for each sample point. The effect is a reflection coefficient (RC) fourth dimension series.
    -Convolve the RC fourth dimension serial alongside an assumed origin wavelet.


Notes:
Often, several wavelets too wavelet polarities are tested. Positive polarity unremarkably refers to a wavelet for which the positive cardinal peak coincides alongside a positive RC. Conversely, negative polarity way that a negative trough coincides alongside a positive RC after structure of the synthetic seismogram.

Vertical resolution of seismic data:
Vertical resolution tin last idea of as the minimum resolvable bed thickness. There are 2 criteria that are used to define this limit:

    -The Rayleigh boundary – bed thickness (h) is 1/4 of the seismic wavelength (or, two-way fourth dimension thickness is 1/2 of the dominant seismic period). This is the tuning boundary – i.e., maximum constructive interference betwixt the exceed too bottom of the bed takes house for this value of bed thickness.
    -Widess boundary – bed thickness is 1/8 of the seismic wavelength. This is the resolution limit. For example, if the velocity is 4000 m/s too the dominant frequency is fifty Hz, so the seismic wavelength is lxxx m. Hence the resolution boundary is 10 thousand – this is the thinnest resolvable bed thickness for these parameters.


Horizontal resolution of seismic data
The horizontal resolution of unmigrated seismic information is given yesteryear the Fresnel zone, which has a width of:
 whether for hydrocarbon exploration or geotechnical studies  Seismic Interpretation basics
Where:

    Fn = The nth Fresnel Zone radius inwards metres
    d1 = The distance of P from i destination inwards metres
    d2 = The distance of P from the other destination inwards metres
    ╬╗ = The wavelength of the transmitted signal inwards metres


For example, for a velocity of 4000 m/s, a two-way fourth dimension of 1.0 sec too a frequency of fifty Hz, nosotros receive got w = 141 m!

In principle, the horizontal resolution of migrated seismic information is equal to the spatial Nyquist wavenumber (i.e., twice the CMP draw spacing – typically virtually 10 m). In practice, this horizontal resolution is never achieved, too a resolution of virtually 3-4 traces (typically 30-40 m) is to a greater extent than realistic.

Additional notes on tabular array salt domes:

    -Deeply buried tabular array salt is oft less dense than the overlying strata, too is hence dependent area to buoyancy forces.
    -Although inwards manus specimens halite is solid, over geologic fourth dimension tabular array salt behaves similar a gluey fluid.
    -Salt domes tin last subdivided into swells (does non pierce overlying strata) or diapirs (overlying units are breached).
    -Stratigraphic thickening of side yesteryear side beds tin last used to deduce timing of diapirsm.




see also: 
Hydrocarbon traps
The Petroleum System


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