Tuesday, January 27, 2015

Reservoir Simulation Model

The distribution of porosity, permeability, and other properties over the area of a reservoir are incorporated in a reservoir simulation model. The models are updated as additional information becomes available. The validity of a model is judged by how closely actual results (fluid-flow rates, bottom-hole pressuress, and so on) match predictions based on the model.

A general rule of thumb is that a model can be used to predict reasonably well for about the same length of time as it has matched past history. Scenarios of production programs and strategies (for example, varying the number of wells and their locations, preferentially producing different wells, injecting water or gas to enhance the drive, and so on) are run on the model to determine how to optimize the economic return. Obviously, very realistic maps and data are required to achieve this end result.

(Sherrif. 1992. Exploration Seismology)

Monday, January 26, 2015

VSP Vertical Seismic Profile

The traces of the upgoing VSP are often stacked together to yield the pattern of primary reflection for correlating to conventional seismic data.

Corridor stacks are usually better than synthetic seismograms made from well-log measurements for relating reflections to interfaces because the measurements are made at seismic frequencies and are not sensitive to logging uncertainties.

The slope of the first breaks VSP (direct-wave traveltimes) gives the velocity. Reflections have a slope opposite to the first breaks. By using this difference, it is possible to separate downgoing waves (which consist of direct waves and multiples involving an even number of reflection) from upgoing waves . The upgoing waves may be 30dB below the downgoing waves.

(Sherrif. 1995. Exploration Seismology)

Thursday, January 22, 2015

Instataneous Frequency Seismic Attribute

A lowering of instataneous frequency is often observed immediately under hydrocarbon accumulations. Such "low-frequency shadows" seem to be confined to a couple of cycles below (not at) accumulations.  No adequate explanation is available, proposed explanations generally involve either the removal of higher frequencies because of absroption or other mechanisms, or improper stacking because of erroneous velocity assumptions or raypath distortions.

(Sherrif. 1995. Exploration Seismology. )

Seismic Reflection Characters

Parallel reflections suggest uniform deposition on a stable or uniformly subsiding surface, whereas divergent reflections indicate variation in the rate of deposition from one area to another or else gradual tilting. Chaotic reflections suggest either relatively high depositional energy, variability of conditions during deposition, or disruption after deposition, such as can be produced by slumping or sliding or turbidity-current flow.

A reflection-free interval suggests uniform lithology such as a relatively homogenous marine shale, salt , or massive carbonates ; however, distinguishing reflection-free patterns from multiples and noise that obsecures reflections may be difficult.

(Sherrif. 1995. Exploration Seismology)

Erosional Truncation & Toplap

Erosional truncation indicates that the sediment package formerly extended higher than it does today but that portions were removed by erosion,

whereas toplap indicates deposition near sea level and that the sediment package never extended significantly higher in the section.

With good seismic data quality, toplap sometimes can be distinguished from erosional truncation because there were changes in the depostional environment near toplap and consequently reflections are changeable in attitude and character, whereas no such changes occured at erosional truncation terminations.

Erosional truncation is the primary evidence for a sea-level fall. Where sediment packages are thick enough and noise sufficiently low, reflections showing these features can be seen in seismic data and used to determine the sea-level changes.

(Sherrif. 1995. Exploration Seismology)

Onlap & Downlap

Onlap indicates locations that are proximal (deposition close to the source of sediments, that is, on the landward side of a sediment package) and downlap locations that are distal (deposition distant from the sediment source).

(Sherrif. 1995. Exploration Seismology)

Tuesday, January 20, 2015

Classification of Stratigraphic Traps

  • Not adjacent to unconformities

Facies-change traps involving current-transported reservoir rock

  1. Eolian (dunes or sheet)
  2. Alluvial fan
  3. Alluvial valley (braided stream, channel fill, point bar)
  4. Deltaic (distributary mouth or finger bars, sheet, channel fill)
  5. Nondeltaic coastal (beach, barrier bar, spit, tidal delta or flat)
  6. Shallow marine (tidal bar, sand belt, washover, shelf edge, shallow turbidite or winnowing)
  7. Deep marine (marine fan, deep turbidite or winnowing)

Seismic Facies Characteristics

Picture above is the seismic facies characteristics (Fontane et al. 1987)

Sunday, January 18, 2015

Consistency of Seismic Interpretation

It is rare that the correctness or incorrectness of an interpretation can be ascertained because the actual geology is rarely ever known in adequate detail. The test of a good interpretation is consistency rather than correctness (Anstey, 1973). Not only must a good interpretation be consistent with all the seimic data, it also must be consistent with all that is known about the area, including gravity and magnetic data, well information and surface geology, as well as geologic and physical concepts.

One can usually be consistent and still have a choice of interpretations, the more so when data are sparse. The interpreter should explore various possibilities, but usually only one interpretation is wanted, that which offers the greatest possibilities for significant profitable hydrocarbon accumulation (assuming this is the objective).

An interpreter must be optmistic, that is, he must find the good possibilities.whereas a nonoptimistic interpretation may result in abandoning the area. Management is usually tolerant of optmistic interpretations that are disproven by subsequent work, but failing to recognize a possibility is an "unforgivable sin."

(Sherrif. 1995. Exploration Seismology)

Thursday, January 15, 2015

Static Analysis

The statics-analysis program looks for systematic variations such as would be expected if time shifts were associated with particular source activations, particular geophones, and so on.

Preliminary statics, as determined in the field office from first-break information and from the elevation of geophone stations, is usually input before the static analysis, so that the statics analysis determines residual statics errors. The result of this analysis are output on a control plot.

(Sherrif, Exploration Seismology. 1995)

Seismic Depth Migration

Migrated time sections may be simply stretched according to a vertical velocity function to give sections where the vertical scale is linear in depth rather than in time. However, where velocity varies apreciably in the horizontal direction, raypath bending introduces additional complications that depth migration attempts to accomodate.

Hubral (1977) observed that the apex of a diffraction curve is where the image ray, a ray that approaches the surface at right angles, emerges. Therefore, if we follow the image ray as it refracts according to Snell's law down through the earth, it will lead to the correct position of the diffracting point even if velocity surfaces are not horizontal. This concept is the heart of depth migration, migration that accomodates horizontal changes in velocity. Conventional migration collapses diffractions to the image-ray positions, so an additional step is needed to move elements to their correct subsurface locations.

The velocity model defines the major velocity surfaces where significant raypath bending occurs; key horizons on a conventionally migrated time section are mapped assuming that these are the major velocity interfaces. Cleary, defining the velocity model adequately is the key to succesful depth migration. Specifying velocities is a very difficult task because choices are not obvious. Detailed knowledge of the velocity distribution is often not available, especially in the structurally complex areas where depth migration is most needed. However, even though velocity errors create depth and location errors in the final product, the improved structural clarity often makes the procedure worthwhile and an appreciable amount of depth migration is being done today.

Subsalt imaging is important in several areas to locate hydrocarbons trapped beneath salt. Appreciable raypath bending occurs at the large contrast between the salt and sediments and the surfaces of the salt may be quite irregular. Migration is usually done in steps: conventional migration frist defines the top of the salt, then the base-of-salt reflection is defined using the salt veloctiy, and finally migration is completed with sediment velocities. Subsalt imaging provides a severe test of migration accuracy and requires very reliable data, which are usually 3-D data, and processing, often prestack migration.

(Sherrif, Exploration seismology. 1995)

Pada tahapan ekplorasi dimana konsep play menggunakan primary objektif carbonate, depth migration menjadi sangat dibutuhkan. Kontras dan distorsi velocity secara lateral menyebabkan imaging reef menjadi kurang sempurna bila processing hanya pada time migration. 

Sunday, January 11, 2015

Events on a seismic record

CMP gather

a = direct wave, V=650 m/s
b= refraction at base of weathering , V=1640 m/s
c= refraction from a flat refractor, Vr=4920 m/s
d= reflection from the refractor in c , V=1640 m/s
e= reflection from a flat reflector , V=1970 m/s
f= reflection from a flat reflector , V=2300 m/s
g= reflection from a dipping reflector V=2630 m/s
h= multiple of d
i = multiple of e
j= ground roll, V= 575 m/s
k= air wave, V=330 m/s

Reference : Sherrif, 1995. Exploration Seismology.

Quick QC Seismic Processing

To do quick QC of Seismic processing, assumed that we are a geoscientist in Oil Company reviewing the progress of processed seismic by vendor, is to look the prestack seismic sorted into CMP gathers.

CMP gather is one very powerful technique for distinguishing between reflections, diffractions, reflected refractions, and multiples . The CMP gather to do QC must through worfklows like
(a) Weathering and elevatation static corrections, because the correction is the same for all arrival times on a given trace.
(b) Normal moveout.

Provided the correct normal moveout has been removed, reflections appear as straight lines, whereas diffractions and multiples still have curvature, becuase their normal moveouts are larger than those of primary reflections, and refractions have inverse curvature.

(Sherrif, 1995. Exploration Seismology)

Tuesday, January 6, 2015

Sonic Logs effect on Synthetic Seismogram

Sonic logs may indicate sand velocities that are too high because they measure an invaded-zone velocity that exceeds the velocity of uninvaded sand. The editing of sonic-log data for synthetic seismogram manufacture attempts to correct for this. (Sherrif, 1990)