Utility of 3-D Seismic Attribute Analysis and VSP for Assessing Potential Carbon Sequestration Targets on the Rock Springs Uplift, Southwest Wyoming

Abstract

One of the most important steps in characterizing a geologic CO₂ storage site is the construction of 3-D volumes of seismic attributes. Once these seismic attribute volumes are constructed, key attributes can be correlated to core and well-log observations and analytical measurements. These correlations allow standardizations and extrapolations of a variety of determinative rock/fluid characteristics from the well bore of the stratigraphic test well out into the 3-D seismic survey volume, creating a realistic 3-D model of storage reservoirs and seals. This chapter discusses the technique for performing the tasks required to make the seismic attribute volumes; the correlations between key rock/fluid parameters and seismic attributes; and the crucial extrapolations from 1-D core, log, and VSP observations out into 3-D seismic attribute space. Topics covered include surface seismic specifications, seismic data processing, seismic resolution, vertical seismic profiling (VSP) and data acquisition, comparison of VSP and surface seismic data, comparision of VSP and well data (geologic property modeling), horizon mapping and depth conversion, seismic attributes, seismic interpretation, and qualitative permeability determined from seismic attribute analysis.

Glossary of Seismic Terms

Acquisition

the generation of seismic energy waves within the earth and their distant recording

Acquisition footprint

any regularity or pattern in seismic data that is not of geologic origin and can be due to acquisition or processing problems.

Aliasing

an effect that causes some continuous signals to become indistinguishable when sampled.

Azimuth

angle measured clockwise from north to a linear direction.

Bin

an area on the land surface that captures any seismic trace whose midpoint falls into it. All the traces in a bin form a three-dimensional CMP gather.

Common gather

a collection of seismic traces with a common or fixed characteristic.For example, a common shot gather is all traces generated by the same shot.

Component

the directional orientation of a source or receiver during acquisition.Conventional surface seismic involves data recording in one vertical component and two horizontal components.

CMP

(1) literally, common midpoint. The midpoint surface location for a seismictrace that is calculated from source and receiver coordinates. (2) A seismic acquisition and processing technique that yields multiple prestack traces at each midpointlocation.

CMP fold

the number of traces in a CMP gather. Conventional processing involves summation (stacking) of these traces to produce one poststack trace.

CMP gather

gather of all traces in a seismic survey having the same midpoint number or bin.

CMP stack

a stack of all prestack traces associated with the same midpoint number or bin.

Depth structure map

a map in depth units that is generated from a time structure map by means of a time-depth transform function.

Far offset

the distance from source to farthest receiver.

Fold

the number of seismic traces within a gather.

Frequency

the number of wave peaks or cycles per second.

Gather

see Common gather.

Horizon slice

a horizontal display or map view of 3-D seismic data showing a particular reflection, as opposed to a time slice that hasa constant travel time.

Interface

a surface in the earth where velocity, density, or other physical parameters change abruptly. Commonly, but not necessarily, coincident with a change inrock type or pore fluid.

Interpretation

the analysis of geophysical data to generate geologicly reasonable models and predictions about subsurface properties and structures.

Inversion

(1) a process of estimating subsurface physical parameters (velocity,density, etc.) from seismic data. (2) A situation where a low-velocity layer underliesa high-velocity layer.

Lateral resolution

the minimum separation of two features in map view before their individual identities are lost.

Medium

a term designating a specific location (layer or section) within the layeredearth model. Plural is media.

Midpoint

the point on the land surface halfway between source and receiver.

Migration

the process by which seismic events are geometrically re-located in either space or time to provide a more accurate image of the subsurface.

Offset

the distance between the source and receiver associated with a trace.

Phantom horizon

a seismic attribute not coincident with a certain tracked horizon but conformal to it and shifted by a certain number of milliseconds.

P-wave

the primary or fastest wave traveling away from a seismic energy source.This kind of seismic wave excites particle motion that is parallel to the direction of wave propagation. Currently, this is the main wave type used in reflectionseismology.

Poststack

an object or process related to seismic data after CMP stacking.

Prestack

an object or process related to seismic data before CMP stacking.

Processing

a sequence of digital data processing routines followed to enhance reflected seismic signals and to suppress noise.

Receiver

a device for measuring surface motion due to the passage of seismic or sound waves.

Reflected wave

the wave that returns back to the surface after reflection from an interface.

Reflection

(1) the process in which a seismic wave strikes and partially bounces off an interface. (2) An event recorded on the surface as a seismic image (in section or map view) associated with reflection from a particular interface.

Reflection coefficient

the ratio of the amplitude of a reflected wave to the amplitude of the incident wave.

Reflection coefficient series

the reflection coefficient as a function of recording time (depth).

Reflector

the interface associated with a particular reflection event.

Section

a side-by-side display of many seismic traces.

Seismic

the reflection seismology method commonly used to provide information about the subsurface structure of rock formations and the physical properties of both the subsurface rocks and the fluids within them.

Seismic event

any continuous object on a seismic display.

Seismic trace

a curve representing seismic attribute variation with time or depth a ta specific (fixed) location. Usually is plotted with time (depth) increasing downward.

Seismic wave

are energy wave that travels through the earth’s layers. May be of natural (earthquake, volcanic eruption) or artificial origin (explosion, vibrator track).

Shot

a seismic energy source.

Signal-to-noise ratio

ratio of peak signal amplitude to average noise amplitude.

Source

an object or process that generates seismic waves.

Stack

the single output trace created by summing two or more input traces.

Synthetic data

numerically created pseudo-seismic data used for testing processing algorithms or validating interpretation details.

Synthetic seismogram

a simulated seismic trace (at a well location) based on sonic and density logs that is used to associate seismic reflections with geologic horizons.

3-D seismic

the shooting and processing of seismic data in such a way that a cube of data is produced and interpreted for geologic meaning.

3-D survey

a dense grid of parallel source lines (cross-line direction in this study)and receiver lines (in-line direction) established for seismic data acquisition andprocessing.

Time slice

a horizontal display or view of 3-D seismic data having a certain arrival time.

Time structure map

a map generated by tracking that displays vertical reflection time to a horizon of interest.

Trace

a one-dimensional graph of a quantity vs. time or depth.

Tracking

the interpretation process of following a reflection surface throughout a data volume and extracting the travel time to this surface.

Velocity

the speed at which a seismic wave travels. Can vary from one location toanother in the earth.

Vertical resolution

the minimum separation of two features on a seismic trace that correspond to the top and base of a geologic unit or bed.

Vertical seismic profile (VSP)

a seismic acquisition technique in which a surface source shoots into a series of receivers located down a well.

Wavelength

the distance between successive equivalent points on two wave cycles.

Zero offset VSP

a VSP that has one source, located at or near the wellhead.