January, 1998 HGS Meetings

Emerging Technologies Group
Dinner Meeting, Thursday, January 8th, 1998

"Prestack Inversion: An Extension of AVO for Lithology and Hydrocarbon Fluid Quantification"

 
Speaker:
Rick Wallace

Authors:
Rick Wallace, Ulterra Geoscience Ltd., Calgary, Canada and Roger Young, Union Texas Petroleum., Houston Texas

Abstract:

Summary:
Over the past two decades post-stack seismic inversion, the process of deriving rock properties from seismic measurements, has evolved significantly. Recent advances in amplitude versus offset (AVO) technology have demonstrated that significant information is also contained in the prestack seismic data with regards to fluids and lithology.

We present a prestack inversion methodology which augments the qualities of AVO and inversion to accurately quantify sand/shale lithology and hydrocarbon fluid properties directly from prestack seismic data. The method is demonstrated on models and Canadian and international seismic data.

Introduction:
Early methods of recursive inversion converted seismic traces to well log traces, providing a measurement of the "pseudo acoustic impedance". The acoustic impedance could also be expressed as "pseudo-acoustic velocity" by assuming a simple relationship between velocity, density and acoustic impedance. In any event though, the inverted property was still acoustic impedance.

While the property of acoustic impedance is more of a geophysical measurement than a geologic rock property, it did yield some indication of actual rock types. Most importantly, it demonstrated that valuable physical information was present in seismic data which was being overlooked by conventional wiggle traces.

The resolution of recursive inversion was limited to the bandwidth of the seismic data (hence the name bandlimited inversion). By using spike detection algorithms to convert the seismic trace to a high frequency sparse reflectivity series prior to inversion, sparse spike inversion algorithms could achieve high resolution. The "blocky" lithologic boundaries created by sparse-spike methods more accurately modeled actual geologic conditions although the output physical quantity was still "pseudo-acoustic impedance".

Recently, model-based inversion schemes have evolved which essentially rely on the fact that the forward model of a "good" inversion should very closely match the actual seismic data. Using iterative forward modeling schemes, these methods perturb an initial acoustic impedance model until it's forward model matches the seismic traces. These methods have the advantage of allowing some degree of control over the starting point and hence the resulting inversion. Once again though, model based inversions still derive acoustic impedance.

AVO techniques have demonstrated that, with prestack seismic data, measurement of the conversion of compressional energy to shear energy at interfaces can yield information about the fluids and lithology present. More recently, advances in pre-stack imaging and analysis have resulted in significantly improved pre-stack signal quality with better preservation of lithologic information.

We present a pre-stack inversion technique which combines inversion and AVO technology with anisotropic petrophysics. This technique uses pre-stack seismic data as well as sonic, density and gamma ray logs to directly derive clastic rock properties including sand/shale content, gas saturation, water volume, and effective porosity. More recently, we have extended the technique to detect oil versus gas using absorption information.

Theory and Method:
We derive the AVO gradient (G) and the theoretical P-wave stack (P) with a least squares line fit to the trace amplitudes versus incident angle at each time sample (after Shuey): A(angle,t) = P(t) + G(t) sin**2 [angle(x,t)] where x is trace offset.

Using these we can derive pseudo-shear wave reflectivity (S) (after Gelfand and Larner):

S(t) = 1/2[P(t) - G(t)]

Inverting the P and S wave stacks, with low frequency constraints from sonic, density and gamma ray logs, yields P-impedance (IP) and S-impedance (IS). Petrophysical well log analysis, based on volume averaging, allows inversion of the inverse P and S impedance (IIP and IIS) to yield mineral volumes.

IIP = IIPfl*Pe + IIPss*Vss + IIPcl*Vcl IIS = IISfl*Pe + IISss*Vss + IIScl*Vcl

Where, Vss and Vclay are the fraction of sand and clay (respectively) in the matrix, and Pe is the effective porosity of the matrix (the volume not represented by sand or shale). The remaining factors (IIPfl, IISfl, IIPss, IISss, IIPcl, IIScl) are the physical properties corresponding to the impedances of pure water, sandstone and shale. The constants for water and sandstone remain relatively constant while the impedances of shale may vary slightly with the geologic setting and are usually adjusted as part of the calibration as shown in Figure 1

The same analysis technique can be applied to the gas quadrant of the cross-plot resulting in hydrocarbon volume (which can easily be converted to gas saturation Sg).
This inversion is applied to the entire prestack seismic dataset (after careful pre-processing and migration to preserve AVO effects) resulting in sand, clay, fluid and gas volumes for the entire seismic section. The net/gross sand volume can be represented by the following ratio:

N/G = Vss / (Vss + Vcl)

and indicates the quantity of sand present out of the total mineral content.

Examples:
We demonstrate the technique on model, Canadian and international seismic data. Figure 2

shows a prestack inverted section on an international structural dataset. The input gathers were prestack migrated with a Kirchhoff migration algorithm and processed to retain AVO effects. Figure 1

shows the cross-plot used to calibrate the inversion.
The inversion in Figure 2 (best displayed in colour) indicates the gas saturation in red (at the top of the sand member under the well location) and the sand/shale content in shades from yellow (pure sand) to green (pure shale). The prospect, on the down thrown side of the fault, indicates good gas saturation and highly porous sand which pinches out, becoming tighter to the right and forming the trap. This prospect has not yet been drilled. Note the sharp gas/water contact points in the upthrown reservoir at the base of the gas saturated sand (red). The location of the contact point is verified by reservoir pressure measurements.

Current Research:
1) Hydrocarbon Fluids
Current research in this methodology has combined absorption information derived from prestack seismic data with the P and S wave impedance information to accurately separate hydrocarbon fluid information. The method has accurately quantified water, oil and gas levels within reservoirs. 2) Carbonates
By fitting a 2nd order polynomial in the measurement of the AVO response from the seismic data, the rate of change of amplitude with offset (C) can be derived:

A(angle,t) = P(t) + G(t) sin**2 [angle(x,t)] + C(t) sin**4 [angle(x,t)]

Although this requires much more accurately processed seismic data to adequately retain this information, the improved fit of the second order polynomial better estimates the gradient (G) and P-wave (P) terms. The term C provides information about density and we are currently researching its usefulness in extending the method to include carbonates.

Conclusion
Prestack inversion demonstrates that significantly more information is contained in the seismic wavefield than simply acoustic impedance and that we can reliably quantify rock and fluid properties from seismic data. The method has been successfully applied to numerous 2D and 3D datasets from Canada, the US, and international targets.

Acknowledgments:
The authors would like to thank Union Texas Petroleum for their support of this research and for permission to use the structural data examples. We would also like to thank Gordon Holmes of Ulterra for his efforts in producing the model data example.

References:
Shuey, R.T., 1985. A simplification of the Zoeppritz equations: Geophysics, V. 50, p. 609-614.
Gelfand, V., et al, 1986: "Seismic Lithologic Modeling of Amplitude-versus-offset Data", Proceedings of the 56th Annual Meeting of the SEG, Nov. 2-6, 1986, p. 334-336.

Biographical Information:
Rick Wallace graduated from the University of Calgary in 1982 with a B.Sc. in Electrical Engineering. He began his career in geophysics as a Special Project Geophysicist with Western Geophysical in Calgary. In 1983 he joined Veritas as Group Leader of Modeling where he worked closely in the testing and development of Maximum Likelihood Deconvolution, Inversion, and AVO methods.

In 1993, Rick founded Ulterra Geoscience Ltd., a high technology group specializing in inversion, AVO and migration services. Ulterra currently employs 12 people, including one in Houston, and continues to be a leader in high technology processing services and software development.

During his career, Rick has authored and presented numerous papers on 3D inversion, AVO, grid balancing and, most recently pre-stack inversion. Rick served for many years on the CSEG convention committee and in 1995, Rick was presented with the CSEG Best Paper Award for his joint luncheon presentation with Doug Uffen on the Swan Hills 3D Inversion.

HGS Dinner Meeting
Monday, January 12th, 1998

Location:

Topic:
"Worldwide Expansion of the Deepwater Gulf of Mexico Success"

Author:
Dr. Bruce S. Appelbaum, President, Texaco Exploration

Abstract:

Success in the Deepwater Gulf of Mexico came as a result of challenges to the conventional wisdom of the 1970s. This led to developments in technology, changes in economic terms, new types of partnerships, and a different work environment. The knowledge gained from the domestic deepwater success is now being expanded globally. We have identified high-potential areas and are now addressing the challenges that come from working in the international arena.

Biographical Sketch:
Dr. Bruce S. Appelbaum is currently president of Texaco Exploration responsible for worldwide exploration activities. He earned a bachelor of arts degree in geology in 1969 from State University of New Yo rk , an M.S. in geological oceanography in 1971, and a Ph.D. in geological oceanography in 1974 from Texas A&M University. Prior to this appointment Dr. Appelbaum was the president of Texaco International Exploration Division. Dr. Appelbaum joined Texaco in 1990 as division manager of the Offshore Exploration Division. In 1993 he was named offshore division manager for Texaco Exploration and Production Inc., responsible for exploration and production activities in the Gulf of Mexico. Prior to his arrival at Texaco, Dr. Appelbaum worked at several independent oil companies, where he was instrumental in developing successful exploration programs primarily in the Gulf of Mexico, but also including international arenas such as the North Sea and Gulf of Suez. Appelbaum has an active network of contacts both within the industry and in academia. In addition to his civic and business leadership, Appelbaum has served as chairman of the executive committee on exploration of API, industrial liaison committee of AAPG and has been vice chairman of the Offshore Operators committee.

Environmental/Engineering Dinner Meeting
Wednesday, January 14th, 1998

Topic:
"Chronologies of Martian Meteorites: New Developments After the 1997 Mars Mission"

Author:
John H. Jones, Planetary Science Branch, NASA Johnson Space Center

Abstract:
A suite of approximately 12 meteorites (called SNC meteorites) have been identified as having originated from the planet Mars. The assignment of a Martian origin to the 12-meteorite suite is well documented. Members of the suite are assigned on the basis of similar oxygen isotopic compositions. Some individuals within the suite, however, contain trapped gases that are chemically and isotopically indistinguishable from the Viking lander analysis of the Martian atmosphere. This is an important observation as the Martian atmosphere is quite thin. Consequently, its chemical composition is continually being modified and is unique from Earth's and other planets.

The SNC acronym is short for shergottite-nakhlite-chassigny, three different lithologies within the suite, all of which are igneous. The shergottites are basalts or basaltic cumulates , the nakhlites are augitecumulates, and chassigny is an olivine cumulate. All members of the SNC suite have been subjected to isotopic analysis and radiometric dating. The nakhlites and chassigny meteorites yield consistent igneous ages of 1.2-1.3 billion years. This is old for an Earth ro ck , but is quite young for meteorites, which are typically 4.0-4.5 billion years old.

The shergottites, however, have been assigned a variety of igneous crystallization ages that range from 4.5 billion years to 350 million years. It will be argued that combining petrography with isotopic age dating allows a resolution of the shergottite age dilemma. The solution to this problem has interesting consequences for the geologic history of Mars.

Biographical Sketch:
Dr. John H. Jones received his B.S. from the University of Kentucky in 1978, and M.S. and Ph.D. degrees in geochemistry from the California Institute of Technology in 1978 and 1981. After post-doctoral and research associate positions with the University of Arizona, he joined the NASA Johnson Space Center as a staff scientist in 1987. His research interests include comparative planetology, the geochemical evolution of Mars, early history of the Earth-Moon system, meteorites and the origin of the solar system, and experimental trace element geochemistry.

Chairman's Note:
The public interest in Mars may have waned from the past summer's euphoria related to the Pathfinder and Sojourner Mars mission. That gives us a chance to have one of NASA's pre-eminent scientists speak to us about Martian geology. How do scientists know that these meteorites have fallen on Earth after originating from Mars? This dinner meeting will be an opportunity to share your views with others concerning the implications of the Mars research and if they represent transported Martian alien life forms to Earth!

International Explorationists Meeting
Monday, January 19th, 1998

Topic:
"Pliocene Deepwater Sands, Niger Delta, Africa: Sequence Stratigraphy, Depositional Facies, Sand Body Geometry and Stacking Patterns"

Author:
Ronald D. Kreisa. Mobil Exploration and Production Co.

Abstract:

Hundreds of closely spaced wells, combined with thousands of feet of core and recent 3-D seismic data provide an unparalleled opportunity to document depositional patterns of Pliocene deepwater sands of the eastern Niger Delta and have led to a clearer understanding of the factors responsible for these patterns.

The Niger Delta is a mixed-energy delta, with wave, tidal, and fluvial energy in near equilibrium, resulting in a radial pattern of distributaries. In Mobil's joint venture acreage, sand from these distributaries was fed through numerous canyons incised into the shelf edge and upper slope, rather than from a single point source. Most sand deposition occurred in fairways both within canyons and in channel levee complexes on the open slope. Individual channels are straight to sinuous, confined by levee deposits or canyon walls. They show little evidence of lateral migration. The ancient channels broke through levees, yielding anastomosing patterns. Multiple incisions within canyons are common. Deposition was also influenced by subsea bathymetry inherited from an earlier shelf margin collapse and by movement along faults.

Stacking patterns are distinctly cyclic. Allocyclic deposition relates to four low-stands of relative sea level. These are punctuated by higher frequency cycles that are both allocyclic and autocyclic. Within the deepwater succession, grain size is a function of stratigraphic forcing mechanisms and climate cycles. In a typical area, the facies within the channel deposits are composed of upward-fining successions 3 to 40 meters thick. They may contain relatively thin intervals of intraslope slumps and debris flows at the base, overlain by turbidite sands. Turbidite intervals range from graded, pebbly coarse grained sands up to 2 meters thick to fine and very fine-grained sands displaying complete Bouma sequences. Many of the slumps and debris flows were apparently generated by bed shear from the coarse-grained turbidity flows. Mass movements of shelf facies or processes for transport of sand into the basin other than by turbidite flow was rare.

The speaker would like to acknowledge R. B. Bloch, J. B. Paul, D. M. Jurick, and S. D. Joiner for their contributions to this research.

North American Explorationists Dinner
Monday, January 26th, 1998

Topic:
"Stratigraphic Traps in Base-Level Rise Deposits of Braided Alluvial and Arid Coastal Plain Sandstones (Frisco City Sand, Jurassic Haynesville Formation), Alabama"

Authors:
Robert C. Handford, Bureau of Economic Geology, Austin, Texas; and Lawrence R. Baria, Jura-Search, Inc., Jackson, Mississippi.

Abstract:

Stratigraphic traps in the Upper Jurassic Frisco City sandstone surround isolated and buried Appalachian basement highs or inselbergs near the updip margin of the Gulf Coast Basin in southwest Alabama. Reservoir sands and gravels, representing arid coastal plain environments, onlap the basement highs at depths of 9,000 12,000 ft. Traps with four-way closure have formed where the sands and basement highs are overlain by top-sealing marine shales. The largest of these fields discovered to date (North Frisco City) is expected to produce over 24 MMBO from 16 wells.

Ephemeral stream channel and sheet flood processes deposited much of the Frisco City sand. Aprons of coarse sand and metamorphic clasts accumulated around the inselbergs as rock fall and debris-flow deposits. The basinward margin of the clastic wedge was reworked by eolian and marine shoreface processes.

Frisco City sands sharply overlie the Smackover Formation, Buckner anhydrite, or Paleozoic basement, marking a basinward shift of facies and sequence boundary (141.5 Ma) at the contact. Basal Frisco City deposits consist of coarse alluvium, but these pass upward into coastal eolian and shoreface sands, and black marine shales of the middle Haynesville Fo rm ation. This succession indicates that deposition occurred during an overall base-level rise. However, some facies stacking patterns suggest that the overall rise was punctuated by several high-frequency base-level transit cycles or local tectonic relaxations during the final stages of extention in the Jurassic Gulf basin.

High-resolution 3-D seismic lines display stratal patterns indicative of retrogradational shoreface or alluvial lobe sand bodies onlapping basement highs. Differing oil/water contacts between these sand bodies may indicate compartmentalized reservoirs .

Biographical Sketches:
Lawrence R. Baria is a geological consultant and owner of Jura-Search, Inc., a company involved with exploration and research in the Upper Jurassic and Lower Cretaceous section of the Gulf Coast. Recent publications include topics on the sedimentology and seismic stratigraphy of the Cotton Valley, Haynesville, Smackover, and Norphlet Formations, as well as facies and diagenetic studies in the Mooringsport and James Lime. Baria received his B.S. and M.S. degrees in geology from Northeast Louisiana University. His Ph.D. studies at Louisiana State University involved sulfate and carbonate diagenesis.

Robert Handford received a B.S. in geology from the University of Northeastern Louisiana, an M.S from the University of Arkansas, and a Ph.D. from Louisiana State University. He has worked for the research laboratories of Unocal, Amoco, and ARCO, as well as the Bureau of Economic Geology at the University of Texas at Austin. After two years of consulting, he recently returned to the Bureau as a senior research scientist. His main areas of interest are sequence stratigraphy and depositional systems. During 1995 1996, he was an AAPG distinguished lecturer in carbonate stratigraphy.

HGS Luncheon Meeting
Wednesday, January 28th, 1998

Topic:
"Identification of Deltaic Facies with 3-D Seismic Coherency and the Spectral Decomposition Cube: A Study From South Marsh Island Area, Gulf of Mexico"

Authors:
John A. Lopez, Greg Partyka, Norm L. Haskell, and Susan E. Nissen, Amoco, New Orleans, Louisiana and Amoco, Tulsa, Oklahoma.

Abstract:
 

A new technique called the spectral decomposition cube is useful for mapping stratigraphy or reservoir delineation. The spectral decomposition cube is a Fourier transform applied to individual traces within a window around an interpreted horizon. Examination of the amplitude maps of specific frequencies can accentuate geologic features that are tuned to specific frequencies. Regional 3-D seismic in the Gulf of Mexico will be shown to illustrate detailed deltaic stratigraphic patterns. The potential usefulness of these frequency defined map attributes is demonstrated by mapping of a series of shallow horizons on a speculative seismic survey in the South Marsh Island area of the offshore Louisiana, Gulf of Mexico.

Coherence maps derived from 3-D seismic are a major breakthrough in the effective interpretation of 3-D seismic data. Coherence is defined as a quantitative measure of the similarity or dissimilarity of nearby seismic traces and is typically calculated as a post-processing seismic attribute. Coherence algorithms and spectral decomposition cubes are attributes of a rapidly developing expanding family of seismic attributes that are particularly helpful for 3-D seismic interpretation of map-view patterns of faults or stratigraphy. Their usefulness is increased when used in combination with traditional mapping techniques.

The result is a continuous measure of lateral changes in the seismic wavelet within the analysis window. The sinusoidal character of the seismic wavelet is removed, and what remains is principally geologic. Therefore, coherence can be used to map structural and sedimentological features directly. A major benefit to using 3-D coherence is that it can be run without any prior interpretation, thus removing initial interpretation biases and increasing the speed of subsequent interpretations. It is important to note that other seismic interpretation products, such as edge detection, may look similar to coherence, but these attributes may require a well-mapped horizon prior to calculation. Overall coherence seems to be more useful for structural applications. The spectral decomposition technique was developed to address the need for improved automated stratigraphic mapping.

The spectral decomposition cube can supersede coherence mapping when used for stratigraphic detection. The spectral decomposition amplitude maps are more useful for detecting stratigraphic features than the phase maps. The spectral decomposition cube is created by applying a FFT (Fast Fourier Transform) to the windowed trace at the picked horizon. The FFT decomposes the trace into all the separate frequencies of appropriate amplitude and phase which when summed together would re-create the initial windowed trace. The resulting spectral decomposition data is treated as either amplitude or phase cubes in which each cube has two dimensions that are a plane view map and the third dimension that is frequency increments. Scrolling up or down through the spectral decomposition cube allows examination in plane view of either amplitude or phase of each frequency within the trace decomposition. The aim is to accentuate possible stratigraphic features that may be tuned to specific frequencies and are not evident in the initial composite trace. Therefore these data are sometimes referred to as the spectral decomposition tuning cube.

One example is a coherency time slice corresponding to a Pleistocene surface at approximately 1200 m (4000 ft) depth, which displays a complex system of deltaic channels. The nearly identical map of channels can be seen on the spectral decomposition cube at 18 Hz. However, on the spectral decomposition image additional subtle features are seen that help complete the stratigraphic interpretation. A comparison of Pleistocene channel geometry with the modern Mississippi Delta suggests the presence of a paleo-Mississippi trunk channel oriented north-south and its associated distributary channels.

Conclusions:
Seismic at tributes applied to modern 3-D seismic surveys can be used to study ancient depositional systems in remarkable detail. Coherence and spectral decomposition are relatively new seismic attributes that are particularly useful in identifying channels. In general, no single attribute should be considered superior to any other. Different attributes may reveal different elements of the stratigraphy or different geophysical manifestations of the stratigraphy. Spectral decomposition in particular requires reviewing of many images to obtain all the possible information the seismic may hold. Final interpretations should include a composite of geologic information interpreted from many different images. The process is similar to that of satellite image interpretations of multiple-bandwidth images.

• Coherence is a multi-trace calculation that is a measure of the relative lateral variation in the traces. Spectral decomposition is a single trace calculation that decomposes the trace into a range of discrete frequencies, amplitude and phase. Spectral decomposition is a new way to map stratigraphic patterns which otherwise may not be recognized in the wavelet.
  
• 3-D seismic coherence and spectral decomposition are powerful seismic attributes to detect subsurface stratigraphy. These attributes are most useful when viewed in map view, where stratigraphic and structural patterns may be more evident.
  
• Channels in the South Marsh Island area generally appear as sub-parallel traces of low coherence at the channel margins.
  
• Different frequency amplitude maps reveal slightly different stratigraphic information which together give a more complete interpretation.
  
• Some stratigraphic imprints from shallower horizons can be found at certain frequencies from the spectral decomposition cube.

For additional information contact Jana L. DaSilva, SWLGS 3-D Expo Chairman, John E. Chance and Associates, Inc., 200 Dulles Drive, Lafayette, LA 70506; (318)268-3234; fax (318) 237-0011; e-mail: jdasilva@jchance.com