Alistair R. Brown

Seismic attributes and their classification

Attributes have proliferated recently with different selections available on different workstations. What do they all mean? When do we use one and when another? The answers to these questions are not easy but the first step is to understand what our options are, and herein lies the purpose of this article.

Understanding seismic attributes

With the success of 3-D surveys has come the popularity of seismic attributes. Attributes are valuable for gaining insight from the data particularly when displayed spatially over interpreted horizons. However, all the many attributes available are not independent pieces of information but, in fact, simply represent different ways of presenting a limited amount of basic information. The key to success lies in selecting the most applicable attribute for the problem. Furthermore, statistical analysis using attributes must be based on understanding, not simply mathematical correlation.

Interactive seismic mapping of net producible gas sand in the Gulf of Mexico

In the Garden Banks area of offshore Louisiana several gas sands have been drilled and found productive. However, the sands are laterally variable in thickness and effectiveness. An improved understanding of the spatial distribution of net producible gas sand is highly desirable for reservoir management. The bright reflections from the top and the base of each sand were tracked automatically on an interactive interpretation system. This yielded time structure maps and hence isochron maps for each gross sand interval. The horizon Seiscrop™ sections diplaying amplitudes over the sand interfaces were then summed, adjusted for tuning effects, and smoothed to yield estimates of net gas/gross sand ratio over the area under study. By combining these with the corresponding isochron maps and an appropriate gas sand interval velocity, we obtained net gas sand isopach maps which tie acceptably with well data. Integration of these provided total reservoir volumes. ™Trademark of Geophysical Service Inc.

Pitfalls in 3D seismic interpretation Keynote presentation at the 11th Annual 3-D Seismic Symposium, Denver

Todays geophysical workstations are splendid tools but they are only tools. Unfortunately too many interpreters are expecting to find the solution to their problem in the workstation! The skill remains the thoughtful geological interpretation of geophysical data. As a consultant, I am often in a position to review seismic interpretations by others. It gives me the opportunity to reflect on how geoscientists can improve interpretations and avoid pitfalls. All too often I am in contact with seismic interpreters who have misidentified a horizon, failed to understand the phase and polarity of their data, distorted the result with a poor use of color, used an inappropriate attribute, failed to recognize a significant data defect, or are still frightened by machine autotracking.

Interactive interpretation of seismic data

Interactive interpretation is urgently needed to increase the productivity of the world’s hard‐pressed seismic interpreters. This paper describes the use of an interactive system that displays seismic data in color on a television screen. The system is easy to use and, by automatically managing the data base for the interpreter, permits him to spend a larger portion of his time on the thoughtful process of interpretation itself. A 3-D data volume can be studied in both vertical or horizontal section form. The system can equally well handle the irregular grid from a 2-D survey. In the course of interpreting a section on the screen, the interpreter may manipulate portions of that section in a variety of ways. He may make composite displays of multiple pieces of data, track horizons in automatic or manual modes, and zoom portions of the data to any desired extent. An interpretation made on one section can later be viewed on other sections marked at the points of intersection. Data can also be flattened to ai...

Data polarity for the interpreter

Zero phase is universally desirable but difficult to achieve. There are two versions of zero phase, known as American polarity and European polarity. American polarity is described as: An increase in impedance yields positive amplitude normally displayed in blue. A decrease in impedance yields negative amplitude normally displayed in red . American polarity is described as: An …

The value of seismic amplitude

We have always observed amplitude in the seismic signal but, in the early days, we were concerned with its existence and not its magnitude because our objectives were only structural. Digital processing today generally seeks to preserve “true” amplitude so that stratigraphic inferences can be made from it and more subsurface information extracted from our seismic data. But how “true” are these amplitudes and how much can we infer from them? A workshop at the 1985 SEG Annual Meeting in Washington focused on this subject and concluded that our ability to control and understand seismic amplitude is far from perfect. This is undoubtedly true but there is nevertheless much valuable information in seismic amplitude that we must use to the full in our seismic interpretations and in our critical exploration and development decision making.

Fault slicing—A new approach to the interpretation of fault detail

The manner in which a fault intersects a hydrocarbon reservoir affects production characteristics and thus must be understood in great detail. A 3-D seismic data volume can be sliced interactively to yield seismic sections parallel to a fault plane. These fault slices can then be used in several ways for the study of faults. Tracking of correlative horizons on fault slices provides a map of fault throw and permits study of the throw as a function of vertical traveltime and horizontal position. Because a fault slice remains within one major fault block, the study of growth relationships in that block is facilitated. Splinter faults, which are also significant in development and production, can be studied effectively on fault slices because of the uniform proximity of these sections to the parent fault. We conclude that there is some uniformity in azimuth between splinter faults and their parent.

Field appraisal with three‐dimensional seismic surveys offshore Trinidad

A consortium comprising Texaco Trinidad Inc., Trinidad and Tobago Oil Company Ltd., and Trinidad‐Tesoro Petroleum Company Ltd. commenced exploration in the South East Coast Consortium block offshore Trinidad in 1973. After four years of intensive exploration, a gas/condensate discovery was announced in early 1977 on the Pelican prospect. Later that year, in anticipation of the possible future need to site drilling/production platforms, a three‐dimensional (3-D) seismic survey was recorded over the prospect. This survey resulted in improvements in seismic record quality, multiple attenuation, and fault resolution. A coordinated geologic‐geophysical interpretation based on the 3-D seismic survey, a reevaluation of log correlations, and the use of seismic logs differed significantly from earlier interpretations. Because of this, it is anticipated that development of the field will need to be initiated in a different fault block from that previously envisioned. A second 3-D survey contiguous to the Pelican su...

Interperter's corner; picking philosophy for 3-D stratigraphic interpretation

As interpreters seek to extract more information out of seismic data, they must adapt to 3-D, to workstations, to color display, and to many tools and concepts of stratigraphic and reservoir interpretation. Undoubtedly more and more of our prospects are stratigraphic, and some of us are not adapting fast enough. In this article my concern is picking philosophy. Approaches that are adequate — and well established — for structure mapping will often not work for stratigraphy. Let us consider an all‐too‐common procedure for horizon identification at the start of a project.