Exploration Geophysics - Volume 14, Issue 3-4, 1983

During the years since the introduction of digital recording and signal processing methods, the geophysicist has held the technical centre stage. During this period, the interpreter has received the benefit of better sections, but the methods and tools available to him have not altered significantly. With the decline in the cost of computing, and the advent of graphics processors, the interpreter finally has the benefits of computer technology at his disposal. This paper describes the use of a colour video system for structural and stratigraphic interpretation, and indicates how these capabilities will expand to combine lithologic and borehole data with the seismic information. The system is easy to use and manages the data for the interpreter, thus allowing him to spend a larger proportion of his time on the conjectural process of interpretation itself. Features of the system that aid this process include horizon tracking and structural flattening to simulate the structural configuration during deposition. Fault correlation and loop tying are also significantly simplified when using the system. The result is an interpretation that can be produced confidently and quickly, as compared to existing methods. Extension of these systems to the reconciliation of seismic, borehole and lithologic information will enhance the interpreter’s art significantly.

All sedimentary rocks are the product of their provenance, dispersal and depositional environment, and indications of these factors are preserved in the geologic record. The classical Uniformitarian Principle of Geology that ‘the present is the key to the past’ suggests that study of modern sediments, processes and depositional environments can be used to understand and explain ancient sediments. The same may be said of geophysics; examination of the seismic signatures of modern sediments can be applied, by analogy, to the interpretation of the seismic record of ancient sediments. High resolution seismic profiles from late Quaternary marginal-marine and reefal environments of the Queensland continental shelf are presented. It is suggested that a study of the seismic response of these modern sediments, which are uncon-solidated and free from the effects of tectonism, can assist in the genetic interpretation of petroleum seismic data.

Recent developments in the Offshore Gippsiand Basin have suggested that not all large oil accumulations are necessarily restricted to four-way dip, closed structures at Top Latrobe level; the discovery of the Fortescue oil field in September 1978 indicated that further substantial oil accumulations may be trapped by a mechanism of combined structural-stratigraphic closure. Integrated well log correlations, micropalaeontological dating, stratigraphic interpretation, geochemistry and reservoir pressure analyses have all been cited to indicate that the Fortescue oil accumulation consists of a series of discrete sandstone reservoirs lying on the western flank of, but separated from, the giant Halibut oil field. Seismic data have been incorporated with detailed geological data to develop a model by which such structural-stratigraphic plays can be recognised. Truncated seismic events, associated with the stratigraphic truncation of Upper Palaeocene to Lower Eocene delta-top coal swamp deposits, can be mapped across the area of the Fortescue field such that the existence of permeability barriers (basal and lateral seals) can be recognised and predicted. By combining the geometry of such permeability barriers with partial structural reversal at the level of the Top Latrobe unconformity, the geometry of the Fortescue oil field can be predicted. Palaeogeographical studies suggested that similar Palaeocene-Eocene coal swamp deposits might provide adequate permeability barriers to oil accumulations elsewhere in the Gippsiand Basin. The structural-stratigraphic model was therefore extrapolated to other parts of the basin in an attempt to recognise such potential plays. It is suggested that the West Kingfish and Yellowtail oil fields, perhaps not fully defined at Top Latrobe level by complete four-way dip closure, could be further examples of the structural-stratigraphic concept. The Pisces No. 1 well, recently drilled by Union Texas Australia Incorporated and its Joint Venture Partners in Permit Vic P/12 on the southern margin of the Gippsiand Basin, was a further test of the structural-stratigraphic play model. While the well proved to be dry, probably due to the lack of adequate vertical seal, the structural-stratigraphic model was essentially vindicated.

During the 1960s, a large proportion of the sedimentary basins of Australia were covered by aeromagnetic surveys, at least on a reconnaissance basis. But in recent years, although aeromagnetics have been widely used in Australia for mineral exploration, they seem to be largely neglected in current petroleum exploration. The techniques of high resolution aeromagnetic surveys are virtually untested in Australia. It is worth reviewing results from some of the old surveys to see what information they provided, whether this was useful at the time, and remains useful even today. Secondly, current techniques of surveying and data processing are reviewed, to see what might be expected from new and detailed high-resolution aeromagnetic surveys. It is believed that they have a role to play in guiding exploration in particular plays, especially those where basement structure and tectonics are fundamental, or where intra-sedimentary volcanics are involved.

A properly specified gravity survey is a cost-effective approach that can significantly aid the design of a seismic program in petroleum exploration. Gravity survey specifications should take into consideration three sources of error that affect the accuracy and resolution of gravity data: (1) error in the Bouguer anomaly value assigned to a station, (2) errors in interpolation of the gravity field between stations and (3) geological ‘noise’ due to near-surface density variations and deep regional discontinuities. The first error is principally a function of the uncertainty in the station positioning, while the other two are functions of the station spacing and coverage. Various types of gravity surveys are discussed with illustrated examples.

Common depth point data in seismic reflection surveys are now being recorded with ever increasing source-receiver offset distances. For such wide aperture seismic data the traditional methods of interpretation may fail, since velocity analyses and signal enhancement methods based on hyperbolic travel-time curves are no longer appropriate. The present paper shows, however, that even in steeply dipping environments both velocity determination and depth migration determination reduce to simple mathematical expressions. These expressions can be applied to solve complicated overburden problems.

The increasing use in recent years of ‘wavelet processing’ for seismic reflection data marks a shift in emphasis from processing for a predominantly structural interpretation to processing for a stratigraphic interpretation, which requires a high resolution wavelet of more stable phase characteristics. A wavelet processing system must begin with a ‘model’ or concept of the formation of the reflection wavelets, and such models have tended to become more complex as the sophistication of wavelet processing has increased. The advanced system described here is based on a model of the recorded wavelet as the convolution of a number of component wavelets, due to the source, the earth, and the recording system. After attenuation of noise by frequency-wave number (F-K) domain processing, a pre-stack desig-nature process makes use of the whole shot record for maximum statistics in estimating the effective source wavelet, and corrects that part of the total wavelet to a broad-band, zero phase characteristic. Interrelated post-stack processes then analyse the time variant wavelet components, mostly due to absorption and transmission effects, for correction also to a zero phase characteristic. The input reflection wavelet is assumed to be minimum phase, though this may require preliminary correction processes for certain sources. Problems with this minimum phase assumption in more conventional deconvolution processes are shown to be due to their method of estimating the phase spectrum, rather than with the validity of the assumption itself. This system of wavelet processing results in a final section on which the reflection wavelets have a stable broad-band zero phase characteristic, which not only gives the interpreter maximum resolution, but is also necessary for the application of subsequent inversion processing for lithologic interpretation.

The tau-p transform, or slant-stack, converts seismic records (i.e. time versus offset records) into a space in which many seismic events are well separated. In particular, ground-roll transforms to a point at time zero, refractions transform to points at their zero offset intercept times and reflection hyperbolae transform to ellipses. Significantly, the ellipses do not cross one another, even if the reflection hyperbolae do cross each other in record space. The tau-p space has different dimensions than conventional record space. Record space can be thought of as traces with units of time and with each trace representing a given offset x. In the tau-p space, tau (in dimensions of time), actually represents zero offset reflection time. Each trace corresponds to a particular ray parameter, p. This parameter p can be thought of in many different ways: it is a ray parameter sin i/v; it is the dt/dx, or the slope of the arriving event; and it is the horizontal slowness of the event across the recording array. Further, each trace represents a single angle of incidence at the surface; thus, events can be separated according to angle of incidence. Since each trace represents a particular angle of incidence, multiples are exactly periodic in tau-p space. In addition, velocity analysis can be readily performed in the tau-p space. In fact, the Wiechert-Herglotz-Bateman inversion of refraction data is essentially determining velocities in p space. Possible applications of the tau-p transform in a seismic processing sequence include: ground-roll isolation, isolation of refractions, limitation of angles of incidence (beam-steering), separation of P and mode-converted SV waves, combining multi-component recording, interpolation and resampling of the data, multiple attenuation, and velocity analysis. Some of these applications can be readily performed in the normal processing sequence, while others require special considerations in the tau-p space. Many of these applications are possible because an inverse transform, that is a transform from tau-p space back to X-t record space, is readily accomplished. Multiple attenuation in tau-p space is possible in two different modes: (1) multiples are exactly periodic and can thus be addressed precisely using conventional time series analysis, and (2) some multiples are very