Quantative Interpretation Of Seismic Data Using Well Logs
Authors: Gevers E.C.A. , Watson S.W. 
Published: December 2007

Seismic and petrophysical data sets are processed to acoustic impedance. The petrophysical processing is divided into three phases.

The first involves the preparation of a depth-sampled acoustic-impedance log corrected for environmental effects. Re-sampling in time and band limitation of the acoustic-impedance log yields at the end of second-phase processing a synthetic seismic trace that can be used to identify lithological units on a seismic section.

Third-phase processing provides the information necessary for interpreting seismic-amplitude values. As an example, the loop associated with a potential reservoir sand is identified and traced along several seismic sections. Amplitude measurements taken from this loop are interpreted in terms of expected fluid content, and a porefill-distribution map of the reservoir is constructed.

Introduction

Recent improvements in seismic data acquisition and processing have produced a seismic trace which, in favourable cases, closely resembles an acoustic-impedance log (product of velocity and density logs) through the earth. These acoustic-impedance profiles can be interpreted in terms of lithology and porefill. A prerequisite for such detailed analysis is that a data learning set should be available. In the Royal Dutch/Shell Group the necessary information is derived from petrophysical well logs and geological data.

In this paper we shall describe the integration of seismic and petrophysical/geological knowledge to detect and map areas of hydrocarbon-bearing sands developed in a marginal marine facies in the depth range 7 000 - 10 000 ft (2 134 - 3 548 m). Emphasis will be given to petrophysical data processing, which we shall divide into three parts. First-phase processing leads to an acoustic-impedance log sampled in depth and characterizing the formation undisturbed by the drilling process. Second-phase processing involves comparing band-limited acoustic impedance logs in the time domain with seismic acoustic-impedance traces around the well location. A positive identification of lithological units on the seismic sections affords the possibility of quantitatively analyzing the seismic data. In order to be able to interpret the analysis results, third-phase processing is necessary.

The adopted method gives also the nature of the hydrocarbon-fill, provided that allowance is made for fluctuations in reservoir quality and that there is sufficient acoustic-impedance contrast between oil-filled and gas-filled sands. The minimum acoustic-impedance contrast resolvable is dictated by the noise content and frequency bandwidth of the seismic signal.

Geological Background

The hydrocarbon reserves in the field under investigation are contained in roll-over structures that are associated with growth faults. The host sediments are marginal marine deposits of Upper Tertiary age consisting of a large number of sedimentary offlap cycles each of which starts with a marine clay and progressively changes upwards into proximal fluviomarine interlaminated silts, sands and clays.

First-Phase Petrophysical Processing

The digitized well logs with a sampling increment of 0.5 ft (152 mm) were inspected for digitisation and calibration errors1, noise spikes, cycle skips2, wash-outs, shale-alteration effects and filtrate-invasion effects, and the necessary correction procedures implemented along the lines of the methods described by Dominico3 and Ausburn4.