Seismic Dim Spots Too Can Have A Great Reservoir Potential!
Authors: Shrestha Rajendra K , Mennie James R 
Published: May 2016

Deepwater turbidite exploration in Northwest Borneo has targeted amplitude supported structures that resulted in significant discoveries. Seismic dim spots provide opportunities for hydrocarbon exploration (Brown, 2012), however, they are considered higher risk due to the non-unique impedance relationships. This paper presents an example of a gas bearing reservoir represented by both bright and dim amplitude response in Offshore Sabah, Malaysia.

Introduction

Structurally conformable amplitude anomalies are often associated with hydrocarbon bearing reservoirs and have traditionally been the focus in deepwater turbibidite exploration. The acoustic response of an exploration target is non unique with contributing variables such as reservoir thickness, quality, porosity, and fluid type contributing to the response. It is also recognized that the seismic response may be further impacted by acquisition, processing and other geological overprints which potentially can mask the acoustic response of the reservoir. For this reason, the lack of amplitude anomalies or even dims spots can be easily misinterpreted to be indicative of the absence of a prospective reservoir. Therefore, it is necessary to evaluate the seismic anomalies by integrating all available data and tools in the exploration and appraisal phase.

Well A was drilled in deepwater offshore Sabah in the late 1990s targeting a structurally comformable amplitude anomaly (Figure 2). The well encountered a hydrocarbon bearing reservoir deposited in turbidite channels. An appraisal well (Well B) recently tested the extent of the reservoir in an area characterized by a lack of strong top reservoir amplitude response observed in Well A within the field (Figure 2). The appraisal results indicated the reservoir development in this dim area of the field was even better than in the discovery well and upgraded the resource potential of the field.

Bright and Dim Seismic Facies

A zig-zag seismic line that demonstrates the variation of seismic response within the reservoir interval is shown in Figure 1. A well defined seismic flat spot defines the hydrocarbon water contact (confirmed in well penetrations). The high amplitude seismic facies in the right of the seismic panel was targeted by the discovery well A. The primary objective of the subsequent appraisal well (Well B) was to test the dim seismic facies in the left section of the seismic panel (Figure 1). The key uncertainty of this well was that the reservoir would be less developed than in the discovery well.

This appraisal well (Well B) penetrated a well developed reservoir with even higher net-to-gross than the discovery well. A zero-offset VSP was obtained with one of the main objectives to further characterize if the observed seismic response was reservoir derived. A geological side track was subsequently drilled in order to evaluate the extent of the high net-to-gross reservoir penetrated in the appraisal well and to determine if the high quality reservoir extended into a bright seismic facies. The side track demonstrated correlatable reservoir development to Well B (Figure 2 and 3).