Use of seismic attributes at Lake Maracaibo to reduce exploratory risk

Abstract

Abstract Although there is not a unique approach to understand the meaning of seismic attributes, they are becoming a widespread tool in reservoir characterization. In areas where very little petrophysical and production information is available, seismic attributes can play an important role in identifying new prospects and reducing exploratory risks. We present a simple methodology to do such an assessment and examples of its application to a Cretaceous carbonate sequence at Lake Maracaibo, Venezuela, one of the oldest and most prolific oil provinces in the world. The main idea behind the proposed methodology is that independent seismic attributes should be related to different, but not necessarily independent, properties of the reservoir. By sampling the space of seismic attributes as completely as possible, computing as many attributes as possible from the seismic data, we try to increase the probability of deriving a set of seismic parameters that correlates with the parameters that control production. Reservoir quality is derived from comparison of independent seismic attributes related to rock properties. The described methodology was applied in a Cretaceous carbonate sequence at Lake Maracaibo, Venezuela. After a careful examination of the attribute responses and their correlation with production and petrophysical data available, we were able to identify new prospective areas, identify possible producing intervals, and confirm the reservoir engineers ideas about compartmentalization of the field. The proposed methodology was proved useful in decision making processes to reduce exploratory risk in areas with scarce well control but with 3D seismic data. Introduction An area in the Maracaibo Lake had been producing from Eocene sands very successfully. The discoveries of light oil in thedeeper Cretaceous carbonate rocks, at an average depth of 12,000 feet, in the Lake led to exploratory programs searching for oil in these strata. The study area of this paper was no exception, and from 1968, several Cretaceous exploratory wells were drilled. After several successful wells drilled in structural highs (one accumulated 14.8 MMBO), the dry wells and poor producers that were drilled afterwards, showed that the production mechanism was not clearly understood. The exploratory program was stopped until a coherent production scheme was proposed. The task of reaching an understanding of the processes that accumulated good reservoirs in this area, to better define theprospective areas, was assigned to a multidisciplinary team. As part of these efforts, a 250 km2 3D seismic survey was acquired in 1990. This paper presents the results of the study of the seismic attributes and how they were used to matchpetrophysical characteristics with seismic response in order to determine possible prospective areas in a zone with scarce well control. Recently, the use of seismic attributes has proved its value in defining details at reservoir level, extending the information that is in the seismic trace farther than just for structural andstratigraphic purposes. Examples of this use are the so-called Hilbert attributes, or "instantaneous attributes"; amplitude, phase and frequency, where "instantaneous" refers to the fact that they are calculated for each seismic time sample.