Seismic Identification Of Reservoirs: Case Studies From The Gulf Of Mexico
Authors: Lefler W.C. , Neidell N.S. 
Published: October 2008

A method which improves the ability to predict oil and gas reservoirs in sand/shale sequences from seismic data utilizing a high degree of velocity control and color inversion technology is presented. Five field examples taken from different producing trends in the Gulf of Mexico are included to demonstrate the application to exploration and development programs in both mature and frontier petroleum provinces.

INTRODUCTION

Recent applications of Sequence Stratigraphy to modern exploration and development projects has enhanced our ability to accurately predict the distribution of reservoirs beyond the limits of existing well control. Seismic reflection patterns are utilized in conjunction with biostratigraphic and lithologic data to identify the depositional environments which control the geometry and distribution of sediments. Ultimate economic impact of the technology therefore rests with correctly identifying reflections which are associated with the reservoirs.

Objectives of this paper are to define characteristic seismic reflection regions or "Zones" for sand and shale sequences, and "to illustrate an approach which significantly enhances the seismic image, increasing stratigraphic visibility and hydrocarbon detection. People demonstrate the significance of these ideas by reviewing five field examples which are taken from the excellent data base available in the Gulf of Mexico. This approach for establishing key rock properties from seismic and well information in sand/shale sequences has direct application for exploration and reservoir definition both offshore and onshore on a global basis.

SAND/SHALE REFLECTIVITY ZONES

The tertiary section in the Gulf of Mexico is predominately a two-component system of sand and shale with small amounts of carbonates and evaporates. Seismic reflections can therefore be most effectively utilized for prediction of Iithologies as well as indicating the presence of hydrocarbons.

The normal incidence definition of reflection coefficients is a product of both density and velocity (acoustic impedance). In the Gulf of Mexico, as in all other basins, density and velocity of sediments increase systematically with depth and geologic age except in abnormally pressured sections. These normal increases are primarily due to the effects of compaction and cementation.

As illustrated Figure 1, both seismic and well log data indicate that acoustic impedance in shallow, unconsolidated sands is generally lower than the associated shales and produces a negative reflection at a water sand top. In older, more consolidated sands, impedance is higher than the shales resulting in a positive reflection at the sand top. These can be respectively referred to as Reflectivity Zone 1, the under-compacted section, and Zone III, the consolidated or compacted section. As the low impedance sands compact to become the high impedance member, there is a cross-over or transitional interval, Zone II, where impedance for sand and shale are on average equal. Zone 11 can contain sands from Zones I and III as well as sands which exhibit no apparent reflection above the shale background and can also be anomalous in behavior when charged with oil or gas. This Zone designation relates specifically to wet sand reservoirs in normally pressured shale environments.