Precise Target Prediction From 3-D Seismic Data: Case Histories From the Gulf of Mexico

Abstract

Abstract Modern 3-D seismic datasets provide information about the subsurface that goes well beyond the accurate mapping of complex structures. Utilizing attribute analysis of reflection character, it is often possible to peer inside the structure to estimate reservoir quality and fluid content. Carefulapplication of this analysis will lead to more precise prediction of what the drill bit will find and therefore allow the confidence to drill smaller hydrocarbon accumulations. This paper will illustrate such analysis on two prospects in the Gulf of Mexico. In both cases attribute analysis of 3-D data was successfully used to predict the presence or absence of hydrocarbons in specific reservoir intervals with a high degree of precision. Introduction The burden of experience now firmly establishes the importance of 3-D seismic, The first and most obvious benefit of 3-D is the improved structural image of the subsurface as a result of 3-D migration, i.e. the elimination of off-linestructures. A second benefit of 3-D is the ability to visualize the subsurface in 3 dimensions. This allows for a much greater level of certainty in most interpretations, A third benefit of 3-D takes us beyond the realm of mapping reflection times, that is locating the container, to analysis of reflection character in order to see inside the container. Understanding and interpreting reflection character have always been parts of the explorationist 's toolkit. In the past, numerical methods designed to make such analysis moreobjective were plagued by inconsistencies related to the inaccuracies of 2-D data. Our experience is that tools such as numerical inversion (impedance estimation), analysis of amplitude variation with offset (AVO) and geostatistics can be used with some real success once a good 3-D dataset is in hand. The tangible benefit to the exploration process of these numerical tools is the ability to quantify reserve potential and estimate risk. It is obvious that as one tries to justify drilling for smaller targets, one must be able to concurrently increasethe chance of success if reasonable economic hurdles are to be met. The following two case histories demonstrate our experiences with these tools. Case 1: Taking Marginal Prospects To Profitability The first case history focuses on the use of advanced seismic imaging techniques to accurately estimate reserve volumes on prospects thought to have marginal economics. With theseprospects, small reserve fluctuations can take a well from being profitable to one that you wished never had been drilled. This example is from the offshore Gulf of Mexico and involves the analysis of a potential hydrocarbon indicator (" bright-spot") that could not be drilled from an existing platform and therefore, required the setting of a satellitestructure and a tie back to existing production facilities. The potential hydrocarbon indicator is seen in the upper right corner of the seismic line in Figure 1,