Seismic attribute measurements resulting from Complex Trace Analysis assist in defining hydrocarbon production zones, especially in the offshore environment where recording conditions are reasonably uniform. Measurements of reflection strength, instantaneous phase, instantaneous frequency and other quantities, sometimes including stacking velocity and instantaneous velocity, are usually displayed in color-coded form. Their interpretation is discussed.
Pairs of seismic lines across gas and oil production from clastic and carbonate rocks in the Gulf of Mexico and Offshore Europe are examined by these techniques. The color reflection strength and instantaneous frequency displays are especially found to have anomalies which correlate with hydrocarbon accumulations, and phase displays are valuable in delineating structural details.
The first major assist provided by seismic methods to drilling and production technology, other than in the classical role of predicting structure, was the use of velocity analysis data to predict abnormal pressure. Another assist was provided by bright spot technology, which showed that certain hydrocarbon accumulations produce seismic effects such as changed amplitudes and frequency content, lowering of velocity, etc. Improved analysis and display techniques, especially the use of color, aid in detecting and seeing the significance of relatively subtle variations. Modeling helps in correlating geologic variations with seismic observations. Seismic methods also assist in predicting shallow drilling hazards such as fault and slump zones and gas pockets. Wavelet contraction techniques and improved migration methods allow more precise definition of small faults and other structural features which affect accumulations.
Numerous instances of successful predictions as to what wells will encounter based on seismic data have occurred. The techniques cited above are almost standard in some organizations and their use is increasing steadily. However, erroneous predictions have occurred often enough to induce caution. Undefined variables affect what is seen in seismic data and sometimes lead to erroneous conclusions. Theoretical foundations underlying some observations are lacking. Seismic interpretation remains partly deterministic, partly empirical, partly art.
Documentation of the successes and failures in predicting production which have occurred is almost completely lacking. This makes it difficult to determine the circumstances associated with success or failure and to isolate the other variables which may be active, perhaps even dominant. The problem is made ever more difficult because determining the probability of a successful prediction is often a non-sequitor; success is usually measured by economic profitability whereas prediction is usually of porosity and fluid content in a rather generalized way at best.
The examples given here involve clear correlations of seismic anomalies with production. They are thus very encouraging that such anomaly measurements are valuable. However the detailed correlations leave uncertainties, as to how specific they are. For example, in trying to see differences between two spots, one of which produces and the other of which does not produce, we may see clear differences and still not know that the observational differences are related to the reasons for production or lack thereof.
