Calibration of Seismic Attributes for Reservoir Characterization ...

Executive SummaryThe project, “Calibration of Seismic Attributes for Reservoir Calibration,” has completed theinitially scheduled third year of the contract, and is beginning a fourth year at no additional cost,designed to expand upon the tech transfer aspects of the project. We have completed allessential work on four major data sets and have made intriguing conclusions in all of them. Asthe preparation of tech transfer items continues, we expect to find minor additional tasks toperform on some of the individual data sets, for completeness or demonstration purposes.Our description of the work performed will follow each data set, and the discoveries madeassociated with them. A summary will present those same discoveries in terms of theirrelationship to the current state of the art in seismic calibration interpretation.The Stratton data set has provided a challenge in thin-bed reservoir characterization in theabsence of sonic-log calibration data. In general, the seismic character of potential productivezones is obscure in this data set, and horizons containing the pay zones are typicallydiscontinuous. Previous work by other authors has demonstrated the apparent usefulness ofsimple attributes mapped along ‘phantom’ horizons which were tied at one well through a VSPand controlled by a constant offset from a nearby continuously tracked horizon. We havedemonstrated in this project that this apparent correlation is an artifact of isopach changes, whichdominate the interpretations based on simple attributes and on seismic facies analysis. However,if the interpreter understands that the interpretation is based on this correlation with bedthickening or thinning, reliable interpretations of channel horizons and facies can be made.The Boonsville data set has provided another challenge in thin-bed reservoir characterization, inwhich the seismic character of a productive sand zone appears, indistinguishable from thecharacter of a non-productive limestone to most commonly used attributes. In this case, theinterplay of impedance and thickness conspire with tuning to produce the similarity in mostattributes. This problem was attacked with two methods. In the first method, a technique wasdeveloped that made use of the well-log interpretations to divide the area into a number of facies,following a reasonable geological model; then the seismic attributes, including seismic faciesgenerated under various neural network procedures, were used to further subdivide thoseregional facies into productive and non-productive subfacies. In the other method, a newtechnique involving cross-correlation of seismic waveforms was developed to provide a reliablemap of various facies present in the area; we think this technique holds great promise for otherdata sets as well, and it appears to be extremely robust.The Teal South data set has provided a surprising set of data, extremely rich in interpretationpossibilities. We used the limited log data and excellent seismic data of this class III AVO brightspot reservoir in the Gulf of Mexico to develop a robust seismic petrophysics model throughwaveform (stratigraphic) inversion for acoustic impedance. We then used this model, togetherwith a pressure-dependent elastic modulus relationship developed by us, to predict the futureseismic response of the reservoir, as it was produced. Our predictions met the observations withuncanny accuracy. But observations of nearby, unproduced reservoirs also indicated a similarresponse, one that was not predicted with classical reservoir models or simulations. Weconcluded that these nearby reservoirs are undergoing a pressure drop in response to theproduction of the main reservoir, and that oil is being lost through their spill points (as gas comesout of solution), never to be produced. This set of observations may have serious ramificationsfor engineering and exploitation techniques throughout the Gulf of Mexico.Annual Technical Progress Report Michigan Technological University DE-AC26-98BC15135October, 2001 Page 9