Geologic Model-Guided, Progressive Inversion: Key for High-Resolution Reservoir Model from Seismic

Hongliu Zeng, Stephen C. Ruppel, and Charles Kerans
Bureau of Economic Geology, Jackson School of Geosciences,
The University of Texas at Austin

In most carbonate fields, seismic is the only physical data control between and beyond wells. Generating high-resolution reservoir models from seismic has been a major challenge. The main obstacle is that the seismic bandwidth is commonly limited (10–80 Hz), preventing seismic data from imaging detailed geology required for high-resolution model building.

One solution is to use core, wireline logs, and outcrop analogs to build a priori impedance models and conduct model-based seismic inversion. An accurate and high-resolution a priori model can seamlessly integrate detailed geologic information into seismic inversion, creating a broad-banded reservoir model that cannot be achieved from seismic data alone.

The biggest problem in the model-based inversion is that it is very difficult, if not impossible, to build an accurate high-resolution a priori model by interpreting closely spaced geologic boundaries from low-resolution seismic data. Without detailed and geologically correct geometric control over the initial model, inversion results are prone to be either low resolution or erroneous in depicting reservoir architecture.

Progressive inversion helps solve the problem by building multiple initial models and performing multiple inversions. The first initial model would be a less accurate model based on few prominent and reliable geologic boundaries and related seismic horizons. The first inversion, performed on the basis of this initial model, usually reveals more geologic details than the original seismic data. Additional horizons are then added from interpretation of the first inversion to create a new and more accurate initial model that fits better to the local geologic model and the interpretation of core, wireline logs, and analogous outcrop. A new inversion based on this improved initial model would provide yet more geologic details and be yet higher in resolution. This process may be repeated until inversion resolution is satisfactory for reservoir-model-building applications.

We present preliminary results on application of the methodology to an outcrop model and two 3-D seismic data sets in Cogdell and Fullerton fields, all in west Texas. An example from Fullerton field (Figure 1) shows that in the third-round inversion some 20- to 40-ft-thick stratigraphic and reservoir units become recognizable, which is a significant improvement over the 100-ft resolution in seismic amplitude data.