Interactive Map-Based 3D Migration Modeling Approach to Ranking of Exploration Prospects

Zhiyong He
ZetaWare, Inc., Sugar Land, Texas

In the last two decades, significant advances in theory, algorithms and software have been made in basin modeling, with much emphasis on how to correctly model the physical processes involved. Huge increases in computer performance have also made it possible to run near-seismic-resolution migration models with sophisticated compositional kinetics and PVT models.
Despite these advances, in this paper we show that the weakest link that leads to the most uncertainty in our modeling results is often not the lack of sophistication of our models. It is rather the uncertainty of our underlying geological model. For example, 10 °C uncertainty in temperature could cause a factor of 2 to 4 uncertainty in expelled HC volumes and in turn a very different pattern of accumulations. 1 MPa (145 PSI) variation on top seals over the carrier beds can change the number of prospects being charged. The probability of certain traps being charged depends much on how we define salt welds on seismic and whether we believe faults are seals or conduits…For these reasons, we believe we can best address the uncertainties by developing and using tools that can quickly and easily “what-if” the impact of these different geological interpretations.
For example, the existence and extent of the first carrier bed above the source may be un-resolvable using even the best 3D seismic data. If that is true how much chance to we have of estimating the capillary entry pressure of the seal above that carrier? In one of our case studies, by varying the top seal capillary pressure and observe the resulting migration patterns, we were able to not only explain the dry holes and existing accumulations in the area, but also to narrow the range of the capillary pressure of the top seal by best matching observations. Meanwhile, this exercise provided a ranking of likelihood of charge for the remaining prospects, as the prospects that require top seal values below the reasonable range are the highest risk. This approach had a significant impact on the exploration team’s decisions.
Because we often do not know which geological horizon is the first carrier, or the areal extent of the carrier sands, it is useful to perform “what-if” scenarios in which we try out different horizons as the first carrier, and experiment with the areal extent. Many other uncertain aspects of the petroleum system lend themselves to such scenario testing, but this approach requires very fast realization of each scenario. Other examples of successful scenario-testing we have used include a) in the North Slop of Alaska, migration through sub-cropped contacts of the carrier bed and the reservoir compared to migration though leaky faults and b) in the Campos basin, different cut-off values of salt isopach that define salt welds as migration windows and rank prospects based on the thickness cut-off required.
In most situations, there are multiple uncertain aspects that affect whether a prospect gets charged: different carrier beds, top seals capacities and spatial variation, thermal gradient, source facies, richness, etc. Our map-based 3D migration toolkit has focused on the flexibility to allow complex geological scenarios (unconformities, faults, salt, etc.), speed and interactivity (with each scenario realized in a matter of few seconds or minutes). We believe this approach to geological uncertainty compliments the more deterministic and time consuming full-physics models and provides the necessary prospect risk assessments and ranking for the business units of exploration enterprises.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands