Exploration risk assessment is typically achieved by a holistic evaluation of the basin petroleum systems and the combined risks identified for the reservoir, trap, charge and seal. In structurally complex basins, structural segmentation can severely complicate the process. Modeling a representative paleoenviroment often challenges software technology and requires more time and effort than can be justified by the business requirements.

Conventional shortcomings

Exploration risk can be reduced by understanding the likelihood of hydrocarbon presence, type and potential volumes. Seismic data can delineate the trap, and high-quality 3-D seismic might reveal information on reservoir quality or provide an indication of fluid content. However, seismic data alone is not adequate to reveal the complete geological story.

Petroleum systems modeling (PSM), a geological modeling technique that reconstructs basin evolution, can provide a solution. PSM incorporates all of the geologic elements and processes needed for oil and gas to accumulate. The elements are an effective source rock, reservoir, seal and overburden, with the latter burying the source rock to a depth where it can reach thermal maturity. Processes include trap formation; source rock maturation; and generation, migration and accumulation of hydrocarbons. Elements and processes need to occur in a specific relation and order for organic matter in the source rock to be converted to petroleum, stored and preserved. A dynamic model, one that evolves through geologic time, is essential to correctly assess the many interactions as process timing often determines exploration success or failure.

Conventional PSM uses vertical backstripping to describe the structural evolution of a basin. However, this is not sufficient in structurally complex regions. Lateral rock movements mean PSM should be performed using structurally restored models. This requires a specific methodology to simulate physical parameters such as rock stress, pore pressure and compaction within a basin and then to correctly couple the resulting structures with subsequent modeling of the thermal history, source rock evolution and derived hydrocarbon compounds. The PSM must be structurally consistent through geologic time; if the geometric framework is inconsistent, the properties and processes simulated in this framework also will be incorrect. An inconsistent structural framework will lead to an incorrect thermal history, causing erroneous hydrocarbon generation results and spurious migration pathways and accumulation locations.

Solution and case studies

Schlumberger developed the TecLink workflow in the PetroMod petroleum systems modeling software to integrate structurally restored paleogeometries at specified time steps into the petroleum system model. This technique correctly couples the temperature and pressure fields and simulated dynamic fluid movements to the structural component of the model.

The TecLink workflow was applied to a 2-D cross section from the Monagas fold and thrust belt in eastern Venezuela (Figure 1). Although this passive margin setting was tectonically calm during the Mesozoic to Mid-Cenozoic, transpression started in the Late Oligocene, resulting in inversion of the major normal faults. Thrustingrelated tectonic burial caused rapid heating and pore pressure increases within the footwall source rocks, favoring hydrocarbon generation and expulsion. At the same time, folding and faulting created structural traps in the thrusted footwall that received the hydrocarbon charge. All of those petroleum systems processes were directly controlled by the tectonic evolution and have been captured in the dynamic model.

Integrating the structural restoration made it possible to successfully model the charging of the producing Santa Barbara oil field and long-distance migration along the regional foreland bulge into southern oil fields. A prospect north of Santa Barbara, in the vicinity of one of the major thrusts, was investigated. As a result of modeling the geomechanical response to the thrusting, seal integrity was identified as a primary risk. The modeling also indicated that if oil had accumulated in the past, it was most likely cracked into gas under high temperatures. The gas-enriched trend has been confirmed by recent drilling results, which have established extension of the Travi Field to the southwest.

PSM studies can include 1-D, 2-D and 3-D models. However, only 3-D modeling can fully assess and quantify petroleum migration, accumulation and loss because these transport processes are inherently 3-D and 4-D. Modeling 3-D petroleum systems in structurally complex compressional areas is a time-consuming and technically challenging task. Almost all PSM studies to date in basins dominated by compressional tectonics have been limited to using multiple 2-D transects, as in the preceding Venezuela case study.

Centro de Innovación y Tecnología-ICP de ECOPETROL faced this challenge in evaluating the exploration potential of the petroleum system of the northeastern Cordillera and foothills area of the Llanos Basin in Colombia. The area has been subjected to intense compressional tectonics, leading to a high degree of shortening and structural complexity.

PetroMod TecLink technology was employed to build the fully coupled 3-D model needed to properly assess petroleum migration, accumulation and loss. The petroleum systems story is further complicated by long lateral migration pathways of up to 400 km (250 miles) and diachronous episodes of migration and trap formation during the uplift and exhumation of the eastern Cordillera.

Pressure and thermal boundary conditions through geological time were defined to constrain the model, which was calibrated to well control for pressure, temperature and thermal maturity. The dynamic evolution of the basin was modeled, including the interaction of the key petroleum system elements and processes. The consistent 4-D geological model generated for the area identified multiple possible migration pathways from the potential kitchen areas to provide confidence in exploration decisions in one of the most structurally complex areas in Colombia (Figure 2). Detailed hydrocarbon migration studies were performed to understand the filling history and migration pathways with a match to known fields and observed accumulations.

Three key exploration risk factors were determined from the study:

1. Lower and Upper Cretaceous source rocks reached peak hydrocarbon generation between the Turonian and the Paleocene. Transformation ratio progressively decreases toward the east. Peak hydrocarbon generation was reached before the structures were fully formed.

2. Younger Paleocene source rocks contribute significantly to the present-day accumulations and are still in the generation window.

3. The hydrocarbon pathways suggest long-distance migration over a significant period of time. Consequently, there is a significant risk of migration losses due to the complex migration history.

Exploration risk from frontier areas to near-field exploration can now be assessed with greater confidence, and the exploration decisions made are auditable by reference to the 3-D petroleum systems models.

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