Alaska North Slope study area (gray shading) includes seismic lines (gray) and wells (dots). The Barrow Arch is a broad east-plunging anticlinal trend that trapped abundant petroleum as at Prudhoe Bay Field. NPRA = National Petroleum Reserve in Alaska, ANWR = Arctic National Wildlife Refuge. (Images courtesy of WesternGeco)
Surface geophysical methods can image subsurface structures with reservoirs that have the potential to trap hydrocarbons and, in some cases, can indicate the probable presence of hydrocarbons. Even today, establishing fluid content with confidence requires drilling. But significantly, the development of petroleum systems
modeling software technology as part of an integrated workflow has enabled oil and gas companies to estimate undiscovered hydrocarbons to help mitigate risk in exploration prospects prior to drilling.

Petroleum systems modeling

A static view of potential traps on a play fairway map ignores the fact that successful exploration requires many dynamic elements and processes to work for present-day traps to be filled with oil and gas. Petroleum systems modeling, through advanced computer simulation, tracks migration through time and models the composition and volumes of the entrapped fluids. This has resulted in a shift from static to dynamic methods for exploration.

Petroleum systems modeling has become the technology centerpiece for many integrated exploration companies. It reduces exploration risk by incorporating a broad spectrum of geoscience data from seismic measurements, well logs, stratigraphy, geothermics, structural geology, micropaleontology, and geochemistry. It can be used to quantify many of the key aspects of an evolving petroleum province to understand and predict the locations, volumes, compositions, and pressure-volume-temperature properties of petroleum accumulations in a lead or prospect.

Four time slices (millions of years before present) from a 3-D petroleum system model show the dynamic progradation of a sequence of foresets from southwest to northeast across the North Slope of Alaska, which controlled the timing of petroleum generation from source rocks below the Lower Cretaceous Unconformity (LCU) as shown in Figure 3. Coastline is black.
The importance of petroleum system modeling has been realized throughout the industry because it can:
1) Facilitate visualization of the essential risk elements (source, reservoir, seal, and trap) and geological processes of the petroleum system (trap formation and generation-migration-accumulation through time) and thereby enhance communication with stakeholders;
2) Convert static data into dynamic data and interpretations that can be tested numerically to assess the range of possible outcomes; and
3) Provide a consistent approach to compare and evaluate prospects.

In June 2008 Schlumberger acquired IES Integrated Exploration Systems, the company responsible for the development of PetroMod petroleum systems modeling software. PetroMod software uses seismic information for structural control, well data for lithofacies, and overall geological knowledge with the evolution of a sedimentary basin over time to predict if and how a trap was charged with hydrocarbons. This includes the source rock and timing of hydrocarbon generation, migration routes, quantities, and hydrocarbon type in the subsurface or at surface conditions. Combined with seismic-to-simulation software and geomechanics software, this offers a near-complete suite of tools in an exploration workflow for evaluating the trap, reservoir, seal, timing, and source.

Alaska petroleum systems modeling

Calculated conversion (percentage) of source organic matter to petroleum in the Triassic Shublik formation source rock at four different times on the Alaska North Slope. The Shublik formation is located below the LCU and the prograding foresets in Figure 2. Coastline is yellow.

To understand the existing discoveries and future potential of hydrocarbons in Alaska, PetroMod software was used to model a study area on the North Slope).

Seismic data were used to create a 3-D cube of present-day geometry (Figure 1). Forward models simulate the burial history of the rock units and the generation-migration-accumulation of petroleum within the 3-D cube through time (e.g., Hantschel and Kauerauf, 2009). For example, forward modeling requires that each rock unit be de-compacted to restore its properties prior to deposition of overlying units.

A key dynamic aspect of this 3-D Alaska model is that it accounts for progradation of the time-transgressive foreset sequence on the Lower Cretaceous Unconformity (LCU, Figure 1). These overburden rocks control the timing of generation from underlying source rocks.

For example, Figure 2 predicts the thermal maturity of the pre-LCU Triassic Shublik formation source rock. Model output can be compared to measured data from available wells. The model can be calibrated to improve the match between the simulation and measured data. Finally, models can be processed to quantify uncertainties and determine correlations and risks.

A paradigm shift from fairway maps

Petroleum system models offer a clear advantage over play fairway maps because they account for the risk posed by the timing of generation-migration relative to trap formation. At Prudhoe Bay and elsewhere on the Barrow Arch, trap formation preceded generation-migration by several million years, resulting in major oil accumulations as confirmed by the modeling. However, in the Brooks Range foothills in the south-central National Petroleum Reserve in Alaska, the timing risk is high for the structural traps, which can only be filled by re-migration of petroleum from older stratigraphic traps. Timing of generation-migration relative to trap formation is favorable for the stratigraphic traps, although there is significant risk associated with migrating petroleum from the source rock upward through thick Jurassic and Lower Cretaceous mudstone into turbidite sandstones near the base of the foreset sequence above the LCU.

In another study area, fairway mapping defined northern and southern fold belts, both containing abundant traps. Because of shallower water and thus less expensive drilling, the operators were inclined to drill the northern fold belt first, with disappointing results. Petroleum systems modeling clearly showed that traps in the northern fold belt did not form until after generation and migration of petroleum from the source rock. Petroleum modeling for the southern fold belt showed that trap formation occurred prior to generation-migration-accumulation and is therefore a more favorable exploration target. This is validated by recent exploration results.

As petroleum exploration and production remains essential to global economic prosperity, limited alternatives to the convenience and energy content of petroleum guarantee that the search for additional oil and gas resources will continue for many decades. In frontier exploration areas and in known mature exploration areas, it is expected that petroleum systems modeling will play an increasingly vital role in predicting petroleum occurrence, volumes, and composition and in helping to quantify exploration risk.

Acknowledgments
The authors thank Ken Bird and Les Magoon (USGS) for useful discussions.
We also thank Ian Bryant and the team
at Schlumberger for their support.