Modeling Hydrocarbon Migration and Retention in Fine-Grained Clastic Successions: Conventional Approaches to Evaluating Unconventional Resources

Rene Jonk¹, Elizabeth Cassel², and Scott Barboza^1
1ExxonMobil Upstream Research Company, New Play Concepts Group
²Stanford University, Department of Earth Sciences

Introduction
Hydrocarbon production from so-called “unconventional” reservoirs has been rapidly increasing in the last decade. Unconventional resources include tight gas (low permeability sandstone reservoirs), shale gas (gas associated with fine-grained, organic-rich rocks) and oil shale (kerogen-bearing rocks). Exploration for these resources, particularly within the continental U.S., was carried out differently from conventional accumulations, relying instead on identification of enigmatic sweet spots whose profitability was mainly controlled by advances in drilling and stimulation technology. We find, however, that standard play element assessment techniques aid in the delineation of profitable unconventional accumulations much in the same way as is required for assessing conventional resources. An important distinction is that a more fundamental understanding of the subtleties of hydrocarbon saturation and adsorption is required (e.g. Shanley et al., 2004).

In this study, we demonstrate that fine-grained, clay-rich stratigraphic successions (“shales”) are heterogeneous and contain all aspects of plays (reservoirs, seals, sources, traps, and migration) that can be analyzed using conventional methods. Furthermore, these elements may be mapped at an appropriate scale to allow assessment on a basin, play and perhaps even prospect scale.

Methods
Our study focuses on the Mowry Shale Formation exposed within the Powder River Basin, Wyoming. The Mowry Shale is a 200-500ft organic-rich, fine-grained succession deposited during the Cretaceous within the Western Interior Seaway. As with many shale units, a cursory examination may result in mis-characterizing the Mowry as homogeneous “shale”. Methodical field, petrophyscial, and petrologic study, however, reveals several sub-formations and members with distinctly different lithology (Bohacs, 1998).

The work performed involves a detailed stratigraphic description of one core within the basin, selection of samples for porosity, permeability and Mercury Injection Capillary Pressure (MICP) measurements. Lithofacies were correlated to wireline log data and populated with relevant rock properties to build a high-resolution earth model of the succession (Fig. 1). Secondary migration modeling was accomplished using Permedia MPath to evaluate the presence of intra-shale resources.

Results
Figure 2 shows a schematic of a migration simulation result. Typically, accumulations form beneath flooding surfaces of parasequences and parasequence sets where more reservoir-prone lithofacies are overlain by more seal-prone lithofacies. In more proximal lithofacies associations (near the base and top of the succession) these accumulations typically form in bioturbated sandy mudstones (reservoirs) that are overlain by parallel laminated (and partially carbonate-cemented) mudstones (seals). In the more distal lithofacies associations within the organic-rich packages, accumulations occur in (bio)siliceous mudstones that are overlain by regionally extensive ash-fall deposits (bentonites) which can act as very effective capillary seals. We performed several different simulations motivated by alternative interpretations of lateral continuity within certain stratal packages. In particular, sandy mudstones may be discontinous and lensoid in shape, which sets up excellent (albeit small) stratigraphic traps.

Conclusions
Fine-grained stratigraphic successions are much more heterogeneous than typically appreciated. These heterogeneities may have a significant effect on our perception of risk and value within some shale gas plays. Careful study of the fundamental (sequence) stratigraphic stacking (i.e., Bohacs et al., 2009) in such successions is required for assessment of hydrocarbon resource play elements such as source, maturation, migration, reservoir, seal & trap.

References
Bohacs, K. 1998. Contrasting expressions of depositional sequences in mudrocks from marine to non marine environs. In: Schieber, J., Zimmerle, W., Sethi, P., Eds., Shales and Mudstones I. Schweizerbart Stuttgart, 33-78.

Bohacs, K., MacQuaker, J., Lazar, R., Jonk, R., Hemmesch, N., Cassel, E. 2009. Significant variations in hydrocarbon source and mudstone-reservoir character at the parasequence scale: counter-intuitive trends, systematic relations, and economic implications. Annual AAPG Convention, Denver, Abstract.

Shanley, K.W., Cluff, R.M., Robinson, J.W. 2004. Factors controlling prolific gas production from low-permeability sandstone reservoirs: Implications for resource assessment, prospect development, and risk analysis. AAPG Bulletin, 88, 1083-1121.

Figure 1. Schematic showing workflow used in this study

Figure 2. Schematic showing 2D slice through migration simulation result, showing hydrocarbon accumulations color-coded by saturation (red low; blue high). All lithofacies within the model are different types of clay-rich, fine-grained rocks ("shales").

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.