Identifying Shale Reservoir Sweet Spots
Jeffry D. Ottmann
Technical Team Lead Argentina Shale Gas Senior Technical Professional Advisor, ExxonMobil Exploration Company
Commercial production from mudstone reservoirs is a new viable resource type that will provide an important energy supply for a significant time to come. Three underlying fundamental properties are essential to identifying high density recoverable resources or “sweet spots”. Porosity development, rock strength (“brittleness”), and reservoir energy are the elements which are the foundations of sweet-spot development. Modifiers, such as natural fracturing and/or diagenesis, can either enhance or degrade recoverable resource density.
Shale plays, such as the Vaca Muerta, although described as unconventional due to the scale of reservoir parameters and extraction methods, are still within bodies of rock that were deposited under systematic environmental changes in response to variations in accommodation, sediment supply, paleobathymetry, and climate. These processes deposit multiple depositional sequences with predictably distributed system tracts bounded by surfaces with significant breaks in sediment accumulation; these can be systematically sorted into lithofacies families with similar properties. Organic matter distribution and preservation, rock brittleness, and reservoir energy are significantly impacted by the stacking and relative development of system tracts.
Porosity development from enrichment, preservation, and maturation of organic material along with increased brittleness and interparticle microporosity in biogenic-rich intervals are associated with rates of sediment supply and are coeval. Additional controlling elements of development and distribution of depositional facies and system tracts are tied to the bathymetry and configuration of the paleo-basin floor. Positive topography benefits the reservoir formation during optimal sediment rates as focal points for upwelling and organic-matter accumulation. Thin-section and outcrop images of the Vaca Muerta reveal the development of a multi-component porosity system. Several commercially successful mudstone reservoir systems such as the Eagle Ford and Haynesville consist of multiple porosity types. The combination of multiple porosity types provide the permeable pathways necessary for fluids to flow into fractures whether naturally occurring or artificially induced. X-ray imaging illustrates the importance of organic-material saturation and distribution of permeability to the fluid flow. Conversely, other facies types display elements of porosity destruction via diagenesis.
Reservoir potential is greatly diminished within systems tracts with (1) high sedimentation rates due to increased clay-mineral content and (2) low sedimentation rates due to cementation. These system tracts are, however, important elements within the play as they provide hydrocarbon and pressure seals. Reservoir energy is critical to the extraction of the hydrocarbon from these tight reservoirs. Although some of the energy may be the result of source rock maturation, basin history is likely a larger effect. Uplift and erosion are likely causes of overpressure and distribution of commodity type. Retention of pressure depends on the distribution of the non-reservoir clay-rich lithologies. In addition, thermal maturities of the source interval may not accurately describe the commodity distribution as gases are likely to expand during uplift and redistribute in-situ fluids.
AAPG Search and Discovery Article #90165©AAPG 2012 GEOSCIENCE TECHNOLOGY WORKSHOP, 2-4 December 2012, Buenos Aires, Argentina