Outcrop-based, Integrated, Core- to Seismic-scale, Stratigraphic and Depositional Models for Deep Water Reservoirs: New Concepts for Stratigraphic Traps
Anthony R. Sprague
A stratigraphic trap can be usefully defined as a "closure that relies on facies changes rather than structural elements". In this context, “facies changes” includes true lithofacies changes; lateral thinning of strata; and stratal termination by truncation, on-lap, or down-lap. The simple case of a trap, where this “facies change” applies in all four dip closure directions and with no component of structural dip or fault juxtaposition, is associated with significant hydrocarbon migration and charge risk. Traps where one to three dip closure directions have such a “facies change” component have been, and in future will be increasingly important, in the identification, delineation, and production of hydrocarbons.
This is especially true in the case of reservoirs deposited in deep water, slope to basin floor environments. In such depositional settings, subaqueous sediment gravity flows are freely able to interact with sea floor topography. Syn-depositional mobile substrate and fault movement has been documented to dramatically create such sea floor topography, at varying spatial and temporal scales. In addition, the flows themselves can be both strongly erosional and highly depositional, changing from one to the other over very short length scales. All these depositional attributes of deep water reservoirs contribute to multiple opportunities for "facies changes" and the creation of stratigraphic traps.
A visit to almost any outcrop of deep water strata with good exposure will clearly demonstrate the existence of lateral and vertical "facies changes" at a variety of scales. All of which are potentially significant for the creation of stratigraphic traps. The Permian slope to basin floor sandstones and shales of the Karoo Basin in South Africa are no exception. Major efforts have been undertaken in the past 20 years to observe in detail, and map over many thousands of square kilometers, the distribution of these strata.
The observed stratigraphy and lithofacies distributions, along with interpreted depositional environments, of the Karoo deep water strata have been well-documented. These results, integrated with observations and analyses of subsurface data in hydrocarbon-producing deep water basins, suggest that "facies changes" significant to the development of stratigraphic traps are controlled primarily by (1) the physical stratigraphic scale, and (2) the depositional environment. These two attributes of deep water reservoirs can be integrated together to identify and map a variety of stratigraphic traps.
At the exploration stage, ahead of the drill bit, as identified on 2D or low-resolution 3D seismic data, bulk reservoir intervals can be identified and mapped at the composite sequence set, composite sequence, or sequence set physical stratigraphic scale. At these scales, regionally extensive shale packages within the stratigraphy provide vertical "facies changes" that are the seals for the bulk reservoir intervals. Thus defined, these bulk reservoir intervals show lateral "facies changes" that are primarily controlled by gross changes in depositional environment. Mapping such changes includes the recognition of depositional environments ranging from degradational highly confined, aggradational or external leveed high confined, weakly confined, distributive, and unconfined. Reservoir distribution maps as defined by the explicit lateral extent of such depositional environments, integrated with appropriate structure maps, allow for the identification of potential exploration scale stratigraphic traps.
At the development stage, with higher resolution and/or quality 3D seismic data, well-log, and possibly core calibration, individual reservoirs can be defined at the sequence and complex set physical stratigraphic scale. At these scales, internal shale packages may be extensive sub-regionally or locally distributed and eroded. In the former case, such shales may form internal seals and allow for the development of separate reservoir compartments within the bulk reservoir interval. Each reservoir body separately sealed in this way may show lateral "facies changes" significantly different from the bulk reservoir interval, and therefore increase the likelihood of stratigraphic trapping. In such cases, mapping of discrete confined channel complexes, individual distributary channel-lobe complexes, and mass transport complexes (as seals or barriers), for example, will be important to delineate potential stratigraphic traps. At these sequence and complex set physical stratigraphic scales, true lateral lithofacies changes are still composite in nature, reflecting the overall change from a channelized to overbank environment, or from a proximal distributary lobe to a distal fringe environment, for example.
At the production stage, with detailed well-log correlation, and any detailed seismic-based mapping, discrete reservoir bodies can be defined at the complex and storey set physical stratigraphic scale. At these scales, internal shale packages exist as a result of true lateral lithofacies changes within depositional bodies. These internal shale packages can act as barriers or baffle to flow, and in some cases, as local seals. Predicting the vertical and lateral distribution of these shale packages is critical for the placement of paired injector and producer wells for example. Lithofacies changes at the complex and storey set scale are associated with true lateral pinch-out of strata. In the case of confined channel complexes, both down-flow and cross-flow lithofacies changes within lateral accretion packages can result in reservoir bodies that are disconnected at the production time-scale, thereby effectively forming stratigraphic traps. In the case of distributary channel-lobe complexes, autogenic channel avulsion processes can result in discrete reservoir bodies, with separate lateral lithofacies changes. As a result, these reservoir bodies may be disconnected at the production time-scale, thereby effectively forming stratigraphic traps.
AAPG Search and Discovery Article #90204 © AAPG Geoscience Technology Workshop, Stratigraphic Traps and Play Concepts in Deepwater Settings, May 14-15, 2014, Rio de Janeiro, Brazil