ABSTRACT: Structural Interpretation in the Upstream Petroleum Industry: Application of Global Experience and Key Technologies
ANDO, CLIFFORD J. , ExxonMobil Exploration Company, Houston, Texas
The fundamental goal of structural analysis in the petroleum industry is to develop a 3-D geometrically and kinematically consistent interpretation honoring all data, which can be used to evaluate all aspects of the hydrocarbon system that have been influenced by structural history. Those aspects can include trap, seal, source rock distribution, maturation, secondary migration, and reservoir distribution. Structural interpretation is conducted at a wide range of scales between regional and prospect, with large variations in data quality and availability. Where seismic data quality is high, advanced volume and surface interpretation technologies can provide detailed, confident structural interpretations. However, where data are sparse or of lower quality, the interpretations will be more model-based. In all cases, the interpretation process must be underpinned by the sound application of fundamental geological and physical principles.
Two decades ago, traditional, descriptive, analog-based structural styles analysis began to give way to a more quantitative approach using cross section balancing and kinematic modeling. These techniques are used to validate interpretations and test the range of admissable alternatives, and remain an important part of the toolkit. More recent advancements in subsurface technology, including high quality 3-D seismic data, and tools such as discontinuity volumes, horizon slices, and surface visualization techniques allow us to address technical uncertainties and risk with structural analysis in ways not done before. In areas with high-quality seismic data, the tasks of identifying structural style and defining the gross trap container are made easier than ever before, with emphasis shifting from mapping the trap container toward evaluating trap integrity by mapping fault networks and their interaction with the stratigraphic framework. Mapping the fault network and understanding stratigraphic variations in faulted traps is essential for predicting reservoir and seal juxtaposition relationships, and the likelihood of compartmentalization. In areas with complex traps and poor seismic imaging, it is especially important that all available data are incorporated to support the structural interpretation. Many diverse data types must be integrated to address the inherent risks associated with structural complexity. In these cases, the non-unique nature of structural interpretations necessitates special consideration of the full range of admissable alternatives.
The structural interpretation, at global, regional, and trap scales, impacts a wide range of derivative analyses, and must be accurate and consistent enough to meet the business need at hand. Studies of sedimentation patterns, heat flow, fluid flow, and seal integrity depend upon a rigorous structural interpretation. In addition, a consistent fault framework should be carried through the various technical stages of hydrocarbon column prediction, prospect assessment, geologic modeling, and reservoir simulation. Despite advances in seismic imaging, complexities and imageability of faults at the trap scale continue to dominate uncertainty in maps, and complicate predictions of reservoir performance during planning for development and production. Key learnings in those areas, for example improved understanding of how fault architecture controls reservoir segmentation, are leading to more rigorous, realistic treatment of faults in geologic models and reservoir simulations. In regional analysis, experience in complex basins, such as those with linked structural/depositional histories and mobile salt or shale substrates, has led to a clearer understanding of controls on hydrocarbon play elements. In these types of basins, the interplay between structural evolution and sediment dispersal has a profound effect on trap formation, reservoir distribution, and hydrocarbon migration. Regional structural analysis, when integrated with other core technical pursuits, provides a powerful means to identify and pursue viable hydrocarbon plays.
Effective structural interpretation on a global basis requires considerable effort. A potential strength for companies with worldwide activities is the utilization of cumulative learning from basins with similar structural styles; this also represents a substantial challenge. In an effort to optimize the use of global experience and application of core technologies, ExxonMobil has created and disseminated best practices for certain key technical areas. Such an approach for structural interpretation integrates workflows for structural trap and seal analysis, spanning the business cycle from regional/play analysis to mature field production. Thorough execution of basics, application of global experience, and attention to the range of alternative interpretations can support confident business decisions in areas with significant technical hurdles.
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