Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Houston Geological Society, December 2004
Quantitative Seismic Geomorphology of Clastic Reservoirs and Systems
Lesli J. Wood
The discipline of geomorphology has a long and illustrious history. More recently tangential application of geomorphic principles to the study of stratigraphic sequences, reservoir heterogeneity and geobody formation has begun to recognize the additional insight that can be developed when we apply knowledge gained through modern geomorphic study to interpreting older strata and processes (Carter, 2003; Posamentier and Kolla, 2003; Posamentier, 2003). Evolving image technologies (3D seismic, multicomponent seismic, visualization, attribute analyses, laser outcrop imaging, etc.) now enable geoscientists to see in greater detail than ever before how seascapes and landscapes have evolved through time. There is little doubt that seismic geomorphology, when integrated with seismic and sequence stratigraphy, is a powerful tool for understanding basin evolution. However, applying geomorphic principles and laws to simply understanding the gross history of basin evolution is to only scratch the surface of what this new approach can bring to the table.
Opportunity exists to take a more quantitative approach in the integration of geomorphology and seismic data. Quantitative seismic geomorphology (QSG) is a new direction in the application of Seismic Geomorphology that will create a step-change in our knowledge, characterization and understanding of older clastic environments. QSG is defined as “Quantitative analysis of the landforms, imaged in 3-D seismic data, for the purposes of understanding the history, processes and fill architecture of a basin.” (Wood, 2003). QSG uses 3-D seismic data integrated with core and logs to investigate the nature and architecture of reservoirs through quantitative data collection on the system’s morphometrics, and through analyses of the spatial and temporal variability of reservoirs. Several recent papers, poster presentations and oral talks have discussed this approach to quantification of morphometrics in clastic reservoirs.
Fachmi and Wood (2003) used a large 3D mega-merge, seismic dataset in the West Natuna Basin of Indonesia to examine the paleogeomorphic evolution of the basin, to assess the impact of tectonic overprint on the character of the geomorphic systems and to create quantitative probability models for exploration and development in this complex system. The West Natuna Basin is a prolific hydrocarbon basin whose reservoirs are deltaic and fluvial. Reservoir systems are architecturally complex and incredibly well imaged in two 3-D seismic volumes that have been merged to cover 3,154 km2. Fifteen well log suites located within the study boundaries provide deterministic data on lithology. The geomorphology of the preserved channel reservoir system has been quantified by measuring channel element sinuosity, meander wavelength and radius of curvature, channel and valley width:thickness, meanderbelt width and rates and directions of meander migration. These quantitative values are used by the authors to examine the size range of reservoir elements, calculate reservoir rock volumes, and ascertain drainage radius.
Zeng, Wood and Hentz (2001) and Wood (2003) in a study in the northern Gulf of Mexico (Vermillion Island and South Marsh Island) shelf used 360 sq km of 3D seismic and 155 well logging suites to examine environments that included fluvial, deltaic, and shallow marine, as well as shelf-edge, slope and fan systems. Three specific fluvial incision types, creeks and distributaries, bypass fluvial systems and aggradational fluvial systems were examined through QSG methods and found to show their own unique sinuosity, channel widths, meander lengths and meander belt widths. Vshale calculated from well data showed these channel and valley types to each be unique in their fill type, with sand:shale ratios that corresponded to mixed load, bed load and suspended load systems, as defined by Schumm (1977). These results suggest an ability to utilize seismic facies morphometrics to identify fill type within fluvial incisions, similar to techniques employed in the classification modern fluvial systems.
Geomorphic principles of channel form versus nature are applicable not only in subaerial and deltaic fluvial systems but also in deep marine channel systems. Moscardelli, Wood and Mann (2004) and Mize and Wood (2004) working in offshore eastern Trinidad, used 8,000 plus sq km of 3D seismic data and over 200 shallow dropcore to examine shelf-edge deltas, slope-leveed channel systems, debris flows, and associated environments and processes. The study area is characterized by right-lateral transpressional structuring along the Caribbean-South American plate margin and is a world-class mobile shale basin showing extensive development of mud diapirism, mud volcanos and mud walls. These variables have a significant affect on deep-marine debris flow character and distribution, as well as levee channel morphometrics, evolutionary patterns and levee character.
Detailed mapping of debris flows show significant scouring at the base of these flows and significant internal structuring due to internal compressive forces within the flow. Underfilled accommodation formed by the passing debris flow provides accommodation for later occupation by leveed channel systems. Leveed channels show significant variability in depth and sinuosity due to local slope changes, a fact contrary to the classic proximal to distal decrease in levee heights and sinuosity seen in many systems. Measurements show a steady decrease in sinuosity as the systems evolves with regions of highest sinuosity migrating in the landward direction. Among the many observable relationships is that between radius of curvature and meander belt width. This relationship is particularly important because the incised meander belt is the container and the radius of curvature is a measure of the channel reservoir extent within the “container”. In addition, zones of high radius of curvature show a propensity to have more crevassing and overbank flow.
These types of seismically-derived quantitative data provide a rich data set for development of probabilistic approaches to reservoir uncertainty. Cumulative probability curves (see Capen, 1994) can be used to illustrate the probability distribution of reservoir and seal morphologies. As an example, data collected from Pliocene-age, northern Gulf of Mexico incised channels and valleys show the channel width P50 = 840 meters, P10 = 330 meters and P90 = 1250 meters (Wood, 2003). The morphometrics of modern analogs being used in reservoir assessment can be examined in the context of these quantitative seismic morphometrics allowing geoscientists to gauge the appropriateness of the analog for a specific field. These data show that 85% of the channel widths in this Pliocene-age reservoir facies are less than 1100 meters wide. Over 20% of the widths are less than 500 meters wide. These data provide some sense of reason for chosing the morphometrics of reservoir bodies for reservoir rock volume calculations, development well design, correlation distances and drainage radius’s, as well as building reservoir models and designing field floods.
The future of seismic geomorphology
will continue to grow and develop into increasingly quantitative methods
as data quality and imaging and visualization techniques improve. The
ability to collect large quantitative data bases from one’s own
reservoir interval is currently only hampered by the lack of tools in
existing workstation software packages and geoscientist’s limited
knowledge of how those morphometrics can be used in their exploration
and development process.