Improving Reservoir Characterization of Karst-Modified Reservoirs with 3-D Geometric Seismic Attributes Susan E. Nissen1, E. Charlotte Sullivan2, Kurt J. Marfurt3, and Timothy R. Carr4 1Consultant, McLouth, KS 2Pacific Northwest National Labs, Richland, WA 3College of Earth and Energy, University of Oklahoma, Norman, OK 4Department of Geology and Geography, West Virginia University, Morgantown, WV

Outline Characteristics of karst-modified reservoirs Multi-trace geometric seismic attributes Seismic-based examples of Collapse structures Polygonal features Oriented lineaments Interpretation workflow for karst-modified reservoirs Conclusions

Karst Modified Reservoirs Carbonate reservoirs Rocks modified by dissolution during subaerial exposure May also have hydrothermal and tectonic overprints

Solution-enlarged fractures Examples of karst features that can affect reservoir performance Cockpit karst, Jamaica www.cockpitcountry.com Residual paleo-highs May be hydro- carbon traps Solution-enlarged fractures Loess-filled fractures, Missouri Fluid conduits (if open) or barriers (if filled) Collapse features Compartmentalize reservoir Affect deposition of overlying strata Cave collapse facies in image log Ft. Worth Basin, Texas

Interpretation of Karst Features Well data alone is insufficient for identifying the spatial extent and distribution of local karst features. Karst features with substantial vertical relief can be readily identified using 3-D seismic. Critical features relating to reservoir character are often subtle and not readily detected using standard 3-D seismic interpretation methods. Multi-trace geometric seismic attributes can help!

Multi-Trace Geometric Seismic Attributes Calculated using multiple input seismic traces and a small vertical analysis window The analysis "box" moves throughout the entire data volume => attributes can be output as a 3-D volume Provide quantitative information about lateral variations in the seismic data

Multi-Trace Geometric Seismic Attributes Coherence - A measure of the trace-to-trace similarity of the seismic waveform Dip/azimuth - Numerical estimation of the instantaneous dip and azimuth of reflectors Curvature – A measure of the bending of a surface (~2nd derivative of the surface) Reference Trace Instantaneous dip = Dip with highest coherence Dips tested

Mid Continent examples - Collapse structures - Polygonal features - Oriented lineaments Central Kansas Uplift Ord. Arbuckle Mississippian Ft. Worth Basin Ord. Ellenburger We will look at examples of the application of volumetric seismic attributes to three areas An Ordovician Ellenburger aquifer in the Fort Worth Basin, north Texas . An Ordovician Arbuckle reservoir in Kansas And a Mississippian reservoir in Kansas

Collapse Features – Fort Worth Basin vertical seismic section Pennsylvanian Caddo Collapse features are visible as depressions on the 3-D seismic profile Collapse features extend from the Ellenburger through Pennsylvanian strata ~2600 ft Collapse features Vertical cross section from a 3-D survey in the Fort Worth Basin of north Texas. Here, collapse features extend from the Ordovician Ellenburger carbonates through Mississippian and Pennsylvanian shales, siltstones, and limestones- a vertical distance of about 2600 ft. Ordovician Ellenburger

Attribute time slices near the Ellenburger Amplitude Coherence fault N Dip/Azimuth Most Negative Curvature W E Collapse features Different attributes show different information about the collapse features Coherence better at showing features than conventional amplitude slice Dip/azimuth shows that they are depressions, sense of motion on faults Volumetric curvature shows more detailed, somewhat polygonal features not evident on the other attributes S 3 mi

Collapse features line up at the intersections of negative curvature lineaments Coherence Most Negative Curvature Time = 1.2 s 1 mi

Polygonal Features Ordovician Arbuckle Kansas Ordovician Ellenburger Fort Worth Basin 1 mi 1.6 km 1 mi 1.6 km Comparison of polygonal features in Kansas Arbuckle and Fort Worth Basin Ellenburger. Diameters ~700-900 ft Diameters ~1400-1600 ft Diameters ~1200 -3500 ft Vertical relief generally 2 ms (~15 ft) or less

(After Cansler and Carr, 2001) Arbuckle Polygonal Karst -- Cockpit Karst Cockpits (After Cansler and Carr, 2001) Cockpit karst doline A horizon structure map of the Arbuckle surface from a 3D seismic survey in Kansas shows approximately 100 ft (30 m) of local relief, with northwest to southeast-trending ridges. Individual cones and dolines can be seen, some with diameters <1000 ft (300 m). Most Positive curvature extracted along the Arbuckle horizon shows a network of polygonal features with average diameters of approximately 750 ft (230 m). Most of these features have no apparent relief on the seismic structure map. This seismic geomorphic landscape is reminiscent of polygonal or cockpit karst, as described by Williams (1972). Polygonal karst forms in uplifted low-relief strata that have been fissured by a system of joints. Stream sinks are initiated at locations of maximum fracturing. Scattered small depressions expand and capture smaller neighbors until the entire surface is occupied by adjoining polygonal depressions. Polygonal karst has been identified by Cansler and Carr (2001) on the Arbuckle erosional surface elsewhere in Kansas using well data. In their study area, dolines are up to 250 ft (75 m) deep and are localized in areas as wide as 1 mile (1.6 km). Typically, they are 10-60 ft deep (3-20 m) with diameters of 1000-2000 ft (300-600 m). Cansler and Carr (2001) concluded that it is likely that the surface is pitted with a large number of smaller dolines that are too small in area to delineate with well spacing or < 10 ft (3 m) in depth. We appear to be imaging just such features with our seismic curvature map. cone Morphological map of karst area in New Guinea (Williams, 1972) Arbuckle structure overlain with paleotopographic divides in Barton Co., KS (Cansler, 2000) Arbuckle time structure overlain by most positive curvature

Ellenburger polygonal karst - tectonic collapse structures Collapse feature at topographic high Faults Collapse Features Coincide with Deep Basement Faults Polygonal geomorphology, similar to the cockpit karst identified in the Kansas Arbuckle, is also seen on the surface of the Ordovician Ellenburger horizon in the Fort Worth Basin. Most negative curvature time slices near the Ellenburger show polygonal geometry and corresponding coherence slices show circular collapse features that line up at the intersection of curvature lineaments. However, when we examine the 3D visualization of coherence extracted along the top of the Ellenburger, we see that the collapse features occur near the tops of topographic highs, as well as at valley heads and that the rims of the “cockpits” are rather wide. Although the presence of subaerial karst is well established in the Ellenburger by the presence of cavern collapse facies in conventional cores, this karst forms a pervasive background and is not limited to areas of the large collapse features. Many of the collapse features coincide with deep basement faults, or occur along Pennsylvanian age fractures and small faults. In addition, dolomite and native copper cements in fracture fill indicate flow of burial fluids. These lines of evidence indicate that the polygonal geomorphology and extensive collapse features in the Fort Worth basin data set are more likely controlled by tectonic processes than subaerial weathering. Implications from these studies are that tectonic and subaerial karst processes may be linked, with subaerial karst forming at intersections of tectonic joints. Reactivation of zones of weakness allows migration of fluids (meteoric and hydrothermal) along the same vertical pathways through time. N

Oriented lineaments -- Kansas Mississippian Lineament trend vs. oil/water production In a Mississippian reservoir in central Kansas that is subjacent to a pre-Pennsylvanian unconformity and karst surface, lineaments in the long wavelength Most Negative curvature volume, extracted along a horizon corresponding to the base of the aquifer supporting the reservoir, are correlated with fluid production in the reservoir. These lineaments are dominated by two orthogonal directions (northeast and northwest), which line up with regional structural trends. Wells situated near northeasterly oriented lineaments have lower oil production and a thicker basal conglomerate above the unconformity surface than do wells more distant from the northeasterly trending lineaments. The presence of rotated blocks of dolomite and green shale in cores is suggestive of low permeability debris fill. The northeasterly trending lineaments may relate to a high concentration of shale-filled fractures that either degrade the quality of the limestone reservoir or serve as compartment boundaries. Proximity to northwesterly trending lineaments correlates with higher water production, but has no relationship with oil production, suggesting that northwesterly trending lineaments correspond to open fractures that connect directly to the underlying aquifer. 0.5 mile

Workflow for Identification of Karst Overprints Using Multi-Trace Attributes Interpret features relating to structure, geomorphology, and reservoir architecture on attribute slices Extract attributes along horizon or time slice Measure distance from oriented lineaments. Outline potential reservoir compartment boundaries (fluid barriers) Identify areas of enhanced or occluded porosity/permeability Separate subaerial karst from tectonic overprint Core and log data Horizon picks Predict general production performance based on type of karst overprint Volumetric attributes Identify dominant karst geomorphology (e.g., polygonal karst vs. groundwater-sapped plateaus) Production data Identify preferred orientations of fluid conduits vs. barriers

Conclusions Coherence, dip/azimuth, and curvature extractions are valuable for establishing seismic geomorphology Different attributes reveal different details about karst features A workflow utilizing multi-trace attributes, along with geologic and production information, can improve characterization of karst-modified carbonate reservoirs

Acknowledgements Devon Energy Grand Mesa Operating Company John O. Farmer, Inc. Murfin Drilling Company IHS - geoPLUS Corporation Seismic Micro-Technology, Inc. U. S. Department of Energy Petroleum Research Fund State of Texas ATP Kansas Geological Survey, University of Kansas University of Houston