David L. Carr

3-D seismic evidence of the effects of carbonate karst collapse on overlying clastic stratigraphy and reservoir compartmentalization

A multidisciplinary team, composed of stratigraphers, petrophysicists, reservoir engineers, and geophysicists, studied a portion of Boonsville gas field in the Fort Worth Basin of north‐central Texas to determine how modern geophysical, geological, and engineering techniques can be combined to understand the mechanisms by which fluvio‐deltaic depositional processes create reservoir compartmentalization in a low‐ to moderate‐accommodation basin. An extensive database involving well logs, cores, production, and pressure data from more than 200 wells, 26mi2 (67km2) of 3-D seismic data, vertical seismic profiles (VSPs), and checkshots was assembled to support this investigation. We found the most important geologic influence on stratigraphy and reservoir compartmentalization in this basin to be the existence of numerous karst collapse chimneys over the 26-mi2 (67km2) area covered by the 3-D seismic grid. These near‐vertical karst collapses originated in, or near, the deep Ordovician‐age Ellenburger carbonate ...

Use of 3D digital analogues as templates in reservoir modelling

Geological analogues may be used to rigorously interpret three-dimensional (3D) subsurface reservoir geometry by combining the capabilities of graphics workstations with digital outcrop data collection. Digital data from sedimentary environments and outcropping geological formations are interpreted in a 3D viewing environment to construct 3D templates of analogues for reservoir bodies. These 3D geometries and associated scaling parameters are then available to build 3D digital hypotheses concerning subsurface reservoir geometries. Two examples serve to illustrate this approach. Data from a modern fluvial system in the USA are used to construct digital templates. Interpretation of this dataset enables the relationship between 3D external geometries of sedimentary units to be rigorously defined and related to internal sedimentary structures. The location of gas-filled reservoir compartments in the Carboniferous Bend Conglomerate reservoirs of the Boonsville Field in North Texas are then interpreted using such analogue-derived digital templates.

3-D seismic imaging and seismic attribute analysis of genetic sequences deposited in low‐accommodation conditions

A multidisciplinary team, composed of stratigraphers, petrophysicists, reservoir engineers, and geophysicists, studied a portion of Boonsville gas field in the Fort Worth Basin of North‐Central Texas to determine how modern geophysical, geological, and engineering techniques could be combined to understand the mechanisms by which fluvio‐deltaic depositional processes create reservoir compartmentalization in a low‐ to moderate‐accommodation basin. An extensive database involving well logs, cores, production, and pressure data from 200‐plus wells, 26-mi2 (67km2) of 3-D seismic data, vertical seismic profiles (VSPs), and checkshots was assembled to support this investigation. The reservoir system we studied was the Bend Conglomerate, a productive series of gas reservoirs composed of Middle Pennsylvanian fluvio‐deltaic clastics 900 to 1300 ft (275 to 400 m) thick in our project area. We were particularly interested in this reservoir system because evidence suggested that many of the sequences in this stratigr...

Reservoir systems of the Pennsylvanian lower Atoka Group (Bend Conglomerate), northern Fort Worth Basin, Texas: High-resolution facies distribution, structural controls on sedimentation, and production trends

This study defines the depositional systems of mature lower Atoka Group reservoirs, structural influence on their sedimentation, and sand-transport patterns at a higher degree of resolution and over a significantly larger part of the play area than previously conducted. The reservoir systems are characterized by pronounced variations in depositional style, even between stratigraphically adjacent systems. They represent a variety of on-shelf siliciclastic depositional facies, including gravelly braided river, fluvial-dominated delta, and low-sinuosity incised river deposits. Penecontemporaneous, high-angle, basement-rooted reverse faults and genetically associated folds of the Mineral Wells–Newark East fault system exerted direct control on the orientation of complex fluvial-channel and delta-distributary sand-transport pathways and the geometry of deltaic depocenters. Multiple contemporaneous source areas, including the Ouachita fold belt to the southeast, the Muenster arch to the northeast, and the south flank of the Red River arch, also contributed to the complexity of sandstone trends in the lower Atoka play area. Bubble maps of normalized per-well first-year production and total cumulative production allow qualitative conclusions regarding geologic controls on production distribution. Most wells with optimal gas production occur within two northwest-trending production fairways that coincide with primary sandstone trends of one or more reservoir systems. Highest per-well oil production exists where lower Atoka reservoir facies occur above oil-prone Barnett Shale source rocks (vitrinite reflectance 1.1% Ro) in the western and northwestern parts of the study area. Widespread fault-bounded, karst-produced sag structures that extend vertically from the source rocks through the lower Atoka Group most likely served as hydrocarbon-migration conduits and formed traps for both oil and gas.

Reservoir systems of the pennsylvanian lower Atoka Group (Bend Conglomerate), northern Fort Worth Basin, Texas

This study defines the depositional systems of mature lower Atoka Group reservoirs, structural influence on their sedimentation, and sand-transport patterns at a higher degree of resolution and over a significantly larger part of the play area than previously conducted. The reservoir systems are characterized by pronounced variations in depositional style, even between stratigraphically adjacent systems. They represent a variety of on-shelf siliciclastic depositional facies, including gravelly braided river, fluvial-dominated delta, and low-sinuosity incised river deposits. Penecontemporaneous, high-angle, basement-rooted reverse faults and genetically associated folds of the Mineral Wells–Newark East fault system exerted direct control on the orientation of complex fluvial-channel and delta-distributary sand-transport pathways and the geometry of deltaic depocenters. Multiple contemporaneous source areas, including the Ouachita fold belt to the southeast, the Muenster arch to the northeast, and the south flank of the Red River arch, also contributed to the complexity of sandstone trends in the lower Atoka play area. Bubble maps of normalized per-well first-year production and total cumulative production allow qualitative conclusions regarding geologic controls on production distribution. Most wells with optimal gas production occur within two northwest-trending production fairways that coincide with primary sandstone trends of one or more reservoir systems. Highest per-well oil production exists where lower Atoka reservoir facies occur above oil-prone Barnett Shale source rocks (vitrinite reflectance 1.1% Ro) in the western and northwestern parts of the study area. Widespread fault-bounded, karst-produced sag structures that extend vertically from the source rocks through the lower Atoka Group most likely served as hydrocarbon-migration conduits and formed traps for both oil and gas.

Integration of analog data for building testable, deterministic geological models in a common interpretation environment: An example from the Atokan Boonsville gas field, Fort Worth Basin, Texas

A fundamental problem facing the petroleum industry is to effectively use the large amounts and diverse types of data that are collected to define and exploit stratigraphic and structural compartments that contain undrained hydrocarbons. The key is to build data-driven, deterministic geological interpretations to intelligently target infill wells. This approach is fundamentally different from that used in geostatistically-driven approaches which interpolate the sparse data support points without maximizing the value of the data that has been collected or geological knowledge. We describe a workstation tool and interpretation method for that allows one to combine, in 3D, well-based interpretation and quantitative analog information from fields or outcrops to make testable predictions about the location of geological bodies that are prospective infill drilling locations. This tool combines (1) 3D visualization in a common viewing environment of diverse data that are viewed at true scale (e.g., 3D surface seismic; vertical seismic profiles, reservoir simulation results, conventional wireline and borehole imaging logs, core photographs); (2) a well-based interpretation environment; and (3) an archive of digital 3D geological analog shapes and textures that one can use to relate textures seen in image logs or core images to those observed in analog data, i.e., other fields or outcrops. These shape and texture analogs can then be used to place geological bodies that can be rescaled and oriented in 3D. Comparison with seismic data may then support or refute these interpretations. We have applied this tool to the interpretation of the Atokan Bend Conglomerate in the Boonsville Field in north Texas, a mixed siliciclastic-carbonate succession containing deltaic, estuarine, and fluvial valley-fill sandstone reservoirs. The key to exploitation of this field is identifying sandstone bodies within the lowstand, incised valleys, which are commonly less than 500 meters in width, and 20 meters in thickness. By combining core, image log, and seismic information in the 3D visualization tool, we have recognized Bend Conglomerate reservoir sandstone bodies and further, we have been able to define them in inter-well space using sandbody shape analogs from the tools digital archive. The key advantages of this approach are that we preserve information about the interpretation process and multiple hypotheses; we see all data at the appropriate scale; and we view the implications of the deterministic geological interpretations within the same data volume as our measured data. This provides a means of capturing and applying geological knowledge of analog formations, as well as tracking the steps in the interpretation process.

Integration of Sequence Stratigraphy and 3-D Seismic Imaging in Low-Accommodation Basins

Pennsylvanian-age rocks in several areas of the Midcontinent of the United States were deposited in low-accommodation basinal settings. Many sequences in these low-accommodation environments exhibit severe lateral heterogeneity because they have been extensively reworked by repeated transgressions and regressions. Consequently, the distinctive geometries of relic depositional features tend to be distorted or totally destroyed, in contrast to such geometries in high-accommodation basins where depositional topography, once buried, is rarely exposed to erosional processes. Our objective is to show how these thin and obscure low-accommodation sequences can be identified in well control and interpreted in 3-D seismic data volumes. Numerous, deep-rooted karst-collapse zones affected the areal continuity of many sequences in some Midcontinent basins. We combine sequence stratigraphy with 3-D seismic imaging to document that many of these karst-collapse zones originate at deep Ellenburger ( ) levels and then extend vertically for a distance of 2,000 ft (600 m) or more into Pennsylvanian-age rocks. We also offer evidence that properly chosen seismic attributes, calculated in thin, accurately defined seismic time windows that correspond to log-defined sequences, show compartmented reservoir facies in low-accommodation basins.

Surface-bounded reservoir compartmentalization in the Caddo Conglomerate, Boonsville (Bend Conglomerate) Gas Field, Fort Worth Basin, Texas

Interpretation of cores and logs from 222 wells and a 26 mi[sup 2] 3-D seismic survey in the Boonsville (Bend Conglomerate) Gas Field indicates the Caddo Conglomerate zone (Atoka) contains two reservoir sandstone bodies which are physically separated by a key chronostratigraphic erosion surface. The oil-productive Lower Caddo sandstone represents a southward-prograding, strike-oriented highstand delta system. Downdip wells have encountered both oil and gas in a younger valley-fill sandstone complex comprising the Upper Caddo lowstand systems tract. Abandoned delta-platform limestones at the top of the Lower Caddo highstand tract were truncated during lowstand valley incision prior to Upper Caddo sandstone deposition. The limestones do not occur above the sharp-based, blocky to upward-fining Upper Caddo valley-fill sandstones, and underlying Lower Caddo sandstones typically display upward-coarsening, progradational patterns. Significant gas reserves in Upper Caddo wells located structurally downdip to the Lower Caddo oil accumulation indicate the two units are hydraulically separate reservoir compartments. Both reservoir compartments have been successfully imaged using 3-D seismic attributes analysis, confirming the original, log-based interpretation and providing a powerful infill drilling and reservoir management tool.

High-Resolution Reservoir Characterization of Midcontinent Sandstones Using Wireline Resistivity Imaging, Boonsville (Bend Conglomerate) Gas Field, Fort Worth Basin, Texas: ABSTRACT

In the absence of abundant core data, Formation MicroScanner* (FMS) and Fullbore Formation Microlmager* (FMI) wireline logs from 3 wells in Boonsville Field provided continuous geologic information in a 1000-foot thick, Pennsylvanian (Atoka) interval. Cores provided the most detailed sequence-stratigraphic information, but only 358 ft of core from 4 wells was available to evaluate the 30 mi[sup 2] project area. The FMS and FMI logs thus served as continuous, oriented [open quote]virtual cores[close quote] that expanded our stratigraphic database and improved our interpretations, which included the identification of key chronostratigraphic surfaces, lithofacies, sedimentary structures, faults, and fractures. Paleocurrents inferred from the FMS and FMI images suggest that most Bend Conglomerate sandstones are lowstand valley-fill deposits derived from the Muenster and Red River Uplifts, rather than Ouachita-derived deltas. Combined analysis of cores and wireline resistivity imaging technology enabled the development of a fine-scale, sequence-stratigraphic framework which formed the basis for correlation and mapping of the major Bend Conglomerate reservoir zones, and helped us to identify compartmentalization mechanisms within these complex reservoirs.

Combining sequence stratigraphy with 3-D seismic imaging in low-accommodation basins

Pennsylvania-age rocks in several areas of the Midcontinent of the United States were deposited in low-accommodation basinal settings, that is, in basinal areas where only modest verticle reliefs could accept the sediment influx. Many thin Pennsylvanian sequences in these low-accommodation environments exhibit severe lateral heterogeneity because they have been extensively reworked by repeated transgressions and regressions of a fluctuating sea. Consequently, the distinctive geometries of relic depositional features (such as meandering channels) tend to be distorted or even totally destroyed, as compared with how such geometries appear in high-accommodation basins where depositional topography, once buried, is rarely exposed to destructive processes. Our objectives are to show examples of 3-D seismic images of several depositional topographies in a moderate- to low-accommodation basin and to explain how these thin sequences can be identified in well control and interpreted in 3-D seismic data volumes.