Timothy R. McHargue

Transient fan architecture and depositional controls from near-surface 3-D seismic data, Niger Delta continental slope

A near-surface, three-dimensional seismic data set from the Niger Delta continental slope, offshore Nigeria, reveals important stratigraphic and architectural features of channel and fan systems in intraslope basins and permits the development of predictive models for application to deeper reservoir systems. Synsedimentary extensional faulting and mud diapirism control slope gradient, but erosion and deposition from sediment gravity flows tend to smooth the depositional profile and establish an equilibrium profile that adjusts to the changing slope gradient. Architectural features and sediment deposits interpreted from seismic character and seismic stratigraphy, in the absence of borehole data, include mass-transport complexes, distributary channels, submarine fans, and hemipelagic drape complexes. Leveed channel complexes are absent in this study area. These architectural features reflect a combination of active (sediment input from channel systems) and relatively passive (slope failures and slumps) sediment supply systems. Deposition of sandy fans is caused by a hydraulic jump at an abrupt reduction of slope gradient. Channel incision results from knickpoint migration headward from an abrupt increase of slope gradient. Submarine fans that show evidence of channel incision and bypass are termed “transient,” whereas fans without channel incision and bypass are termed “terminal.” This distinction has implications for both exploration and reservoir management. The presence of incised channels in transient fans indicates bypass of significant sand volume to a basinward location. If the transient fan is a hydrocarbon reservoir, the incised channel, which commonly is shale filled, may compartmentalize the reservoir. Dayo Adeogba has had 11 years of experience as a petroleum geologist with ChevronTexaco, mostly in development geology and reservoir management. He earned a B.Sc. degree from the Obafemi Awolowo University (Nigeria) and an M.S. degree from Stanford University. He currently focuses on deepwater depositional systems, seismic geomorphology, seismic stratigraphy, and stratigraphic analysis to solve complex reservoir development and fluid-flow issues.Tim McHargue is a research consultant at ChevronTexaco and a consulting faculty at Stanford University. He received his B.S. and M.A. degrees from the University of Missouri and his Ph.D. from the University of Iowa. His research interests are in sequence stratigraphy, seismic stratigraphy, exploration, and reservoir characterization. Currently, Tim is coordinating geological research on turbidite reservoirs at ChevronTexaco. Steve Graham is a professor in the School of Earth Sciences, Stanford University. He teaches courses in sedimentary geology, seismic interpretation, sedimentary basin analysis, and petroleum reservoir characterization. His current research projects include studies of sedimentary basins in eastern Asia, South America, and the western United States, as well as studies of the sedimentology and stratigraphic architecture of deepwater deposits.

Tectonostratigraphic development of the Interior Sudan rifts, Central Africa

Abstract In the Muglad, Melut and Blue Nile rift basins of Interior Sudan three major episodes of rifting, concomitant subsidence and nonmarine/nonvolcanic sedimentation are recognized. These three rifting cycles, which spanned 140 to 95 Ma (Fl), 95 to 65 Ma (F2), and 65 to 30 Ma (F3), resulted in the accumulation of up to 5400, 4200 and 5400 m of sediments, respectively. In the Muglad Basin, the best known and largest of the Sudan rift basins, each rifting cycle consists of (1) a basal sandstone unit (at least near rift margins), that is followed by (2) an upward coarsening section of lacustrine shale grading through marginal lacustrine mudstone and sandstone into fluvial mudstone and sandstone, and (3) a capping blanket of fluvial and alluvial sandstone. The shale-dominated portions of these cycles were deposited in a closed-drainage basin during active faulting. The fluvial and alluvial blanket sands were deposited in an open-drainage basin during the thermal sag phase following each tectonic cycle. The Early Cretaceous F1 intracontinental rifts of Interior Sudan were linked to major rifts/spreading centres in the Proto-South Atlantic by the dextral WSW-trending Central African Shear Zone and to the Indian Ocean via the NW-trending Anza rift in Kenya. In the Muglad Basin, F1 deformation involved high strain rates, rapid syn-rift crustal stretching and subsidence, and the formation of deep, fault-bounded tensional and transtensional pull-apart basins. During the F2 and F3 deformations, the rates of subsidence and stretching were much lower and were focused within smaller geographic areas. Structural elements include asymmetric half-grabens and less common full-grabens with central highs. The three superimposed tectonic episodes resulted in the subsidence of NNW- to NW-trending rift sub-basins; this gave rise to a wide variety of normal fault geometries, displacements, and growth histories. Planar domino-style and listric normal F1 fault arrays are modeled. The rotated F1 basement blocks typically are asymmetric and low-standing, and favour NE-directed growth and material transport. The F2 and F3 normal faults, which have both NE- and SW-directed polarities, often are listric and decouple younger syn-rift strata from older F1 rotated and locked planar basement faults. Palinspastic restorations and forward modeling of three regional cross-sections suggest that F1 faults account for 65–80% of total crustal extension. Stretching factors across the Interior Sudan rifts vary between 1.25 and 1.40. This range is considered to be a minimum since restored sections do not cross F3 depocenters. Total crustal extension across the Muglad plus the Melut rift basins exceeds 75 km.

U-Pb zircon ages from the southwestern Karoo Basin, South Africa - Implications for the Permian-Triassic boundary

U-Pb ages determined using sensitive high-resolution ion microprobe-reverse geometry on 205 single-grain zircons from 16 ash beds within submarine fan deposits of the Ecca Group provide the first evidence of a marine Permian-Triassic (P-T) boundary in the Karoo Basin of South Africa. These U-Pb ages provide an objective basis for correlating the deep-marine sediments of the southwest Karoo Basin with fluvial-deltaic deposits in the central and eastern parts of the basin where the P-T boundary is recorded in a diverse macrofauna. Furthermore, these new zircon ages and their correlation imply asymmetric subsidence and variable sedimentation rates across the basin.

Internal Geometry, Seismic Facies, and Petroleum Potential of Canyons and Inner Fan Channels of the Indus Submarine Fan

The Indus Fan, the second largest submarine fan in the world, covers 1,250,000 km2 (500,000 mi2) and contains sediment more than 7 km (23,000 ft) thick. Multichannel (24-fold) CDP seismic data provide the bases for evaluating the Indus Fan and consist of four seismic facies. Of these, only the high-amplitude, discontinuous (H-D) facies is thought to contain reservoir-quality sandstones. The H-D facies is confined to the axes of leveed channels. Canyon-channel systems that fed the fan in the past can be divided into three zones. The degradational zone is composed of an erosional canyon complex filled by prodelta mud. The transitional zone, located near the canyon mouth, consists of superimposed channels that initially were erosional but eventually aggraded and developed levees. The headward termination of the H-D facies occurs in this zone. The aggradational zone consists of superimposed leveed channels confined solely by their own levees. The H-D facies forms extensive interconnected bodies several kilometers (about 2 mi) wide and 100 m (330 ft) thick in aggradational channels. The proximal termination of the H-D facies near canyon mouths implies the presence of reservoir-quality sandstone surrounded by source/seal mudstone in the transitional zone. This stratigraphic trapping geometry and structural leads may represent a vast, untapped petroleum province.

The elusive character of discontinuous deep-water channels: New insights from Lucia Chica channel system, offshore California

New high-resolution autonomous underwater vehicle (AUV) seafloor images, with 1 m lateral resolution and 0.3 m vertical resolution, reveal unexpected seafloor rugosity and low-relief (<10 m), discontinuous conduits over ∼70 km2. Continuous channel thalwegs were interpreted originally from lower-resolution images, but newly acquired AUV data indicate that a single sinuous channel fed a series of discontinuous lower-relief channels. These discontinuous channels were created by at least four avulsion events. Channel relief, defined as the height from the thalweg to the levee crest, controls avulsions and overall stratigraphic architecture of the depositional area. Flow-stripped turbidity currents separated into and reactivated multiple channels to create a distributary pattern and developed discontinuous trains of cyclic scours and megaflutes, which may be erosional precursors to continuous channels. The diverse features now imaged in the Lucia Chica channel system (offshore California) are likely common in modern and ancient systems with similar overall morphologies, but have not been previously mapped with lower-resolution detection methods in any of these systems.

The natural range of submarine canyon-and-channel longitudinal profiles

We differentiated 20 submarine canyon-and-channel longitudinal profiles across various types of continental margins on the basis of relative convexity or concavity, and according to their similarities to best-fitting mathematical functions. Profiles are visually differentiated into convex, slightly concave, and very concave groups, each of which generally corresponds with a continental-margin type and distinct depositional architecture. Profile groups generally reflect the competing influences of uplift and construction of depositional relief of the seafloor and its degra da tion by erosion related to mass wasting. Longitudinal-profile shape provides a basis for classifying deep-sea sedimentary systems, linking them to the geomorphic processes that shape continental margins.

Postavulsion channel evolution: Niger Delta continental slope

Channel avulsion is fundamental in defining submarine fan morphology yet, as a process, is poorly understood. The postavulsion evolution of five channel-levee systems, documented from both the shallow subsurface and the sea floor, is marked in the early stages by relatively wide axial channel belts containing sinuous channel elements. The axial channel belt in each system narrowed through time in association with levee aggradation, which resulted in increased channel confinement. Of the five systems studied, four avulsed from a radial avulsion node at the mouth of the basin feeder-channel complex, which is the entry point to the basin. Only one avulsion occurred at an avulsion node downflow of the mouth of the feeder-channel complex. The degree of channel instability in three of the four systems before an avulsion event was increased by channel-floor aggradation caused by the backfilling of channel-confined turbidity current deposits. Channel-floor aggradation reduced the confinement relief of the systems, thereby increasing the probability of avulsion during an outsized flow event. The backfilled deposits in the channel belts display relatively high seismic-reflection amplitudes inferred to be coarser grained (more sand rich) than their surroundings, that is, out-of-channel deposits. Overbank cyclic steps are exceptionally well preserved on subsurface levees, and their potential function in promoting an avulsion event is discussed. The actual process of avulsion is caused by the flow itself instead of a reduction in confinement relief, and although outsized flows are the likely trigger, depending on the degree of this relief in the channel, multiple small flows could also be responsible for levee breaching, resulting in avulsion. The process of channel-system evolution resulting in avulsion can be applied to other subsurface data where compensating high-amplitude channel belts are recognized. In the context of hydrocarbon exploration, investigating up depositional dip to identify avulsion nodes increases the chance of locating sand-rich deposits, especially where multiple channels converge on one point.

Event-Based Geostatistical Modeling: Description and Applications

Event-based methods provide unique opportunities to improve the integration of geologic concepts into reservoir models. This may be accomplished over a continuum of rule complexity from very simple geometric models to complicated dynamics. Even the application of simple rules, including few conceptual interactions based on an understanding of stratigraphic relationships and parametric geometries for event scale depositional and erosion features, have been shown to efficiently produce complicated and realistic reservoir heterogeneities. In more complicated applications, initial and boundary conditions from analysis of paleobathymetry and external controls on sediment supply and the event rules may be informed by process models. These models have interesting features that depart from typical geostatistical model; they demonstrate emergent behaviors and preserve all information at all scales during their construction. These models may be utilized to produce very realistic reservoir models and their unique properties allow for novel applications. These modeling applications include; impact of model scale, seismic resolvability, value of information, flow relevance of advanced architecture, iterative and rule-based conditioning to sparse well and seismic data, numerical analogs for architectural concepts, statistical analysis and classification of architectures, unstructured grid construction and utilization as training and visualization tools.

Simulating depth-averaged, one-dimensional turbidity current dynamics using natural topographies: FLOWS THROUGH NATURAL TOPOGRAPHIES

This study simulates turbidity currents through natural submarine topographies using a steady, one-dimensional, depth-averaged model to determine if modeled flows might traverse the length of channel forms observed at the seafloor or in shallow seismic data sets. To accomplish this, we calculated flow dynamics based on 50,000 sets of initial conditions drawn randomly between prescribed bounds and identified those conditions that allowed flows to traverse the naturally observed systems. We also used flow height and velocity to rule out initial conditions that produced flows that would be broadly accepted as unrealistic. We found that a small percentage (2.3–9.7%) of flows traversed the measured portion of these natural systems and maintained plausible peak depth-averaged velocities when laboratory-derived clear-water entrainment rules were used. However, even these flows reached peak heights that were many times (10–200) greater than that of the channel bottom to levee crest relief. When clear-water entrainment was removed from the model, a larger percentage of flows (34.5–41.6%) traversed the measured channel geometries, maintained lower ranges of flow height, and typically had higher flow velocities. Alternate entrainment relationships allowed flows to maintain realistic flow heights and velocities. We speculate that the unrealistic flows produced using clear-water entrainment rules arise because flow loss through stripping and/or overbank collapse is neglected in this one-dimensional model, or extrapolating laboratory-measured clear-water entrainment rules to the field is problematic.

Seismic stratigraphy and seismic geomorphology of a slope depositional environment — Case study from offshore Angola, West Africa

Deep-water Slope and basin floor deposition characterizes the stratigraphy of offshore Angola, West Africa. The section can be subdivided into two parts. A lower part characterized by nearly horizontal slope at the time of deposition, and an upper part characterized by a slope of ~1.5 degrees. The lower part can be described as a low accommodation section wherein multiple channels closely spaced are observed. In contrast, the upper part can be described as a high accommodation section wherein channels are observed to be widely separated by slope deposits. Turbidite channels within the low accommodation section are commonly not deeply incised into the substrate and are of relatively low relief from channel axis to marginal levee crest. In the high accommodation section similar channels are observed, however larger slope valleys are also present. Both organized and disorganized channel complexes are observed in both sections. Conspicuous by their absence are mass transport deposits. These are observed only along the flanks of larger slope valleys and appear to be restricted in their distribution to valley margins.