The three-dimensional geometry of fluvial channel bodies and valley fills has received much less attention than their internal structure, despite the fact that many subsurface analyses draw upon the geometry of suitable fluvial analogues. Although channel-body geometry has been widely linked to base-level change and accommodation, few studies have evaluated the influence of local geomorphic controls. To remedy these deficiencies, we review the terminology for describing channel-body geometry, and present a literature dataset that represents more than 1500 bedrock and Quaternary fluvial bodies for which width (W) and thickness (T) are recorded. Twelve types of channel bodies and valley fills are distinguished based on their geomorphic setting, geometry, and internal structure, and log-log plots of W against T are presented for each type. Narrow and broad ribbons (W/T 1000, respectively) are distinguished. The dataset allows an informed selection of analogues for subsurface applications, and spreadsheets and graphs can be downloaded from a data repository. Mobile-channel belts are mainly the deposits of braided and low-sinuosity rivers, which may exceed 1 km in composite thickness and 1300 km in width. Their overwhelming dominance throughout geological time reflects their link to tectonic activity, exhumation events, and high sediment supply. Some deposits that rest on flat-lying bedrock unconformities cover areas > 70,000 km2. In contrast, meandering river bodies in the dataset are < 38 m thick and < 15 km wide, and the organized flow conditions necessary for their development may have been unusual. They do not appear to have built basin-scale deposits. Fixed channels and poorly channelized systems are divided into distributary systems (channels on megafans, deltas, and distal alluvial fans, and in crevasse systems and avulsion deposits), through-going rivers, and channels in eolian settings. Because width/maximum depth of many modern alluvial channels is between 5 and 15, these bodies probably record an initial aspect ratio followed by modest widening prior to filling or avulsion. The narrow form (W/T typically < 15) commonly reflects bank resistance and rapid filling, although some are associated with base-level rise. Exceptionally narrow bodies (W/T locally < 1) may additionally reflect unusually deep incision, compactional thickening, filling by mass-flow deposits, balanced aggradation of natural levees and channels, thawing of frozen substrates, and channel reoccupation. Valley fills rest on older bedrock or represent a brief hiatus within marine and alluvial successions. Many bedrock valley fills have W/T < 20 due to deep incision along tectonic lineaments and stacking along faults. Within marine and alluvial strata, upper Paleozoic valley fills appear larger than Mesozoic examples, possibly reflecting the influence of large glacioeustatic fluctuations in the Paleozoic. Valley fills in sub-glacial and proglacial settings are relatively narrow (W/T as low as 2.5) due to incision from catastrophic meltwater flows. The overlap in dimensions between channel bodies and valley fills, as identified by the original authors, suggests that many braided and meandering channel bodies in the rock record occupy paleovalleys. Modeling has emphasized the importance of avulsion frequency, sedimentation rate, and the ratio of channel belt and floodplain width in determining channel-body connectedness. Although these controls strongly influence mobile channel belts, they are less effective in fixed-channel systems, for which many database examples testify to the influence of local geomorphic factors that include bank strength and channel aggradation. The dataset contains few examples of highly connected suites of fixed-channel bodies, despite their abundance in many formations. Whereas accommodation is paramount for preservation, its influence is mediated through geomorphic factors, thus complicating inferences about base-level controls.

http://www.zenzebra.net/fluvial/gibling.pdf

Width and Thickness of Fluvial Channel Bodies and Valley Fills in the Geological Record: A Literature Compilation and Classification

More than 100 offshore mass-movement deposits have been studied in Holocene and Pleistocene sediments. The processes can be divided into three main types: slides/slumps, plastic flows, and turbidity currents, of which 13 main varieties have been recognized. The three types are differentiated mainly by motion, architecture, and shape of failure surface. For slides, the morphology of deposits can usually be linked to a process, but for plastic flows and turbidity currents, information about the motion is mainly provided by the sedimentary record. A static classification based on these features is given, and is related to a dynamic classification system to try to underline the morphological transformation of an offshore event from initiation to deposition.

Classification of offshore mass movements

A primary objective in exploration for and development of fluvial reservoirs is determining the thickness and width of sandstone-conglomerate bodies (mainly channel-belt deposits). Most of the existing techniques for estimating the dimensions of fluvial reservoirs have major drawbacks. A fresh approach to the problem is made using recent theoretical, experimental, and field studies. This new approach involves (1) new models for the lateral and vertical variation of lithofacies and petrophysical-log response of river-channel deposits with explicit recognition of the different superimposed scales of strata, (2) distinction among single and superimposed channel bars, channels, and channel belts, (3) interpretation of maximum paleochannel depth from the thickness of channel bars and the thickness of sets of cross-strata formed by dunes, and (4) evaluation of various methods for estimation of widths of sandstone-conglomerate bodies that represent either single or connected channel belts (outcrop analogs; correlation of sandstone-conglomerate bodies between wells; use of empirical equations relating channel depth, channel width, and channel-belt width; theoretical models; and three-dimensional seismic data).

Interpreting the Dimensions of Ancient Fluvial Channel Bars, Channels, and Channel Belts from Wireline-Logs and Cores

Turbidite channel reservoirs—Key elements in facies prediction and effective development

Meso-Cenozoic geodynamic evolution of the Paris Basin: 3D stratigraphic constraints

The Cenomanian of the Arabian Peninsula comprises a carbonate platform setting with rudists, characterized by gradual lateral facies changes including the interfingering of carbonate reservoirs (Natih and Mishrif formations) and source rocks. In order to be more predictive with regard to the distribution and the geometrical aspects of the reservoirs and source rocks, a high resolution sequence stratigraphic study has been carried out in the Adam Foothills of Northern Oman. Based on detailed field sections a correlation scheme covering a transect of 100 kilometers (km) has been established. Three orders of stacked depositional sequences have been found based on the reoccurrence of facies. During long-term increase of accommodation the depositional environment was separated in basinal and platform facies. In contrast, during longer term sea level fall, i.e. long-term decrease of accommodation space, prograding shelfal units extended platform facies over a large part of the basin. The most heterogeneous facies associations are found in times of minimal accommodation space, when incisions and subaerial exposure produce lateral variable strata (e.g. top Natih E). The organic matter is found at the base of two of the three longer term (3rd order) depositional sequences. The organic carbon is contained in marl-limestone couplets (small-scale cyclicity) with a high abundance of oysters and monospecific brachiopod faunas (coquinas). Rudists are found in the progradational part of these sequences, and occur mostly as reworked rudstone layers in meter to decimeter scale, high frequency cycles. The detailed regional correlation depends on the identification of medium- to small-scale (4th to 5th order) depositional sequences which are bounded by regional shifts of the facies belts. The distinct hierarchical organization of the depositional sequences in the Cenomanian, and the relative stability at that time of the Arabian Peninsula, implies a strong correlation potential and thus a broad regional similarity of the architecture of the petroleum systems at that time.

/pdf/high-resolution-sequence-stratigraphy-of-the-natih-formation-4dmakcytwc.pdf

High Resolution Sequence Stratigraphy of the Natih Formation (Cenomanian/Turonian) in Northern Oman: Distribution of Source Rocks and Reservoir Facies

Downstream evolution of turbiditic channel complexes in the Pab Range outcrops (Maastrichtian, Pakistan)

The EAGE Conference held in Algiers in April 2005 was the opportunity for 23 attendees to participate in a post-conference field-trip organized in the Tassili N’Ajjer outcrops in southern Algeria. The stratigraphic succession from the infra-Cambrian to the Middle Devonian is exposed in these exceptional outcrops, desert conditions favouring continuous exposure. The field trip started in the city of Djanet in the Infracambrian granites and shales and ended five days later in Illizi in the Lower Devonian Emsian sandstones (Figure 1). During the trip,
these outcrops were described in terms of sedimentology, fracturation, and
sequence stratigraphy. The outcrops of this area are of particular interest for petroleum geologists as the same sedimentary succession produces hydrocarbons in the nearby Illizi
and Berkine basins and in the Hassi- Messaoud area. The Cambrian, Ordovician, and Devonian sandstones observed in the Tassili outcrops have an equivalent in the reservoir units in the subsurface. The Silurian hot shales are also the prolific source rock of the petroleum system in these basins. The succession presented in the outcrops can then be directly compared with the subsurface data of the Illizi and Berkine basins. The area was first surveyed by Killian (1922) who described the sedimentary succession of the region. Then, the lithostratigraphy and the Siluro-Devonian succession in the outcrops were
established by Dubois et al. (1967). The reference study of the Lower Paleozoic outcrops in Algeria was published by Beuf et al. (1971). These authors proposed a lithostratigraphic nomenclature for the Lower Paleozoic and a sedimentological description of the formations.
They also identified and interpreted the main unconformities in the Lower Paleozoic with a modern approach, discussing the relative role of tectonics and climate during sedimentation. A detailed mapping of the area was also performed by Sonatrach and Beicip under the supervision of the Ministere de l’Industrie of Algeria. However, since these pioneer studies, little has been published about the outcrop geology although hydrocarbon exploration and development has been very active in the nearby Illizi and Berkine basins. The objective of
this paper is to propose a reference section for the Lower Paleozoic in the western Tassili with an updated sedimentological interpretation. An interpretation in terms of sequence stratigraphy will also be proposed. We then summarize the lithostratigraphy in use in the outcrops and propose a correspondence with the sub-surface.

The Lower Paleozoic succession in the Tassili outcrops, Algeria: sedimentology and sequence stratigraphy

Reservoir modeling of the Chaunoy field was performed by combining a sedimentological study, a sequence stratigraphic analysis, geostatistical simulations, and the analysis of production data and fluid-flow simulations. The reservoir corresponds to the distal part of a Middle Triassic alluvial fan system in the Paris basin (France), and is extremely heterogeneous and layered. The reservoir mostly consists of small ribbon channel deposits interbedded with flood-plain and lacustrine mudstones. The channel amalgamation rate varied with cyclic lake-level variations, which directly controlled the reservoir geometry. Within a base-level cycle, during periods of low accommodation, channels were amalgamated, forming highly heterogeneous sand sheets. As the accommodation increased, channels became progressively isolated within flood-plain mudstones. Finally, a lacustrine transgression deposited lacustrine mudstones and induced thin but widespread vertical permeability barriers across the field. As accommodation started to decrease, considerable pedogenetic alteration occurred, as shown by dolocretes and groundwater dolomites. Five cycles that constituted the reservoir layering framework were identified. Geostatistical simulations of lithotype distribution within these units were computed using the truncated random Gaussian function method. Horizontal and vertical lithotype proportion curves and variograms were calculated from well data. Because of the wide well spacing, it was not possible to determine the range of horizontal experimental variograms. Three lithotype realizations were simulated within a high-resolution grid to compare short, medium, and long correlation lengths. After assigning petrophysical properties to the lithotypes and upscaling, fluid-flow simulations were performed for the three realizations. The three flow simulations were then compared to the 10-yr production history of the field. The simulations showed quite a good match regardless of the variogram range, except in the northern part of the field, indicating a problem in the reservoir layering in this area. This relative insensibility of the flow simulation to the correlation length probably is due to the high net pay within the amalgamated channel reservoir units and to the high number of conditioning wells; however, the flow simulation performed with the longest correlation length showed the best fit with the production history.

Combining Sequence Stratigraphy, Geostatistical Simulations, and Production Data for Modeling a Fluvial Reservoir in the Chaunoy Field (Triassic, France)

Proportions curves which were introduced at the end of the 80’s, have proved to be a powerful tool for sequence stratigraphy analysis and for quantifying lithofacies. In this paper we discuss both the qualitative and quantitative aspects via several examples coming either from analogue studies or from real oil fields.

R. Eschard

Papers

High Resolution Sequence Stratigraphy of the Natih Formation (Cenomanian/Turonian) in Northern Oman: Distribution of Source Rocks and Reservoir Facies

Downstream evolution of turbiditic channel complexes in the Pab Range outcrops (Maastrichtian, Pakistan)

The Lower Paleozoic succession in the Tassili outcrops, Algeria: sedimentology and sequence stratigraphy

Combining Sequence Stratigraphy, Geostatistical Simulations, and Production Data for Modeling a Fluvial Reservoir in the Chaunoy Field (Triassic, France)

Quantification of Facies Relationships Via Proportion Curves