Sedimentological Model Research Articles

HYDROCARBON PROSPECTIVITY OF THE DEEPWATER GIPPSLAND BASIN, VICTORIA, AUSTRALIA

After 35 years of successful exploration and development, the Gippsland Basin is perceived as a mature basin. Several world class fields have produced 3.6 billion (109) BBL (569 GL) oil and 5.2 TCF (148 Gm3) gas. Without additional discoveries, it is predicted that further significant decline in production will occur in the next decade.However, the Gippsland Basin is still relatively underexplored when compared to other prolific hydrocarbon provinces. Large areas are undrilled, particularly in the eastern deepwater part of the basin. Here, an interpretation of new regional aeromagnetic and deep-water seismic data sets, acquired through State and Federal government initiatives, together with stratigraphic, sedimentological and source rock maturation modelling studies have been used to delineate potential petroleum systems.In the currently gazetted deepwater blocks, eight structural trapping trends are present, each with a range of play types and considerable potential for both oil and gas. These include major channel incision plays, uplifted anticlinal and collapsed structures that contain sequences of marine sandstones and shales (deepwater analogues of the Marlin and Turrum fields), as well as large marine shale-draped basement horsts.The study has delineated an extensive near-shore marine, lower coastal plain and deltaic facies association in the Golden Beach Subgroup. These Late Cretaceous strata are comparable to similar facies of the Tertiary Latrobe Siliciclastics and extend potential source rock distribution beyond that of previous assessments. In the western portion of the blocks, overburden is thick enough to drive hydrocarbon generation and expulsion. The strata above large areas of the source kitchen generally dip to the north and west, promoting migration further into the gazetted areas.Much of the basin’s deepwater area, thus, shares the deeper stratigraphy and favourable subsidence history of the shallow water producing areas. Future exploration and production efforts will, however, be challenged by the 200–2500 m water-depths and local steep bathymetric gradients, which affect prospect depth conversion and the feasibility of development projects in the case of successful exploration.

Structures and grain size distribution in surge deposits as a tool for modelling the dynamics of dilute pyroclastic density currents at La Fossa di Vulcano (Aeolian Islands, Italy)

Turbulent, dilute pyroclastic density currents (surges) represent the most common and hazardous type of eruptive event at La Fossa di Vulcano. During the volcanic history these currents have frequently passed over the topographic barrier of La Fossa caldera, maintaining a high velocity. We interpret lateral facies variation in terms of fluid-dynamic behaviour by analyses of structures, thickness and grain size of correlated surge layers from the Palizzi succession. Using these data we validate a sedimentation model of traction fed by a low concentration, turbulent and stratified current. By the use of a sedimentological model, we have then reconstructed the fluid-dynamic parameters of the current and calculated the maximum horizontal distance that solid particles travelled before deposition. Results have been checked against actual deposit features. Consistency of data indicates that the method used in this study, even if it is approximate, and needs further refinement by specific experimental approaches and numerical simulations, gives a reasonable indication of surge characters such as velocity and density, which are relevant for hazard calculation and risk assessment.

Tertiary tectonic and sedimentological evolution of the South Carpathians foredeep: tectonic vs eustatic control

A detailed seismic sequence stratigraphy study based on a dense network of seismic profiles is integrated with structural observations from interpreted geological sections to derive a tectonic and sedimentological model for the Miocene–Pliocene evolution of the South Carpathians foredeep (Getic Depression). Following Paleogene and older orogenic phases, the first tectonic event which affected the studied area was characterised by Early Miocene large scale extension to transtension which is responsible for the opening of the Getic Depression as a dextral pull-apart basin. Further Middle Miocene contraction caused WNW–ESE oriented thrusts and associated piggy-back basins. The last tectonic episode recognised in the studied area relates to general transpressive deformations during the Late Miocene–Early Pliocene interval, a first NW–SE oriented dextral episode is followed by second N–S sinistral deformations. The detailed sequence stratigraphy study allows for the definition of the dominant tectonic control of the sedimentary sequences in foreland basins. A eustatic control may be associated, but has a clear subordinated character.

Mode of subglacial transport deduced from till properties, Mýrdalsjökull, Iceland

Abstract A multi-methological study of recent deposited till yields new insights into the mode of subglacial transport. At Slettjokull, the northern margin of the Mýrdalsjokull ice-cap, the ice-flow direction, transport distance, transport path, and glacier regime are known for a well defined point source. This knowledge permits an investigation of the lateral development of till properties and the reconstruction of a sedimentological model comprising some of the parameters affecting the modification of sediment in subglacial transport. One of the most remarkable results from this investigation is the very short transport distance that is required for the different till properties to reach a mature state. Within the first 250 m of transport distance significant changes occur in the grain-size distribution, clast-morphology, geochemical and mineral magnetic properties. However, the variation beyond 250 m suggests that till properties are likely to be a function of parameters other than transport distance. It is inferred on the basis of a renewed depositional model that the physical processes and conditions in a deformable bed beneath Slettjokull are highly responsive to any change in type of substratum. Furthermore, it is proposed that the ability of comminution processes to act on the sediment in transport is controlled by the prevailing subglacial drainage conditions and its influence on the rheological behaviour of a deformable bed. As a consequence, it may be demonstrated that the sedimentary properties of the deforming bed till might change significantly over very short distances without direct correspondence to transport distance.

Reservoir development, sequence stratigraphy and geological modelling of Westphalian fluvial reservoirs of the Caister C Field, UK southern North Sea

The earliest Westphalian B reservoir of the Caister C Field (informally named the Caister Sandstone Unit consists of erosive conglomeratic sandstones deposited in stacked low sinuosity fluvial channels. Channel types and architectural elements demonstrate an upward change to more sinuous forms. The Caister Sandstone Unit is believed to contain incised valley-fill sediments deposited as late lowstand and transgressive systems tracts when rising base level generated accommodation space. The dominant control on reservoir quality is primary depositional texture with the best permeability found in higher energy coarse sandstone and pebbly intervals associated with basal channel-fill units; crevasse channel and splay sandstones offer poor reservoir properties. The robust correlation framework, which is based upon laterally extensive coal seams, enables a sequence stratigraphic model of the field to be developed, thereby allowing a better understanding of reservoir extent and sandbody continuity. A 3D geocellular geological model is built with reservoir architecture elements controlled by the sedimentological model. This is used for volumetrics calculations, input to a reservoir simulator and in the locating and planning of in-fill wells.

Cretaceous Depositional Systems in the Norwegian Sea: Heavy Mineral Constraints

Deep-water Cretaceous sandstones in the Norwegian Sea were deposited by a number of distinct sediment transport systems tapping different sediment source terrains. Three distinct sandstone types (K1, K2, and K3) have been identified, distinguished, and mapped on the basis of a combination of heavy mineral parameters. Sandstone type K1 occurs on the Trondelag Platform, Halten Terrace, and Nordland Ridge, but does not appear to be present in the deeper water Voring Basin. Sandstone type K1 was ultimately derived from the Scandinavian landmass with detritus sourced from metasediments of the Caledonian fold belt, intrusives of the Trans-Scandinavian Igneous belt, and, to a smaller extent, Svecofennian basement. K1 sandstones were deposited on an unstable slope by debris flows and slumps, with minor reworking by bottom currents. Sandstone type K3 was derived from the Western Gneiss region farther south on the Scandinavian landmass. Sandstone type K2 occurs in more basinal locations in the Norwegian Sea and was not supplied by the systems operating along the Scandinavian margin because its mineralogy contrasts with that of K1 and K3. K2 mineralogy is not consistent with a source in the Lofoten area or East Greenland; therefore, K2 is believed to represent the deposits of a separate axial transport system fed by a source that lay in northeast Greenland. K2 zircon ages indicate involvement of Early Proterozoic (approximately 2000 Ma) and Archean basement, together with metasediments of the Caledonian fold belt. Previous sedimentological models for the Cretaceous of the Norwegian Sea suggest that sand deposition occurred as slumps and debris flows along the continental slope, resulting in discontinuous and unpredictable sandstone units, unless they become amalgamated into thick reservoir sequences. The mineralogical evidence indicates that this model can be applied only to sandstone types K1 and K3. By contrast, sandstone type K2 represents sediment introduced from the conjugate margin of the basin and occurs up to 200 km from its detrital source region; therefore, K2 is likely to occur as more predictable, sheetlike bodies on the basin floor.

Paleoenvironmental reconstruction of human activity in Central Arizona using shell chemistry of Hohokam canal ostracodes

The prehistoric Hohokam are the only known culture to develop large-scale irrigation systems before the arrival of Europeans to the United States. Hohokam social evolution was based, in part, on intensive agriculture. To date, most models of Hohokam irrigation agriculture (i.e., hydrologic and sedimentologic models) fail to address dynamics like flow regime, seasonality, canal use and abandonment (Henderson and Hackbarth, 1939). Biota recovered from Hohokam canals often include ostracodes, carbonate-secreting microcrustaceans that are sensitive to hydrochemical changes. m(Mg/Ca)v and m(Sr/Ca)v ratios in the ostracode Limnocythere staplini can be converted into approximate temperature and salinity values at the time of calcification. Ostracode shell chemistry is becoming a powerful tool for understanding the history of canal water chemistry, intensity of land use, and human impact on soil. This study uses multiple regression models to generate the first paleoenvironmental estimates of temperature and salinity from ostracode shell chemistry. Temperature estimates suggest that most canals were used between late winter (February–March) and the premonsoonal season (May–June). Salinity estimates apparently vary over time. Both paleoenvironmental estimates are in good agreement with history records for the Phoenix area. © 1997 John Wiley & Sons, Inc.

The Role of Geology in the Behavior and Choice of Permeability Predictors

Summary For effective flow-simulation models, it may be important to estimate permeability accurately over several scales of geological heterogeneity. Critical to the data analysis and permeability prediction are the volume of investigation and sampling interval of each petrophysical tool and how each relates to these geological scales. We examine these issues in the context of the As Sarah Field, Sirte Basin, Libya. A geological study of this braided fluvial reservoir has revealed heterogeneity at a series of scales. This geological hierarchy in turn possessed a corresponding hierarchy of permeability variation. The link between the geology and permeability was found to be very important in understanding well logs and core data and subsequent permeability upscaling. We found that the small scale (cm) permeability variability was better predicted using a flushed-zone resistivity, Rxo, tool, rather than a wireline porosity measurement. The perm-resistivity correlation was strongest when the probe permeabilities were averaged to best match the "window size" of the wireline Rxo. This behavior was explained by the geological variation present at this scale. For the larger scale geological heterogeneity, the production flowmeter highlighted discrepancies between flow data and averaged permeability. This yielded a layered sedimentological model interpretation and a change in averaging for permeability prediction at the bedset scale (ms-10 × ms).

Coupled two-dimensional diagenetic and sedimentological modeling of carbonate platform evolution

We have used a new coupled two-dimensional diagenetic and sedimentological model to investigate the evolution of a simple aggrading carbonate platform subject to glacio-eustatic sea-level fluctuations. The model employs variable hydrological zones, internally defined from platform exposure and climate, to provide a framework within which diagenetic processes are simulated at specified rates. Simulations show a clear stacked sequence of diagenetic zones, which are readily recognizable because of their lateral continuity and distinct trends in diagenetic evolution from platform interior to margin. However, spatial associations between unconformity surfaces and underlying diagenetic zones that suggest causality are misleading. The interaction between high amplitude sea-level fluctuations and subsidence results in substantial overprinting and a complex diagenetic history that could not be unraveled by traditional stratigraphic and sedimentological methods.

Lithospheric modeling of the Ordovician foreland basin in the Puna of northwestern Argentina: on the influence of arc loading on foreland basin formation

In a qualitative sedimentological model of the Ordovician basin in the Puna of northwestern Argentina, the temporal and spatial distribution of predominantly marine volcanoclastic facies was interpreted to indicate a basin evolution from extensionally related subsidence to that driven by crustal down flexure. Sedimentological data were taken to reflect a Middle Ordovician eastward thrusting of an extinct Early Ordovician arc over part of its back-arc basin in the course of the collision of the allochthonous Arequipa Massif terrane with the South American margin. To test the original qualitative sedimentological interpretation of the Puna basin, an infinite beam elastic plate loading model was applied using the constraints of the field data. Contrary to the original interpretation, results show that the Puna basin can be modeled as resulting entirely from the effects of lithospheric loading caused by the progressive contruction of a static load, i.e., a volcanic arc. Two loading events, one in the Early Ordovician, and one in the Llanvirnian, with tectonic load thicknesses of 6000 and 2000 m, respectively, best explain the observed basin geometry. The onlap patterns on the distal margin on the peripheral bulge are reproduced well using a flexural rigidity of 10 22 N m representing an equivalent elastic thickness of 14 km of the loaded plate. Higher values for the flexural rigidity are considered unlikely, as they result in an enhanced peripheral bulge which uplifts marine near-shore deposits on the distal margin to unrealistic subaereal positions. The model results fit a foreland basin development where crustal thickening was caused by the construction of a static volcanic arc on a previously passive margin. Modeling the movement of thrust loads over parts of the basin according to traditional models of foreland basin formation in this case leads to model results which are inconsistent with field data, even when moderate thrust rates are considered.

Sedimentological aspects of the Humber Group (Upper Jurassic) of the South Central Graben, UK North Sea

ABSTRACTComplex structure, poorly understood sedimentology and poor biostratigraphic control make the Upper Jurassic Humber Group of the South Central Graben one of the least understood and most complex hydrocarbon reservoirs of the North Sea. Detailed logging of available core from 19 exploration wells has been combined with an improved understanding of the relevance of trace fossils and a recognition of important base‐level variations to provide a greatly enhanced understanding of the depositional system active within the area at that time. A new sedimentological model, based upon the distribution of facies and facies associations, illustrates that Upper Jurassic structure and consequent basin geometry were the principal controls upon the distribution of depositional environments. Rifting and second‐order transgression controlled the back‐stepping onlap patterns observed and higher frequency base‐level fluctuations controlled the internal architecture of individual sandbodies. The model presented accounts for features of these deposits that were previously considered anomalous, such as the thickness of bioturbated sandstones, paucity of foreshore deposits and complex age relationships of sands.

Lake Maracaibo Bachaquero Area, Bloque IV Field: A Sparse Logged Area with Additional Reserves?: ABSTRACT

The results of the high resolution logging throughout the Lagunillas Inferior reservoir, in the VLD-1112 and VLD 1152 wells in the Bloque IV area of Lake Maracaibo, Venezuela, are described in this work. Parts of the reservoir were previously overlooked due to the use of simple petrophysical evaluation using very sparse old logs ( SP, Gr, Res.) which show high water saturation`s ( Sw>0.5) and very low porosity`s (O <0.1). In the intervals with thin laminations, the use of microresistivity images define the clean sands of the shaly section, where pressure tests were taken. With these logs, was possible to define local vertical permeability barriers, which separate the productive well known high permeability zones from other zones which could represent additional reserves. A new sedimentological model was defined using core information and the high resolution microresistivity images, subdividing the reservoir into 12 layers, replacing the old simple 3 layer model. Open hole evaluation of the new wells and cased hole measurements using a saturation log indicate that gas has preferentially moved into high permeability zones, leading to bypassing of oil in moderate to low permeability layers. With this study the possible amount of oil which was overlooked by Maraven couldmore » be about 23 %, compared to the estimations derived from the simple petrophysical evaluation, in the shaly sands, which have a big impact in future completion programs . Finally these results were used to locate and drill the first horizontal well in Bloque IV area.« less

Improved Reservoir Description of the Heidrun Field, Norway, using a Combination of Sedimentological, Geochemical and Pressure Data: ABSTRACT

The Tilje Fm. on the Heidrun Field was deposited in a shallow marine to parallic setting dominated by tidal processes. The resulting, often heterolithic, sedimentary sequences can potentially act as barriers to fluid flow due to lack of lateral continuity of sand bodies, and restricted vertical permeability due to clay drapes and tentacular bedding. Based on the sedimentological model and RFT pressure data, a number of potential barriers to fluid flow were identified. In order to substantiate the existence of these barriers a suite of core samples was collected for geochemical analysis. Using core extracts instead of test oils allows for a denser sampling of the reservoir, providing a more detailed description of any variations in reservoir fluid composition. Gas chromatograms of the core extracts show the presence of two oil types in Tilje reservoirs: (A) a strongly biodegraded oil, (B) a mixture of degraded and non-degraded oil. Intervals containing different oil types are usually sharply delineated, with no suggestion of a gradual change from one type to the other. Both oil types are also found at different relative positions, i.e. type A is found both above and below type B within one well. These observations can be explained bymore » a dual charge model: (i) a first charge of biodegraded oil filling the entire Tilje reservoir; (ii) a second (or continuous) charge of non-degraded oil, arriving after the reservoir has passed the temperature threshold for biodegradation (approx. 70{degrees}C), which was unable to penetrate the entire reservoir. The resulting distribution pattern clearly outlines the presence of barriers to fluid flow. The reservoir units thus identified agree in many cases with the model based on the sedimentological and pressure data. In a few cases a different depth is indicated for a barrier, while also some additional barriers are indicated by the geochemical data.« less

The application of physical measurements to constrain reservoir-scale sequence stratigraphic models

Abstract Sequence and parasequence boundaries are characterized by lateral extents that are typically greater than development well spacings and frequently more extensive than individual oil and gas fields. These surfaces result from shifts of facies belts that generate laterally extensive changes over widespread areas. Commonly only facies interpretations of lithological signatures (e.g. hardgrounds) and biostratigraphic signatures (e.g. faunal changes) are used to identify these boundaries in both surface outcrops and subsurface reservoirs. However, a wide variety of physical measurements may also be made, especially in the subsurface, that may be used as additional criteria to recognize surfaces with sequence stratigraphic significance and hence to infer reservoir architectures. Palaeomagnetic reversal stratigraphies may assist in establishing a chronostratigraphic framework within which to identify significant hiatuses and to constrain sequence stratigraphic interpretations, of both outcrops and subsurface reservoirs. In reservoirs, sequence and parasequence boundaries may be detected by a variety of other methodologies, all of which rely upon the spatial coherence of changes across these significant stratal surfaces. The geochemistry of both formation waters, and hydrocarbons, may show significant changes across these surfaces as a consequence of original variations in pore-water chemistry and/or the inability of diffusive mixing processes to equilibrate chemical compositions across such laterally extensive barriers to vertical communication. Similarly, formation pressure profiles, measured after depletion from initial reservoir pressures, frequently show significant discontinuities across these surfaces. Seismic data record the varying strengths of reflections that emanate from geological interfaces as a consequence of varying contrasts in acoustic impedance across geological surfaces. These surfaces (seismic reflectors) are commonly assumed to be of chronostratigraphic significance. It follows that spatially coherent patterns of variation in the physical properties of sediments between these surfaces may serve to delineate the location of facies belts (systems tracts) during the period of geological time represented by the sediments between reflectors. In favourable circumstances these patterns are imaged with a high degree of spatial precision by 3D reflection seismic data. The integration of the geometry of reflections with variations in reflection strength or other seismic attributes, may therefore serve to define both the chrono- and lithostratigraphic framework of reservoir sequences. It follows that qualitative inferences regarding reservoir architecture that are derived from sequence stratigraphic principles may be tested, refined and given improved spatial definition by the careful integration of diverse measurements with conceptual sedimentological models.

Influence of Lithofacies and Diagenesis on Norwegian North Sea Chalk Reservoirs

The depositional mechanism of chalk is a key influence in the chalkAEs ultimate reservoir quality. Classically, the depositional mechanism is interpreted from core descriptions. Where core data are lacking, dipmeter and borehole imagery logs have proven useful in making lithofacies assessments. Criteria for recognition of three chalk categories are established. Category III chalks correspond to those chalks that have been deposited by gravity flows or slumping and tend to have the best reservoir parameters. Category I chalks are most often affiliated with pelagic deposition and tend to have the poorest reservoir parameters. Category II chalks are intermediate between I and III. Anomalously high primary porosities have been maintained in Norwegian North Sea chalks where the effects of mechanical and chemical compaction have been limited. The diagenetic pathway of a chalk reflects changes brought about by mechanical and chemical compaction. Five factors most heavily influence the diagenetic pathway: (1) burial depth, (2) chalk type, (3) overpressuring, (4) presence of hydrocarbons, and (5) original grain size. Assessments of the sedimentological model, diagenetic pathway, and resultant reservoir quality are provided in case studies of Edda, Tor, and Eldfisk fields. Because the distribution of chalk is largely independent of existing structures, most fields have a component of stratigraphic/diagenetic trapping. Each case study shows unique examples of how petrophysical and reservoir engineering data can be incorporated in assessments of chalk type and the diagenetic pathway and how they may affect reservoir parameters and productivity.

A complete methodology for reservoir characterization through a 3D high-resolution grid

An accurate reconstruction of reservoir heterogeneity is an important goal of a reservoir study. The methodology presented here is aimed at providing a complete petrophysical characterization of the reservoir, particularly when the heterogeneity cannot be adequately described using traditional methods. The first phase of the methodology is integrated petrophysical characterization. This is performed by a statistical analysis of log and core data using a 'cluster analysis' algorithm. This allows an objective classification of the reservoir components into homogeneous subsets (lithofacies), each with specific lithological and petrophysical characteristics. The second phase is simulation of the reservoir heterogeneity. Here, the 3D distribution of the lithofacies recognized at the wells is performed consistently with the sedimentological model by means of geostatistical simulations carried out using a high-resolution grid. The porosity and permeability values characterizing each lithofacies are then assigned to the simulated images to obtain a highly detailed 3D petrophysical description of the reservoir.

The origin and distribution of porosity in the Zechsteinkalk (Upper Permian) of Hewett Field, southern North Sea

The Zechsteinkalk of the Hewett Field is characterized by extremely heterogeneous porosity distribution. The two most closely spaced wells (only 200 m apart) show one of the largest differences in average porosity (1.8 to 7.6%). In order to locate future wells and achieve 'reasonable' OGIP estimates, it is therefore necessary to model porosity distribution. Facies type has some influence on porosity development. Tidal flat muds developed at the shoreward margins of shoals have only very low porosity, whereas inter-shoal and main shoal facies have moderate porosity, on average. Extreme porosity variation within facies is caused by differential diagenesis. Porosity has been created by leaching and dolomitization and destroyed by early carbonate cementation and by anhydrite cementation. Anhydrite is volumetrically the most significant, occluding 20% (bulk volume) porosity in many intervals and completely cementing many fractures. The scale of the variability that can be derived from modelling of porosity-affecting diagenetic processes is mostly much greater than that found between closely spaced wells, particularly in the case of anhydrite cementation. In spite of this uncertainty the geological model represents an improvement on simple independent porosity contouring. The gross trends of porosity distribution, defined by the sedimentological model, have helped enhance confidence in a seismic porosity model and have helped with estimates of gas resources.

A sedimentological model of the Callovian oolite reservoir of the Villeperdue oil field, Paris Basin (France)

The Villeperdue oil field is a composite stratigraphic and structural trap. The study focuses on the stratigraphic control, i.e. the relationship between stratigraphy and sedimentology, and a sedimentological model constrained by core data, outcrop analogues, and thorough petrographic analyses. The Callovian oolite limestones are subdivided into three stratigraphic units each with its own oolitic shoal facies; the upper two units form two separate complex reservoirs with porosity zones which are hard to correlate though they are probably interconnected.

Block hydraulic conductivity of cross‐bedded fluvial sediments

This paper concerns the three‐dimensional block hydraulic conductivity of blocks of limited size in cross‐bedded sediments of meandering rivers. Based on a simplified sedimentological model of a single point bar, a stochastic model is proposed to simulate the spatial distribution of core scale conductivities within point bar deposits. The required parameters for these simulations are obtained from actual hydraulic conductivity measurements and sedimentological field data. The simulations are then used to calculate the three‐dimensional block conductivity tensor with a numerical upscaling procedure. The stochastic simulation model is used to investigate the influence of slope and aspect angle of the cross‐beds and the core scale anisotropy ratio on the block hydraulic conductivity. It is expected that the off‐diagonal elements of the block conductivity tensor generally cannot be neglected. The core scale anisotropy ratio, which reflects the small scale layering within the cross‐beds, can potentially have a significant impact on the elements of the block conductivity tensor.

Sequence stratigraphic and sedimentological model for estuarine reservoirs in the Lower Cretaceous Glauconitic member, southern Alberta

ABSTRACT The Lower Cretaceous Glauconitic member in the Badger, Little Bow, Retlaw and Turin fields in southern Alberta is interpreted to consist of stacked, partially overlapping, unconformity bounded sequences each of which comprises valley-fill and inter-valley strata. Incised valley-fills are up to 33 m thick and 2.5 km wide. Inter-valley successions occur outside (between) the incised valleys; they lie atop stratigraphically flat-lying portions of sequence boundaries, are up to 7 m thick, can extend more than 8 km laterally away from associated valleys, and commonly overlie paleosols. Sedimentological and ichnological analysis of cores indicates that both valley-fill and inter-valley strata accumulated dominantly in estuarine environments. Each sequence boundary is interpreted to have formed during a fall of relative sea level with valley incision accompanied by soil development on subaerially exposed interfluves. Valley-fill and inter-valley strata of each sequence together represent a transgressive systems tract which was deposited during a rise of relative sea level. Valley-fill deposits accumulated dominantly in inner to middle estuarine environments, whereas inter-valley sediments were deposited in broad outer estuarine embayments. Reservoir sandstone bodies in valley-fill and inter-valley strata have different geometries. Valley-fill sandstones are thick, elongate pods which formed from longitudinal sand bars in tidal bay-head deltas when inner-middle estuarine sedimentation was confined within valleys. Inter-valley sandstones are thin sheets which formed from coalesced sand shoal and tidal channel deposits when outer estuarine embayments spread across interfluvial areas adjacent to the valleys. RESUME Le membre Glauconitic du Cretace inferieur dans les champs petroliferes Badger, Little Bow, Retlaw et Turin du sud de l'Alberta est interprete comme etant compose de sequences empilees limitees par des discordances et qui se chevauchent en partie, chaque sequence comprenant des couches de remblai de vallee et de sediments inter-vallee. Les remblais des vallees encaissees ont jusqu' a 33 m d'epaisseur et 2,5 km de largeur. Des successions de sediments inter-vallee se trouvent en-dehors (entre) les vallees encaissees: elles se trouvent recouvrant les parties stratigraphiquement horizontales des limites de sequence, ont une epaisseur allant jusqu' a 7m, peuvent s'etendre lateralement sur plus de 8 km au-dela des vallees qui leur sont associees, et recouvrent generalement des paleosols. L'analyse sedimentologique et ichnologique de carottes indique que les couches de remblai de vallee et de sediments inter-vallee s'accumulerent principalement en milieu estuarien. Chaque limite de sequence est interpretee comme s'etant developpee lors d'une baisse du niveau marin relatif, l'incision de la vallee etant accompagnee par la formation de sol sur les surfaces interfluves subaeriennes. Groupees ensembles, les couches de remblai de vallee et de sediments inter-vallee de chaque sequence representent un cortege de depot transgressif qui fut depose durant une hausse du niveau marin relatif. Les depots de remblai de vallee s'accumulerent principalement dans les parties interne a moyenne du milieu estuarien, tandis que les sediments inter-vallee furent deposes dans de vastes baies de sa partie externe. Les reservoirs de gres dans les couches de remblai de vallee et de sediments inter-vallee presentent des geometries differentes. Les gres des remblais de vallee sont des masses allongees epaisses, qui se formerent a partir des bancs de sable longitudinaux dans des deltas situes au fond de baies tidales, lorsque la sedimentation dans les parties internes a moyennes de l'estuaire etait restreinte aux vallees. Les gres inter-vallee sont de minces nappes qui se formerent a partir de haut-fonds sableux et de depots de chenal de maree qui s'unirent, lorsque les parties externes des baies estuariennes s'etendirent a travers les regions interfluviales adjacentes aux vallees. Traduit par Marc Charest. End_Page 332------------------------