Incised-valley Deposits Research Articles

Geospatial modeling for groundwater potential zoning using a multi-parameter analytical hierarchy process supported by geophysical data

Groundwater plays a crucial role in Hungary sustaining ecosystems and meeting the growing demand for freshwater to fulfill domestic and agricultural needs. This study employs the analytical hierarchy process (AHP) methodology to delineate groundwater potential zones in the Debrecen area, Hungary. To ensure the robustness and reliability of the potential zoning, geophysical data are utilized for validation purposes. In the AHP modeling seven groundwater conditioning factors are integrated, including geology, topography, slope, land use/land cover, precipitation, drainage density, and lineament density. The integration of the normalized weights for each factor identified three groundwater potential zones (GWPZs) assigned as moderate, high, and very high potential. The result of the AHP model is further validated with geophysical data of gravity and wireline logging. Gravity data is subjected to spectral analysis and forward modeling to map the lineaments and detect the thickness of the sedimentary sequences. The forward modeling indicated that the thickness of these sequences varies between 1.25 and 2.7 km, with a deep local basin delimited by normal faults situated in the eastern part of the study area. Additionally, the analysis of the well-logging data using the Csókás method provided a continuous estimation of petrophysical and hydrogeological parameters along the main hydrostratigraphical units. Accordingly, a high and uniform distribution of hydraulic conductivity is observed in the eastern part of the study area due to the presence of coarse-grained incised valley deposits. The results of geophysical modeling showed a close agreement with that of AHP models. This interdisciplinary approach advanced the mapping of potential groundwater zones and provided valuable insights into the hydrogeological characteristics of the groundwater aquifers in the Debrecen area.

Linking between sequence stratigraphy and reservoir quality of Abu Madi Formation utilizing well logging and seismic analysis at Abu Madi and El Qar’a fields, Nile Delta, Egypt

This work has been carried out to interpret the link between sequence stratigraphy of the incised valley deposits with the reservoir quality in the Abu Madi Formation in the Abu Madi and El Qar’a Fields, onshore Nile Delta, Egypt based on borehole and seismic data. The distribution of sand-body in the Abu Madi Formation revealed that the fluvial channel sandstones are not randomly distributed but are controlled by the fluctuation of the sea level (regression and transgression). Interpretation of sandstones distribution of the Abu Madi incised-valley within a sequence stratigraphic framework allows a better understanding of their characterization during burial. The Abu Madi Formation is made up of two sequences: the lower sequence-1 (SQ 1) and upper sequence-2 (SQ 2). Sequence-1 forms the basal part of the Abu Madi Formation. It consists of two systems tracts in all the studied wells, in which the lowstand systems tract is overlained by a transgressive systems tract (TST-1), except at AM-12 and AM-15 wells in the western and eastern sides of the studied area, respectively where it consists of only transgressive system tract (TST-1). The upper sequence-2 (SQ2) is recorded in all the studied wells except at AM-12 and AM-15 wells. It is composed of two systems tracts (LST-2 and TST-2); the lower lowstand systems tracts contains thin sandstone channel-fill interpreted as LST-2, above this sandstone channel-fill the tidal deposits interpreted as TST-2. The lowstand systems tracts of the studied Abu Madi Formation is considered good reservoir target in the studied area, its porosity ranged from 15 to 22%. It was formed due to a rapid fall in relative sea level below the shelf edge formed valley incision due to subaerial exposure of the continental shelf. The floodplain deposits of TST represent a good seal separating LST reservoirs and water bearing zone of lower sequence (SQ1) and upper sequence (SQ2) in each well.

Sequence stratigraphy, sedimentary systems and implications for hydrocarbon exploration in the northern Xujiaweizi Fault Depression, Songliao Basin, NE China

Recent drilling data from the northern Songliao Basin reveal considerable hydrocarbon exploration potential in deep-seated small-scale faulted basins. In this paper, the Shahezi Formation of the northern Xujiaweizi Fault Depression within the Songliao Basin was investigated as a case study. Sequence stratigraphy, sedimentary systems and the hydrocarbon exploration potential of the small-scale faulted basin were investigated through observations and descriptions of cores; analyses of conventional log curves, imaging data and logging data; and interpretations of 3-D seismic profiles. Four third-order sequences, mainly controlled by sediment supply and fault activity, were identified based on unconformities at the basin margins and in sedimentary successions. System tracts in the third-order sequences were also recognized through lithological associations and geophysical response characteristics. The distributions of sedimentary systems, including fan deltas, braided deltas, lacustrine deposits, subaqueous fans, volcaniclastic deposits and incised-valley deposits, in the sequence framework are associated with basin evolution. During the early stage of the Shahezi Formation (SQ1 and SQ2), the sedimentary area was mainly covered by fan deltas, and lacustrine deposits were not very developed. Additionally, incised-valley systems formed on basin margins and subaqueous fans formed along basin-controlled faults. During the middle stage (SQ3), fan delta deposits on gentle slopes were gradually replaced by braided delta deposits, and the sizes of lacustrine facies increased with basin expansion. In the late stage (SQ4), the basin and lacustrine deposit areas reached their maximum sizes, and braided deltas became the dominant sedimentary systems on gentle slopes. With continuous sediment supply, fan and braided deltas migrated forward, and the lacustrine system was locally distributed. In this small-scale faulted basin, the lacustrine mudstones and coal seams widely developed in SQ4, especially in SQ4-HST, and they also exhibited good source rock properties for gas generation. Sandbodies of fan deltas and braided deltas in the upper Shahezi Formation, exhibiting good contact relationships with underlying source rocks, can be favourable reservoirs for hydrocarbon accumulation, as confirmed by the newly drilled Well Songs9H (with a daily gas production of 20×104 m3). However, due to low porosity and permeability (averages of 4.7% and 0.28×10−3µm2, respectively), these reservoirs may need fracturing for commercial hydrocarbon production.

A massive input of coarse-grained siliciclastics in the Pyrenean Basin during the PETM: the missing ingredient in a coeval abrupt change in hydrological regime

Abstract. The Paleocene–Eocene thermal maximum (PETM) is represented in numerous shallow and deep marine sections of the south–central and western Pyrenees by a 2–4 m thick unit (locally up to 20 m) of clays or marly clays intercalated within a carbonate-dominated succession. This unit records a massive input into the Pyrenean Gulf of fine-grained terrestrial siliciclastics, attributed to an abrupt hydrological change during the PETM. However, the nature of such a change remains controversial. Here we show that, in addition to fine-grained deposits, large volumes of coarse-grained siliciclastics were brought into the basin and were mostly accumulated in incised valleys and in a long-lived deep-sea channel. The occurrence of these coarse-grained deposits has been known for some time, but their correlation with the PETM is reported here for the first time. The bulk of the incised valley deposits in the PETM interval are cross-bedded sands and pebbly sands, almost exclusively made of quartz. The criteria for indicting a relation to the PETM include their stratigraphic position between upper Thanetian and lower Ilerdian marine carbonates, organic carbon isotope data, and a high percentage of kaolinite in the clay matrix. The axially flowing deep-sea channel existed throughout Paleocene times in the Pyrenean Basin, within which coarse-grained calciclastic and siliciclastic turbidites were accumulated. This Paleocene succession is capped by thickly bedded quartz sandstones and pebbly sandstones, probably deposited by hyperpycnal flows, which are here assigned to the PETM based on their stratigraphic position and organic carbon isotopic data. The large and simultaneous increase in coarse- and fine-grained terrestrial siliciclastics delivered to the Pyrenean Gulf during the PETM is attributed to an increased intra-annual humidity gradient. During the PETM a longer and drier summer season facilitated the erosion of landscapes, whereas a dramatic enhancement of precipitation extremes during the wet season led to intensified flood events, with rivers carrying greater volumes of both bed and suspended loads. This scenario argues against the possibility that PETM kaolinites indicate a coeval warm and humid climate in northern Spain. Instead, the kaolinite reflects the erosion of thick Cretaceous lateritic profiles developed on the Hercynian basement.

Hydrocarbon migration system of Fulaerji oil field, Songliao Basin, NE China

Abstract Through sedimentologic and petroleum geologic analysis of over 300 exploration wells in the north part of the west slope in the Songliao Basin, the conduit role the incised valley played in the oil and gas accumulation of the Fulaerji oil field is investigated. The major pays in the Fulaerji oil and gas field are the second and third members of the Cretaceous Yaojia Formation, where the hydrocarbon comes primarily from the Qijia-Gulong sag about 80 km east. The pathway of long distance hydrocarbon migration is the overlapping sandstone complex which fills the incised valley. The incised valley deposits overlapped on the delta deposits toward basin, and connected with shore-shallow lake sandy beach-bar formed during the highest lake flooding period toward the basin margin, constituting a sand rich sedimentary complex of delta - incised valley filling - shore sandy beach bar. Towards the basin, delta sand branches and pinches out in mudstone (mature source rock), transporting the hydrocarbon generated in the effective hydrocarbon kitchen to the incised valley filling body, the incised valley filling deposits transported hydrocarbon for a long distance to the updip pinching sandy beach bar lithologic trap, giving birth to the Fulaerji oil and gas reservoir. In the sand-rich sedimentary system related to the incised valley filling, the effective hydrocarbon transporting system is limited along Well Du 412 - Du 71 - Du 65 - Jiang 39 - Jiang 41 and Fulaerji oil and gas field. The pure hydrocarbon transporting layer is a water production layer with a high degree of hydrocarbon impregnation and low residual oil saturation. Baiyinnuole, Erzhan, Alaxin, Jiangqiao oil and gas fields distributed like a string of beads in NW are closely related to this effective hydrocarbon transporting system.

Facies architecture and depositional model of a macrotidal incised-valley succession (Qiantang River estuary, eastern China), and differences from other macrotidal systems

Filling of the modern coastal-zone portion of the Qiantang River (QR) in eastern China was initiated by marine inundation during the transgression after the Last Glacial Maximum and has continued during the Holocene sea-level stillstand. The early part of the fill is transgressive in character, while the younger part is regressive. This paper deals with the sedimentary facies, surfaces, architecture, and depositional model of the QR incised-valley fill based on the detailed analysis of the newly drilled core SE2 and its correlation with more than 800 boreholes. The incised-valley deposits are grouped into five stacked facies: amalgamated channel, floodplain and channel, paleo-estuary, offshore shallow marine, and present-day estuary. A paleo-estuary facies had never been identified before, making these observations novel. This facies is characterized by a sedimentary succession and sediment distribution that are distinct from those of the present-day estuary because of a change in sediment supply from the sea floor beyond the estuary mouth. It also contains large numbers of tidal-channel sand bodies that are significant reservoirs for shallow biogenic gas. Since the last glaciation, there have been three stages in the development of the QR incised-valley fill sequence: (1) a formation stage associated with sea-level fall, (2) a filling stage during the early transgression with rapid sea-level rise, and (3) a burial stage corresponding to the slowing of sea-level rise and the onset of progradation. This fill consists of four longitudinal segments, each of which is distinguished by a distinct stratigraphic succession and different degrees of marine and fluvial influence. The basal erosion surface and sidewalls of the incised valley, the top of stiff clay on the interfluves, and the top of falling-stage fluvial-terrace deposits compose the sequence boundary. The initial flooding surface and maximum flooding surface are located within the amalgamated channel and offshore shallow-marine sediments, respectively. These observations indicate that: (1) relative sea-level changes determine the stratal stacking patterns, but local environmental factors, such as physical processes, accommodation, sediment supply, and coastal configuration control the nature of the facies, surfaces, and sediment-distribution patterns; and (2) the tripartite facies organization and wave ravinement surfaces typifying wave-dominated or mixed-energy (wave and tide) incised-valley fills are absent in the macrotidal QR succession, and the tidal ravinement and erosion surfaces of the macrotidal QR incised valley are more extensive and numerous than those of the wave-dominated or mixed-energy succession.

Sequence stratigraphic controls on reservoir characterization and architecture: case study of the Messinian Abu Madi incised-valley fill, Egypt

Abstract Understanding sequence stratigraphy architecture in the incised-valley is a crucial step to understanding the effect of relative sea level changes on reservoir characterization and architecture. This paper presents a sequence stratigraphic framework of the incised-valley strata within the late Messinian Abu Madi Formation based on seismic and borehole data. Analysis of sand-body distribution reveals that fluvial channel sandstones in the Abu Madi Formation in the Baltim Fields, offshore Nile Delta, Egypt, are not randomly distributed but are predictable in their spatial and stratigraphic position. Elucidation of the distribution of sandstones in the Abu Madi incised-valley fill within a sequence stratigraphic framework allows a better understanding of their characterization and architecture during burial. Strata of the Abu Madi Formation are interpreted to comprise two sequences, which are the most complex stratigraphically; their deposits comprise a complex incised valley fill. The lower sequence (SQ1) consists of a thick incised valley-fill of a Lowstand Systems Tract (LST1)) overlain by a Transgressive Systems Tract (TST1) and Highstand Systems Tract (HST1). The upper sequence (SQ2) contains channel-fill and is interpreted as a LST2 which has a thin sandstone channel deposits. Above this, channel-fill sandstone and related strata with tidal influence delineates the base of TST2, which is overlain by a HST2. Gas reservoirs of the Abu Madi Formation (present-day depth ∼3552 m), the Baltim Fields, Egypt, consist of fluvial lowstand systems tract (LST) sandstones deposited in an incised valley. LST sandstones have a wide range of porosity (15 to 28%) and permeability (1 to 5080mD), which reflect both depositional facies and diagenetic controls. This work demonstrates the value of constraining and evaluating the impact of sequence stratigraphic distribution on reservoir characterization and architecture in incised-valley deposits, and thus has an important impact on reservoir quality evolution in hydrocarbon exploration in such settings.

Sequence Stratigraphy of Miocene Tide-Influenced Sandstones In the Minas Field, Sumatra, Indonesia

Abstract The Miocene Sihapas Group in the Minas field, Indonesia, is composed of a succession of tens-of-meters-thick, erosionally based, tide-influenced sandstone intervals interbedded with marine shale-dominated intervals. Despite more than 1600 wells drilled into this 12-km-wide by 30-km-long mature super-giant field, complex internal tidal facies architecture within these sandstone intervals makes predicting local connectivity within the reservoirs challenging. Past interpretations of Minas field suggested that reservoir sandstone intervals are incised-valley deposits, based on assumptions that abrupt coarsening at the base of reservoirs reflects river incision into the shelf and tidal reservoir facies formed in coastal embayments during transgression. In this paper a new interpretation is proposed that suggests that shorelines remained strongly tide-influenced throughout regressive–transgressive depositional episodes. Pronounced discontinuity surfaces in these deposits are interpreted to have formed by a variety of mechanisms: (1) tidal erosion of the sea floor in front of regressing shorelines; (2) lowstand fluvial valley incision; and (3) the reworking of deposits during transgression of the shelf. Consideration of a wider variety of regressive and transgressive tidal depositional settings and erosional processes allowed revision of Minas field stratigraphy to better explain the observed complex facies variations that control reservoir production behavior. Minas field is divided into five major reservoir intervals that overlie low-grade metamorphic basement. The older deposits, comprising the S and D reservoir intervals, contain multiple upward-coarsening tide-influenced regressive shoreline successions, locally cut out by tide-influenced channel sandstone bodies, and capped by thin beds of marine shelf transgressive sandstone and overlying marine shale. Seismic images indicate that these deposits filled erosional basement topography during a period of long-term relative sea level rise. The upper three reservoir intervals (B, A2, and A1) are each interpreted to be fourth-order regressive shoreline deposits that were variably reworked by tidal currents during transgression. Successive intervals record progressively lower average rate of accommodation development, more rapid falling-stage regression, deeper lowstand incision, and greater reworking during intervening transgressions. Together, these three intervals comprise a third-order, flooding-surface-defined, forward-stepping sequence set deposited on a low-gradient continental shelf. Paradoxically, because intervals deposited during more rapid progradation tend to be thinner, in most wells more regressive sandstone intervals contain proportionally more open marine shelf deposits. This reflects that valley incision is localized, and regressive deposits that formed in lower long-term accommodation growth rate settings tend to be more completely reworked by tidal currents during subsequent transgression.

Fluvial response to tectonic deformation: the Ambra Plio-Pleistocene incised-valley deposits (Tuscany, Italy)

Fluvial response to tectonic deformation: the Ambra Plio-Pleistocene incised-valley deposits (Tuscany, Italy)

Seismic stratigraphy of the Deglacial deposits of the Rhône prodelta and of the adjacent shelf

In order to achieve a synthesis of the stratigraphic organization of the Deglacial deposits of the inner/middle shelf in front of the Rhône delta plain, we merged high resolution (Mini-sparker and Sparker), and very high resolution (chirp and mud-penetrator) seismic data into a single seismic database. Thus, the merged seismic database improves the lateral correlation between eastern and western parts, separated by the Rhône Incised Valley deposits. As a result the interpretation of seismic units in relation to local and global environmental changes was refined. The Deglacial deposits rest on a basal erosional discontinuity capping a complex of Pleistocene prograding wedges. The identified units make up Transgressive and Highstand Systems Tracts, and are bounded by flooding surfaces. The main flooding surfaces are (1) the transgressive surface (D200) which forms the lower boundary of the Deglacial deposits and (2) the maximum flooding surface which forms the boundary between the Transgressive and Highstand Systems Tracts. Regarding the data set the study area is divided into three parts depending on the stacking pattern and main control factors. In the western area the units present an aggradational stacking pattern and the rate of sediment supply and dynamic conditions seems coupled with glacio-eustacy role on sedimentary units building and evolution. In the central area the units present an overall backstepping pattern mainly controlled by glacio-eustacy. In the eastern area the units present an aggradational stacking pattern and both the glacio-eustacy and rate of sediment supply have an important role. The lower parasequence of the Transgressive Systems Tract (U200) is due to the reworking of Würmian terraces. Above, the parasequence U300 is a transgressive body formed during a rapid sea-level rise. The two upper parasequences are interpreted as ancestral coastal systems with a backstepping pattern. The first coastal system (U400) is due to a decrease in the rate of sea-level rise that occurred during the Younger Dryas. The second coastal system (U500) may be due to a similar event or to an increased rate of sediment supply during a constant sea-level rise. Finally the Highstand Systems Tract is composed of three units: (i) unit U601, located to the West and interpreted as a subaqueous delta, (ii, iii) units U600 and U610, located to the East and interpreted as prograding deltaic lobes.

Diagenesis and Reservoir-Quality Evolution of Incised-Valley Sandstones: Evidence from the Abu Madi Gas Reservoirs (Upper Miocene), the Nile Delta Basin, Egypt

Abstract Elucidation of the distribution of diagenetic alterations in incised-valley sandstones within a sequence stratigraphic framework allows a better understanding of their reservoir-quality evolution during burial. Gas reservoirs of the upper Miocene Abu Madi Formation (present-day depth 3350 m, temperature 116°), the Nile Delta Basin, Egypt, consist of lowstand systems tract (LST) fluvial and transgressive systems tract (TST) estuarine sandstones deposited in an incised valley. These sandstones have a wide range of porosity (2 to 29%) and permeability (0.01 to 6071 mD), which reflect both depositional facies and diagenetic controls. Diagenetic events that influenced the reservoir-quality evolution include mainly the formation of extensive grain-rimming Fe-Mg chlorite, mechanical compaction of ductile grains, pressure dissolution of quartz, and quartz cementation. The chlorite rims, distributions of which are not clearly related to sequence stratigraphy, have preserved reservoir quality by inhibiting quartz cementation but have increased microporosity values in the reservoirs. Calcite cementation (18O = −13.5‰ to −6.0‰, δ13C = −14.3‰ to −1.0‰, and 87Sr/86Sr = 0.707124 to 0.708181), has not influenced the overall reservoir-quality evolution of the sandstones because of its localized occurrence. Other diagenetic events that have little influence on reservoir quality include the more frequent formation of kaolin in the LST fluvial sandstones and formation of pyrite in the TST estuarine sandstones. This study demonstrates the possibility of constraining and evaluating the impact of diagenetic alterations on reservoir-quality evolution in incised-valley deposits and their sequence stratigraphic distribution, and thus has an important impact on hydrocarbon exploration in such settings.

Controls on the Geometry of Incised Valleys in the Basal Quartz Unit (Lower Cretaceous), Western Canada Sedimentary Basin

In southern Alberta, multiple episodes of fluvial erosion occurred during Lower Mannville, Basal Quartz time (Early Cretaceous), creating a series of incised-valley systems. This study examines two of these valley networks, in order to illustrate the geomorphic and genetic complexity of incised-valley systems. Both valley networks are filled by a transgressive succession of fluvial to estuarine deposits. Isopach mapping of these deposits over an area of approximately 10,000 km 2 in southern Alberta shows that both systems are dendritic to rectilinear in character, with the trunk valley flowing from south to north along the length of the Western Canada Sedimentary Basin. These systems were fed from the west, south, and east by a network of tributaries. Eustatic sea-level changes may have played a role in initiating river incision, but regional tectonic movements had an important influence on the history of accommodation change. The periods of erosion appear to have been relatively brief because the valleys are narrow, although confinement within cohesive, mudstone-rich paleosols of the underlying incised-valley deposits may have slowed valley widening. A relatively wet paleoclimate may also have contributed to the formation of narrow valleys by favoring the existence of single-channel rivers. The general morphology of the Alberta foreland basin, together with the distribution of unfilled relief associated with a preceding valley system and/or of less resistant, slightly older deposits, determined the location of the trunk valleys. Uplift of the Bow Island Arch (Sweetgrass Arch) to the east may have been responsible for a shift in the valley headwaters between the formation of the two valley systems. Synerosional block faulting in response to flexural loading had a significant influence on the geometry of the valley network through its control on the location and geometry of major tributary valleys. The trunk valley is also significantly narrower and more deeply incised where it crosses the most pronounced uplifted block. Enhanced erosion at tributary junctions and valley bends produced localized areas (ca. 2-3 km in diameter) where the valley fill is up to five times thicker and considerably coarser grained than the adjacent valley deposits. Such localized scour fills represent prime petroleum-exploration targets.

ABSTRACT: Distal Incised Valley Deposits- Starfish Field, Offshore East Coast Trinidad

ABSTRACT: Distal Incised Valley Deposits- Starfish Field, Offshore East Coast Trinidad

Depositional sequences controlled by high rates of sediment supply, sea-level variations, and growth faulting: the Quaternary Baram Delta of northwestern Borneo

Abstract The shelf off the Baram Delta is 50–70 km wide, and is underlain by 8–9 km of post-Eocene upper slope, prodelta, delta-front, fluvial, shelf–shoreface, and estuarine (incised-valley and tidal-embayment) deposits. The shelf break is defined by a prominent fault scarp ∼130 m below sea level. Beyond, the seabed descends at an average of 2–3° to the Borneo Trough at >2750 m depth. The outer-shelf Quaternary succession is locally >1 km thick. Uppermost Quaternary units then thicken by a factor of 2–5 across an en echelon set of shelf-edge growth faults. Five widespread ‘key’ reflectors, traced with the aid of 1500 line-km of high-resolution seismic profiles, are downlap surfaces beneath clinoforms on the shelf. Two of these reflectors directly overlie fluvial channels. These are interpreted as erosional ravinement surfaces that formed during shelf-crossing post-glacial transgressions, and correlate with surfaces of maximum regression in the slope succession. Acoustic facies on the shelf, calibrated by 15 60–100 m-long geotechnical borings, are interpreted as muddy prodelta, transgressive, and incised-valley deposits; forced-regressive and lowstand fluvial channel sands; shelf-edge sandy deltas; and reworked sands along ravinement surfaces. Muddy prodelta deposits drape the slope. Isopach maps of strata between ‘key’ reflectors reveal shingled highstand to lowstand delta lobes. A widespread 4th-order Lowstand–Bypass Sequence developed during the 120–10 ka sea-level cycle. It reaches 400 m thickness and resembles 3rd-order sequences of Vail and coworkers, but with a thinner Transgressive Systems Tract and a thick Forced Regressive Systems Tract (FRST). The FRST includes sandy shelf-edge deltas deposited by small rivers after the Baram system began to bypass the shelf through an incised valley. The incised valley belongs to a contemporaneous Valley–Canyon Sequence and is filled with lower FRST to Lowstand Systems Tract fluvial deposits, and upper backstepping deltaic deposits of the retreating Baram Delta.

Anatomy of Extremely Thin Marine Sequences Landward of a Passive-Margin Hinge Zone: Neogene Calvert Cliffs Succession, Maryland, U.S.A.

ABSTRACT Detailed examination of Neogene strata in cliffs 25-35 m high along the western shore of Chesapeake Bay, Maryland, reveals the complexity of the surviving record of siliciclastic sequences 150 km inland of the structural hinge zone of the Atlantic passive margin. Previous study of the lower to middle Miocene Calvert (Plum Point Member) and Choptank Formations documented a series of third-order sequences 7-10 m thick in which lowstand deposits are entirely lacking, transgressive tracts comprise a mosaic of condensed bioclastic facies, and regressive (highstand) tracts are present but partially truncated by the next sequence boundary; smaller-scale (fourth-order) cyclic units could not be resolved. Together, these sequences constitute t e transgressive and early highstand tracts of a larger (second-order Miocene) composite sequence. The present paper documents stratigraphic relations higher in the Calvert Cliffs succession, including the upper Miocene St. Marys Formation, which represents late highstand marine deposits of the Miocene second-order sequence, and younger Neogene fluvial and tidal-inlet deposits representing incised-valley deposits of the succeeding second-order cycle. The St. Marys Formation consists of a series of tabular units 2-5 m thick, each with an exclusively transgressive array of facies and bounded by stranding surfaces of abrupt shallowing. These units, which are opposite to the flooding-surface-bounded regressive facies arrays of model parasequences, are best characterized as shaved sequences in which only the transgressive tract survives, and are stacked into larger transgressive, highstand, and forced-regression sets. Biostratigraphic analyses by others indicate that this onshore record contains the same number of third-order ( 1 my duration) units as present offshore, and so thinning landward of the hinge zone was accomplished not by omission or erosion of entire cycles of deposition, but instead by omission of some subsidiary elements (e.g., lowstand tracts), by erosional shaving of sequence tops (removing the entire regressive tract in some sequences), by a reduced number of component high-order cycles surviving per larger set, and by qualitative changes in the anatomy or composition of elements (e.g., condensed transgressive tracts; shaved sequences rather than parasequences). All of these differences can be attributed to limited accommodation, ut preservation of an onshore record of each baselevel cycle was probably also favored by the large amplitude and rapidity of eustatic fluctuations during the Miocene.

Influence of relative sea-level on facies and reservoir geometry of the Middle Jurassic lower Brent Group, UK North Viking Graben

Abstract Detailed sequence stratigraphy of several neighbouring fields in the East Shetland basin indicates that a higher order of cyclicity is superimposed onto the lower Brent Group. Both relative falls and rises in sea-level affected the strongly progradational nearshore/barrier-island system and induced important variations in the geometry, distribution and stacking patterns of the reservoir facies. The majority of the shallow marine strata belong to the transgressive and highstand systems tract and consist of the lower-shoreface Rannoch, the upper shoreface and strand-plain Etive and the back-barrier and lagoonal Lower Ness. High frequency increments of shoreline progradation are preserved in the Rannoch as gently seaward-dipping bedsets. The small-scale units mark asymmetric cycles in grain size and mica content and together, with stratiform cement, impart a strong permeability anisotropy on the Rannoch. The nearshore Etive Formation represents a sediment-laden, barred nearshore complex and is subdivided into (1) a uniformly thick upper shoreface and (2) an overlying emergent strand plain. The two subfacies are commonly separated by a thin interval of foreshore which is locally enriched in heavy minerals. The shoreface deposits show systematic variations in stacking geometries ranging from strongly progradational to aggradational. Two relative falls in sea-level led to the subaerial erosion of nearshore Etive deposits and the juxtaposition of coastal-plain mudstones, coals and fluvial deposits with shale-clast lags on lower shoreface Rannoch strata. Although lithostratigraphically part of the Etive, these deposits bear no resemblance to the well ordered nearshore-Etive deposits. The channel complexes are vertically and laterally heterogeneous and are confined to incised valleys separated by non-depositional interfluves. In Cormorant and Tern fields, well-performance anomalies such as early water breakthrough and unpredictable water injection and oil production rates are associated with these lowstand incised valley deposits. An enigmatic coarse-grained unit occurs in the lower Rannoch in Eider field and is provisionally interpreted as the distal expression of the lowstand shoreface deposits associated with the older incised valley. Following the lowstand deposition nearshore-marine sedimentation reestablished during the transgressive/early highstand systems tract. Under the influence of rising relative sea-level the barrier complex aggraded in place with minimal landward translation of the shoreline position. The volume of sediment supplied to the Etive shoreface was sufficiently high to keep pace with phases of increased accommodation. The shoreline system subsequently stacked with a strongly progradational geometry as the high sediment supply filled the available accommodation space during the latter part of the highstand systems tract.

Reservoir Performance of Late Eocene Incised Valley Fills, Cusiana Field, Llanos Foothills, Eastern Colombia.: ABSTRACT

The Cusiana Field is located in the Llanos Foothills of Eastern Colombia. The principal reservoir is the late Eocene Mirador Formation which comprises >50% of reserves. Currently the Mirador reservoir is providing nearly all of the 150,00bopd of production from the Cusiana Field. The Mirador reservoir comprises a stack of incised valley deposits. The fills of the valleys are dominated by quartz arenite sandstones. The average porosity of the valley sandstones is 8% which reflects abundant quartz cement ({approximately}14%) and significant compaction during deep burial ({approximately}20,000feet). Single valleys are up to 70 feet thick and exhibit a distinctive bipartite fill that reflects changing energy conditions during filling. Bases of valleys have the coarsest grain size and have sedimentological and trace fossil evidence for deposition in highly stressed, brackish water environments. The upper parts of the valleys are typically finer grained and were deposited in more saline settings. Despite the low porosity of the Mirador valleys, drill stem tests and production log data show that they have phenomenal performance characteristics. Rates of {ge}10,000bopd are achieved from single valleys. Bases of the valley fills are the key contributors to flow. Integration of detailed core and pore system analysis with the reservoir performancemore » data shows that the permeability fabric of the Mirador can be explained by original depositional architecture and simple loss of primary porosity. Comparison of Cusiana with other quartz-rich sandstones from around the world suggests that it`s low porosity/high performance is predictable.« less

The Lower Cretaceous Chinkeh Formation: A Frontier-Type Play in the Liard Basin of Western Canada (1)

The Lower Cretaceous Liard basin in western Canada covers an area of 9500 sq km (3668 sq mi) but is relatively unexplored despite its size. The present-day expression of the basin, which formed during the latest Cretaceous to early Tertiary, trends north-south and is delineated by the outcrop of the coarse-clastic Upper Cretaceous Dunvegan Formation. The lowermost Cretaceous unit, herein named the Chinkeh Formation, is up to 32 m (105 ft) thick and unconformably overlies older Paleozoic strata. The Chinkeh Formation contains four major lithotypes: (1) conglomeratic breccia interpreted as debris-flow or talus deposits, (2) interbedded coal, carbonaceous shale, rooted sandstone, and conglomerate interpreted as nonmarine valley fill or channel deposits, (3) conglomeratic lag related to marine transgression, and (4) upward-coarsening sandstone interpreted as abandoned shoreline deposits. The lower part of the Chinkeh Formation consists locally of an angular-chert breccia overlain by medium- to coarse-grained sandstone and conglomerate with coal and rooted beds. The uppermost Chinkeh Formation consists of a basal conglomeratic layer overlain by an upward-coarsening, fine- to medium-grained, well-sorted marine sandstone, with bioturbated shale. Sediment was recycled repeatedly with the dominant sediment source being from the east with perhaps some western sources. Cretaceous strata in the Liard basin have good petroleum source-rock and reservoir potential, and hydrocarbons may be present in sandstone of the Chinkeh Formation. Potential play types include stratigraphic traps formed by incised-valley deposits and shallow-marine sandstone pinching out laterally into marine shales of the Garbutt Formation. A potential structural play may occur along the Bovie fault zone where reservoirs may abut against a shale seal on the eastern side of the fault. Most of Lower Cretaceous strata falls within the hydrocarbon generation window, with increasing maturity to the southwest. Pyrobitumen is locally abundant and constitutes 30% of the bulk rock volume in one sandstone. Potential source rocks include the lowermost Garbutt Formation and underlying Triassic oad and Grayling formations. The Chinkeh Formation sandstone has porosity values of 8-18%.