Candidate Sequence Boundaries Research Articles

Bio-electrosequence interpretation of late cretaceous sediments of the Southern Bornu Basin, Nigeria

Integrated analysis that involves physical sedimentological, standard palynological and electrofacies analyses on ditch cuttings and suite of wireline logs from Gaibu–1 Well, southern Bornu were examined to identify critical sequence elements and construct a bio-sequence stratigraphical framework. Four (4) palynozones consisting of Triorites africaensis, Cretacaeiporites scabratus - Odontochitina costata, Droseridites senonicus and Syncolporites/Milfordia spp Assemblage Zones construed to be Late Cretaceous – younger successions. Nine (9) depositional sequences each with candidate maximum flooding surfaces (375, 900, 1875, 2250, 2600, 3050, 3400, 3800, 4300 m) marked by marker shales with high abundance and diversity of palynomorphs. Thus, equate with the local lithostratigraphy and global large-scale depositional cycles with candidate sequence boundaries (50, 725, 1625, 2175, 2490, 2850, 3300, 3610, 3960, 4470 m) ranging about 96.28 to 70.07 Ma. The delineated transgressive surfaces along the built sequences mark the subjected onset of marine flooding characterised with interchange of progradational to retrogradational facies. Delineated sequence elements generally show up-hole from progradational to retrogradational and aggradational that represents Lowstand Systems Tracts (LSTs), Transgressive Systems Tracts (TSTs) and Highstand Systems Tracts (HSTs) respectively. The LSTs are seen in form of prograding complex and slope fans, suggestive of good reservoirs. The TSTs consist of channel sand units and shales that depict retrogradational marine units, which could serve as both seals and source rocks for the sand units. The HSTs are made up of interplay of aggradational to progradational sediment packages that could serve as a potential source rock. The palaeoenvironmental indices depict the successions are deposited within continental to open marine settings. 

Sequence Stratigraphic Interpretation of FX-1 and FX-2 wells, Onshore Western Niger Delta, Nigeria

The area of study is a portion of the Greater Ughelli Depobelt in Niger Delta Basin. The main aim of the paper is to interpret the sequence stratigraphy of FX-1 and FX-2 wells by employing data sets from biostratigraphic data and well logs. Standard laboratory techniques were used for data treatment while computer software such as Petrel and StrataBugs were used for data simulation, processing, integration and interpretation. Sedimentology, interpreted gamma ray and resistivity well logs integrated with biostratigraphic data were utilized to define the candidate maximum flooding surfaces and sequence boundaries. The wells have the following distributions of sequences: FX-1 well have five depositional sequences with eight candidate maximum flooding surfaces at depths 10011 ft., 9509 ft., 9437 ft., 6362 ft., 5752 ft., 5507 ft., 5161 ft. and 4816 ft. dated 34.0 Ma, 33.0 Ma, 31.3 Ma, 28.1 Ma, 26.2 Ma, 24.3 Ma, 23.2 Ma and 22.0 Ma and seven candidate sequence boundaries at 9616 ft., 6656 ft., 6116 ft., 5639 ft., 5424 ft., 4859 ft. and 4581 ft. dated 33.3 Ma, 29.3 Ma, 27.3 Ma, 24.9 Ma, 23.7 Ma, 22.2 Ma and 21.8 Ma, respectively. FX-2 well have four depositional sequences, five candidate MFSs were identified at 7764 ft., 7196 ft., 6721 ft., 5862 ft. and 5571 ft. dated 34.0 Ma, 33.0 Ma, 31.3 Ma, 28.1 Ma and 24.3 Ma and five candidate SBs at 6941 ft., 6029 ft., 5688 ft., 5653 ft. and 5542 ft. dated 32.4 Ma, 29.3 Ma, 27.3 Ma, 24.9 Ma and 23.7 Ma respectively. The correlation of the two wells and sequence stratigraphic interpretation is a supplementary understanding of the subsurface geology of the Onshore, western Niger Delta area of Nigeria.
 Keywords: Bio-stratigraphic data, Well logs, Sequence stratigraphy, Well correlation.

An interdisciplinary approach to reservoir characterisation; an example from the early to middle Eocene Kaimiro Formation, Taranaki Basin, New Zealand

The Kaimiro Formation is an early to middle Eocene, NE-SW trending reservoir fairway in Taranaki Basin, and comprises a range of coastal plain through to shallow marine facies. A time of regional transgression is observed across the Paleocene–Eocene transition, which is linked to a general global warming trend and to regional thermal relaxation-related subsidence in New Zealand. The earliest Eocene transgressive deposits pass upwards into a series of cyclically stacked packages, interpreted as 3rd and 4th order sequences. Maximum regression occurred within the early Eocene and was followed by punctuated retrogradational stacking patterns associated with shoreline retreat and subsequent regional transgression in the middle Eocene.The Kaimiro Formation is considered a good reservoir target along most of the reservoir fairway, which can largely be attributed to a consistently quartz-rich, lithic-poor composition and reasonably coarse sand grain size. Correlations demonstrate that within the early Eocene the main reservoir facies are channel-fill sandstones overlying candidate sequence boundaries in paleoenvironmentally landward (proximal) settings, and upper shoreface/shoreline sandstones in relatively basinward (distal) settings. Middle Eocene reservoir facies are not represented in distal wells due to overall transgression at this time, yet they form a significant target in more proximal well locations, particularly on the Taranaki Peninsula.Depositional facies is one of the principal controls on sandstone reservoir quality. However, while reservoir facies have been proven along the length of the reservoir fairway, it is evident that diagenesis has significantly impacted sandstone quality. Relatively poor reservoir properties are predicted for deeply buried parts of the basin (maximum burial >4.5 km) due to severe compaction and relatively abundant authigenic quartz and illite. In contrast, good reservoir properties are locally represented in reservoir facies where present-day burial depths are <4 km due to less severe compaction, cementation and illitisation. Within these beds (<4 km) the presence of locally occurring authigenic grain-coating chlorite (shallow marine facies) and/or well-developed secondary porosity are both favourable to reservoir quality, while pervasive kaolinite and/or carbonate are both detrimental to reservoir quality.These results illustrate how an interdisciplinary approach to regional reservoir characterisation are used to help reduce risk during prospect evaluation. Assessment of both reservoir distribution and quality is necessary and can be undertaken through integrated studies of facies, sequence stratigraphy, burial modelling and petrography.

Early Miocene sequence development across the New Jersey margin

ABSTRACTSequence stratigraphy provides an understanding of the interplay between eustasy, sediment supply and accommodation in the sedimentary construction of passive margins. We used this approach to follow the early to middle Miocene growth of the New Jersey margin and analyse the connection between relative changes of sea level and variable sediment supply. Eleven candidate sequence boundaries were traced in high‐resolution multi‐channel seismic profiles across the inner margin and matched to geophysical log signatures and lithologic changes in ODP Leg 150X onshore coreholes. Chronologies at these drill sites were then used to assign ages to the intervening seismic sequences. We conclude that the regional and global correlation of early Miocene sequences suggests a dominant role of global sea‐level change but margin progradation was controlled by localized sediment contribution and that local conditions played a large role in sequence formation and preservation. Lowstand deposits were regionally restricted and their locations point to both single and multiple sediment sources. The distribution of highstand deposits, by contrast, documents redistribution by along shelf currents. We find no evidence that sea level fell below the elevation of the clinoform rollover, and the existence of extensive lowstand deposits seaward of this inflection point indicates efficient cross‐shelf sediment transport mechanisms despite the apparent lack of well‐developed fluvial drainage.

Biosequence stratigraphical and palaeoenvironmental findings from the Cretaceous through Tertiary succession, Central Indus Basin, Pakistan

Abstract. Integrated analysis of foraminiferal and palynological data from the Duljan-1 well, Central Indus Basin, Pakistan, is used to identify critical surfaces (candidate sequence boundaries (SB) and maximum flooding surfaces (MFS)) and construct a biosequence stratigraphical framework. Within the Barremian through Bartonian–Priabonian? succession 15 depositional sequences have been recognized, each with a candidate MFS. These biosequences are shown to equate with the local lithostratigraphy and tentatively with the ‘global’ large-scale depositional cycles of Haq et al. (1987). Detailed dating has enabled seven candidate MFS to be tentatively equated with MFS identified on the nearby Arabian plate (Sharland et al., 2001). A combination of detailed age dating and palaeobathymetric determinations indicates significant basin uplift and erosion at end Cretaceous and end Eocene times, the latter coinciding with closure of Neo-Tethys. Smaller-scale unconformities are also noted. Multi-disciplinary palaeoenvironmental interpretations enable recognition of detailed changes in water mass conditions. Palynological data suggest these changes result from variations in terrestrial/freshwater input, though evidence of periodically low oxygen bottom water conditions/shallowing of the oxygen minimum zone, possibly ‘Oceanic Anoxic Event-2’ (OAE-2; late Cenomanian–Turonian) is suggested as a further control.

Stratigraphy of the Late Aptian Lower Greensand around Nutfield, Surrey

A detailed study of Lower Greensand deposits exposed in the fuller's earth workings of Nutfield. Surrey, has yielded new information on their lithofacies and stratigraphy. Two informal lithostratigraphic units, representing contrasting depositional regimes, are proposed for the Sandgate Formation of the Nutfield area. These units are of member status but, pending a comprehensive review of Upper Aptian lithostratigraphic nomenclature, they are provisionally assigned the informal terms Nutfield Beds and Redhill Sands. The Nutfield Beds are characterized by muddy, bioturbated, calcareous sandstones, deposited in quiet-water environments; they include economically important beds of fuller's earth, deposited as reworked volcanic ash. The overlying Redhill Sands consist of poorly consolidated, glauconitic sandstones, characterized by large-scale foresets of tidal bar origin. The Nutfield Beds form a geographically restricted lens, up to 15 m thick, lying within a syndepositional basin referred to as the ‘Nutfield Basin’ in this study. The underlying cherty sandstones ( c . 1–2 m), which constitute the top of the Hythe Formation, show a similarly restricted distribution. The Redhill Sands are also thickest ( c . 15 m) in the middle of the basin, but extend beyond the limits of the Nutfield Beds to the west. Relatively rapid subsidence within the Nutfield Basin combined with high sea-level stand during deposition of the Nutfield Beds produced conditions favourable for the accumulation and subsequent preservation of volcanic ash (now fuller's earth) deposits. Five candidate sequence boundaries have been identified in this study. SB1 is placed at a level within the uppermost Hythe. SB la is placed at the base of a thin glauconitic, pebbly sandstone bed that underlies the top-Hythe cherty sandstones. The cherty sandstones and the Nutfield Beds are interpreted as transgressive and highstand deposits within the same sequence. SB2 is drawn at the base of the Redhill Sands, and SB2a at the base of a unit of pebbly, clay-rich sandstone within the upper part of the Redhill Sands. SB3 is taken at the base of the Folkestone Sands, and is marked by an erosional surface with a relief of up to 4.5 m. Biostratigraphic control is not sufficiently precise to allow correlation of the sequences with those identified in the coastal sections of Folkestone.

Extensional Tectonics, Halokinesis, Eustacy in the Norwegian Central Graben, North Sea: a testing ground for sequence and seismic stratigraphic principles: ABSTRACT

The Norwegian Central Graben is a mature hydrocarbon province with proven reserves within the Upper Jurassic succession. Several phases of extensional tectonics ranging from the Permo-Triassic to the Upper Jurassic, and a thick mobile salt section, serve to complicate a clear regional understanding of the area. A full integration of structural interpretation, seismic and sequence stratigraphic principles, biostratigraphy and core studies is required to achieve a realistic interpretation and predict Upper Jurassic facies distributions within this complex area. Utilizing some 30 wells, regional seismic data and biostratigraphy, candidate sequence boundaries and regionally correlatable flooding surfaces (e.g. Eudoxus), have been identified. Horizon flattening on these surfaces has allowed the recognition of thickening-away reflection geometries adjacent to salt features and divergent geometries into graben boundary faults. This facilitates the identification of the dominant local or regional controls on accommodation space creation. Detailed seismic facies analysis was then used to reveal the relative expansion or suppression of depositional systems tracts as a response to either regional or local structural controls. It was subsequently possible to place these systems within a biostratigraphically constrained regional framework. Mapping the base Zechstein, base Jurassic and base Cretaceous horizons has provided a map view of the active faultsmore » and slopes controlling sediment transport at any given time. This provided the third dimension essential in depicting the spatial distribution of depositional systems, and is a crucial component of any sequence stratigraphic interpretation. A regional picture of the progressive evolution of this complex area has been thus been derived, and the effect of both regional and local controls on sequence stratigraphic expressions has been determined.« less

Genetic sequence stratigraphy for the North Sea Late Jurassic and Early Cretaceous: distribution and prediction of Kimmeridgian–Late Ryazanian reservoirs in the North Sea and adjacent areas

Sediments of Kimmeridgian to Late Ryazanian age form a group of key hydrocarbon play fairways in the syn-rift Jurassic of the North Sea. The perceived yet-to-find reserves of these often subtle plays, lying at or below seismic resolution, have attracted considerable industry attention over the past few years. Reserves are currently estimated by BP Exploration at 1 to 5 billion barrels of oil equivalent, reservoired in three play systems: (1) apron fans (e.g. Brae type); (2) basin floor fans (e.g. Miller, Galley, Ettrick and Magnus types); (3) shallow marine shelf (e.g. Ula, Gyda, Fulmar, Piper, Clyde types). In order to assess the future exploration potential of this play fairway, a high resolution, predictive, sequence stratigraphy was erected for the North Sea Late Jurassic. The stratigraphic framework combines data from over 500 exploration wells with seismic and field data (Magnus, Brae, Miller, Ula, Gyda and Clyde). In the Late Oxfordian to Late Ryazanian, a total of 11 genetic stratigraphic sequences have been defined. They are bounded by maximum flooding surfaces which, within the limits of the biostratigraphy, represent basin-wide isochronous events across NW Europe and can be recognized in exploration wells and at outcrop from Greenland to the Wessex Basin. The maximum flooding surfaces have been biostratigraphically calibrated to provide a consistent and easily identifiable stratigraphic framework. Candidate sequence boundaries have been interpreted within this stratigraphic framework, from basin-ward shifts of facies belts, using sedimentological and wireline log data. The combination of these stratigraphic methods has produced a very powerful tool to predict the presence and distribution of potential reservoirs and play types across the entire North Sea Basin from outcrop in East Greenland to the offshore Netherlands. The model suggests that three major cycles of sand input into the basin can be recognized with an overall marked decrease in net sand content with time. Each cycle is bounded by tectonically enhanced maximum flooding surfaces representing major periods of basin floor reorganization. The intervening maximum flooding surfaces temporarily switch off sediment supply to the basin but do not offset depocentres. These events can form important, field-wide permeability barriers. It is proposed that the tectonically enhanced maximum flooding surfaces are a response to tectonic subsidence during maximum relative sea-level rise, whereas maximum clastic progradation occurs from basin margin uplift during relative sea-level fall. The model is considered to have application at regional and field-specific scales; for example, prediction of both basin floor fan distribution and potential intra-reservoir permeability barriers.

Major erosion surfaces in the basal Wealden Beds, Lower Cretaceous, south Dorset

The transition from evaporitic and lagoonal Purbeck Beds into the fluviatile basal Wealden Beds (Ryazanian-Valanginian) has in the past been interpreted as a continuous regressive succession. Mupe Bay and Bacon Hole, in south Dorset, are the only localities in the Wessex Basin of southern England at which the transition strata are well exposed and not extensively faulted. On the basis of facies analysis the lowermost Wealden Beds are interpreted as a regressive-transgressive-regressive package. Environments fluctuated between lagoon, inner-lagoonal shoreline and fluvial flood plain. The overlying succession is remarkable for the presence of two major erosion surfaces which, based on field geometries and facies relationships, are interpreted as unconformities. The lower unconfor­mity is overlain at Mupe Bay by sediments of lagoonal or lacustrine origin. The upper unconformity is expressed as the celebrated Mupe Bay palaeo-oilseep, an oil-cemented conglomeratic sand, overlain by a thick succession of fluvial sediments. A level near the basal sand and the two erosion surfaces are treated as candidate sequence boundaries (WBl-WB3), with the thin intervening ?lagoonal deposits representing peaks of transgression. The pattern of one minor (conformable) sequence boundary followed by two major (unconformable) sequence boundaries bears a strong resemblance to the Early Cretaceous sequence stratigraphy suggested in recent ‘global cycle charts’. However, biostratigraphical calibration of the sections is currently inadequate to allow close correlation with candidate sequence boundaries in other areas.

Recognition and documentation of depositional sequences from outcrop: an example from the Aptian and Albian on the eastern margin of the Wessex Basin

Stratigraphic data relating to the Aptian and Albian sediments around Folkestone in southeast England, on the eastern margin of the Wessex Basin, are plotted graphically to illustrate biostratigraphic gaps and condensed intervals. These are then used to suggest candidate sequence boundaries, notated LG1-4 and Gl-3, which are named after the Lower Greensand and Gault. The biostratigraphic subzones apparently unrepresented in the area are: LG1 obsoletus and bodei; LG2 gracite and debile; LG3 schrammeni; Gl bulliensis. Candidate sequence boundaries G2 (cristatum Subzone) and G3 (auritus Subzone) are erosion surfaces correlating with probable biostratigraphic gaps elsewhere in the basin. The age of LG4 is uncertain but must lie within the mammillatum Superzone. All candidate sequence boundaries correspond closely to those predicted by recent global sea-level cycle charts, and additionally there is a close match between predicted maximum flooding surfaces and the occurrence of condensed intervals. Most of the candidate sequence boundaries could have been formed as a result of rapid, and at least regionally extensive, relative sea-level falls, but the sedimentological contexts of LG3 and Gl suggest that these may have been generated by transgression-related sediment condensation. The minor sequence boundary in the loricatus Zone (intermedius Subzone) illustrated in the cycle charts cannot be unambiguously recognized at outcrop in the Folkestone area. Additional sequence boundaries not shown in the cycle charts may also be present in the Wessex Basin.