Sequence Stratigraphic Interpretation Research Articles

Paleocene–Eocene boundary in the Gabal El-Gir, east Qena, Egypt: Paleoenvironment and sequence stratigraphy interpretations

Comprehensive field, litho-, biostratigraphy, and paleoenvironmental studies of the upper Paleocene–lower Eocene (P–E) boundary interval in the Gabal El-Gir area were conducted. Two selected sections El-Gir-1 and El-Gir-2 were investigated. The prevailed paleoenvironmental conditions were discussed in the frame of the nannofossils, planktonic, and benthic foraminiferal assemblages. Benthic foraminifera assemblages indicate a remarkable turnover across the PETM. The extinction of Angulogavelinella avnimelechi at the base of the PETM represents a significant marker for the base of the benthic foraminifera extinction event at this site and other neritic sites in the Tethyan region. The dominance of calcareous planktonic excursion taxa at the base of the PETM indicates adaptation to extremely warm sea surface temperatures and low nutrient levels. Also, our data suggests that the biotic responses to environmental change in the early Eocene resembled those observed in the early Eocene hyperthermals at the nearbay Dababiya and El-Ballas sections. This could indicate that similar processes characterized the Egyptian sedimentary basin during the early Eocene. Stratigraphically, despite the limited distance between the studied sections, the Dababiya Quarry Member (DQM) in El-Gir-1 resemble those of the Global Stratotype Section and Point (GSSP) but reduced in thickness. While in El-Gir-2, the lower part of the DQM was missing. These findings could indicate that the DQM was deposited in a submarine channel. Three third-order depositional sequences were recognized covering the P–E interval. The reconstructed sea level curve shows the presence of eustatic similarities with the global sea level.

Chapter 12. Well sequence stratigraphy interpretation methodology

Abstract This chapter describes the methodology employed to recognize depositional sequences in well data, involving the integration of wireline logs, biostratigraphy, lithostratigraphy (and lithological interpretations), seismic and sedimentology (particularly facies analysis), with an emphasis on a pragmatic approach to sequence recognition in North Sea Jurassic–lowermost Cretaceous well successions. Wireline log profiles characterizing individual depositional sequences or parts of sequences are illustrated by reference to key North Sea well sections.

A facies- and sequence stratigraphy-based reservoir model for a carbonate reef complex: South Swan Hills oil pool, Alberta

We present a reservoir model for the South Swan Hills oilfield, Alberta, Canada, developed in a Devonian atoll reef, based on facies analysis and sequence stratigraphy. The analysis illustrates the complex reservoir geometries in such a reef, caused by facies variation from reef margin to reef interior and fluctuations in relative sea level. The latter resulted in cyclic deposition of facies and formation of field-scale sequence boundaries. The model is built on lithofacies analysis from high-resolution core descriptions that recorded lithologies, bioclast type and characteristics, and sedimentary structures. Core descriptions were correlated to well logs, enabling lithofacies to be mapped across the field. Lithofacies distributions permit definition of three distinct reef environments: a reef interior, a reef margin and a terminal shoal. Vertical variability is characterized using sequence stratigraphic interpretations. Two 3rd order and four 4th order sequence boundaries were identified and correlated across the reef. The analysis illustrates the complex reef geometries linked to facies variation from reef margin to reef interior and fluctuations in relative sea level. The latter resulted in cyclic deposition of facies and formation of field-scale unconformities. These and the intervening deposits represent the response to fluctuating sea level.The geological analysis was transformed into a 3-D geocellular model using a sequential indicator simulator. The simulator models lithofacies and, in turn, porosity and permeability distributions based on strong relationships between lithofacies and petrophysical properties for the three representative regions; the interior, margin and terminal shoal. The interior deposits formed forms the bulk of non-reservoir material in the centre of the reef build-up. The margin surrounded surrounds the interior and formed forms a continuous 1–2 km band of reservoir around it. Reservoir thickness is 20–30 m and primarily composed of reef flat deposits. The terminal shoal forms a reservoir body that caps the reef complex, but is disconnected from the bulk of the underlying reef by a thick layer of impermeable clay associated with a sequence boundary. The high-resolution realization displays the geometry and continuity of the reservoir and exposes demonstrates the compartmentalized and variable producibility of the atoll.

Integration of Sequence Stratigraphic Analysis and 3D Geostatistical Modeling of Pliocene–Pleistocene Delta, F3 Block, Netherlands

AbstractThis research is focused on the analysis of the sequence stratigraphic units of F3 Block, within a wave‐dominated delta of Plio–Pleistocene age. Three wells of F3 block and a 3D seismic data, are utilized in this research. The conventional techniques of 3D seismic interpretation were utilized to mark the 11 surfaces on the seismic section. Integration of seismic sequence stratigraphic interpretation, using well logs, and subsequent 3D geostatistical modeling, using seismic data, aided to evaluate the shallow hydrocarbon traps. The resulting models were obtained using System Tract and Facies models, which were generated by using sequential stimulation method and their variograms made by spherical method, moreover, these models are validated via histograms. The CDF curve generated from upscaling of well logs using geometric method, shows a good relation with less percentage of errors (1 to 2 for Facies and 3 to 4 for System Tract models) between upscaled and raw data that complements the resulted models. These approaches help us to delineate the best possible reservoir, lateral extent of system tracts (LST and/or HST) in the respective surface, and distribution of sand and shale in the delta. The clinoform break points alteration observed on seismic sections, also validates the sequence stratigraphic interpretation. The GR log‐based Facies model and sequence stratigraphy‐based System Tract model of SU‐04‐2 showed the reservoir characteristics, presence of sand bodies and majorly LST, respectively, mainly adjacent to the main fault of the studied area. Moreover, on the seismic section, SU‐04‐2 exhibits the presence of gas pockets at the same location that also complements the generated Facies and System Tract models. The generated models can be utilized for any similar kind of study and for the further research in the F3 block reservoir characterization.

Sedimentary facies and stratigraphy of the Campanian-Maastrichtian Taloka Formation, southeastern Iullemmeden Basin, Nigeria

The sedimentology of the Campanian – Maastrichtian Taloka Formation of the southeastern Iullemmeden Basin in Northwest Nigeria is poorly understood. There are still major uncertainties regarding its depositional setting. This study presents a detailed facies analysis and sequence stratigraphic interpretation for the Taloka Formation of Rima Group of the southeastern Iullemmeden Basin in Nigeria based on outcrop observations, sedimentary logging and ichnological analyses. Thin sections, representative of delineated facies associations, are included in the data. Twelve lithofacies and four facies associations were delineated. The facies associations are interpreted to represent intertidal flat, lower shoreface, delta-front and tide-influenced channel environments. The facies associations are arranged vertically into a wave- and tide-influenced deltaic facies succession. Paleosols, low-high bioturbation intensity and low ichnodiversity, (e.g., Thalassinoides and Skolithos ichnofossils) were found in the deposits. Furthermore, the stacking patterns of the Taloka Formation (juxtaposed wave- and tide-influenced facies associations overlain by nearshore and restricted marine deposits) reflect an aggradational set of shallowing upward, progradational parasequences, interpreted as lowstand systems tract: deposits formed when supply balanced accommodation, under low relative sea-level rise condition. A mixed-energy delta model is proposed for the Taloka Formation. The Taloka Formation represents an example of a slowly subsiding, highly-moderately embayed coast.

Stratigraphy and Paleoenvironment(s) of the Late Cretaceous deposits in Ohafia area, Afikpo Basin, Southeastern Nigeria

Lithofacies and biofacies data extracted from three outcropping sections of Nkporo and Mamu Formations’ sediments in Ohafia area were used for paleoenvironmental and sequence stratigraphic interpretations. The five (5) lithofacies distinguished from the sections were Dark-Grey to Black Shale Facies (A), Oolitic Limestone Facies (B), Calcareous Siltstone Facies (C), Sandstone Facies (D) and Heterolithic Facies (E). The attributes of these lithofacies, including lithotype, rootlets, bioturbation, reducing conditions and microfossil contents, were used to discriminate the units into various environments of deposition and paleodepths. The oolithic limestone facies for instance was interpreted as deposits in shallow coastal areas with high turbulence, especially, in warm shallow marine environments. Paleobathymetry of the lithofacies ranged from non-marine through coastal deltaic, shallow inner neritic, inner neritic to middle neritic paleodepths. Micropalaeontological analysis of the sediments revealed a complete absence of planktonic foraminifera while agglutinated benthic assemblages (dominated by Haplophragmoides spp., Textularia spp. and Anomalinoides spp.), calcareous benthic foraminifera (Gavelinella guineana and Planulina nacatochensis), Gastropods, Ostracods, and Arenaceous indeterminate were recovered. The recognition of Bolivina afra-Haplophragmoides talokaense assemblage zone in the Mamu Formation conferred Maastrichtian age to the deposits. The presence of palynological assemblages like the Adallussiella polymorpha were used to assign Late Campanian age for the Nkporo Formation. Sequence stratigraphic analysis using the integration of foraminiferal, paleontological and lithofacies data identified two incomplete depositional sequences. The first sequence identified in the Nkporo Formation, comprised a Highstand systems tract (HST) bounded at the top by a sequence boundary (SB) dated 75 Ma. The SB also marked the beginning of the second depositional sequence that occurred in the successions of the Mamu Formation and comprised of Transgressive systems tract (TST), bounded at the top by a maximum flooding surface (MFS) dated 69.8 Ma. The results of this study have indicated the presence marker beds and constrained stratigraphic surfaces which can be used to map the identified play elements (source, reservoirs, traps and seals) in the study area. The surfaces are thus essential ingredients in basin-wide correlations for improved hydrocarbon exploration and development in the Afikpo and other adjoining inland basins of Nigeria.

Anatomy of a platform margin during a carbonate factory collapse: implications for the sedimentary record and sequence stratigraphic interpretation of poisoning events

Sequence stratigraphic interpretations are limited in their prediction of sedimentary architecture and changes in sea-level when used in systems characterized by marked fluctuations in sediment supply, particularly in carbonate-dominated environments. Pronounced variations in sediment supply occurred during the Pliensbachian–Toarcian transition, when significant perturbations of the carbon cycle and intense environmental disturbances led to a global shutdown of carbonate production. We studied the impact of this event on sedimentation and stratal stacking patterns in the Moroccan central High Atlas. We reconstructed the lateral variations in facies and sedimentary geometries along a carbonate platform margin and slope in the field by tracking six key discontinuity surfaces covering the Pliensbachian–Toarcian transition from the platform margin to the toe-of-slope. This work highlights the difficulties in sequence stratigraphic interpretations in cases of neritic carbonate factory collapse without assessments of shoreline movement because the stacking patterns in open marine environments do not necessary reflect the regional variation in base level in disturbed carbonate systems. This study also emphasizes the uncertainties associated with focusing solely on lower offshore strata to assess the history and causes of palaeoenvironmental perturbations because decreases in the rate of carbonate production dampen neritic carbonate shedding and therefore favour the creation of a hiatus in deep water settings.

Record of a dense succession of drowning phases in the Alpstein mountains, north-eastern Switzerland: part I—the Lower Cretaceous Tierwis Formation (latest Hauterivian to latest Barremian)

In the Alpstein massif of north-eastern Switzerland, a complete succession of uppermost Hauterivian to uppermost Barremian condensed hemipelagic sediments crops out. This succession is known as Tierwis Formation, comprising in ascending order, the Altmann and Drusberg members. The sedimentary succession bears a number of fossiliferous glauconite- or phosphate-rich beds. A large number of newly discovered ammonites from these key beds and from several poorly explored levels of the Tierwis Formation allows for a new age calibration. The new dating as well as revised sequence stratigraphic interpretations and geochemistry contribute to a better understanding of the lithostratigraphic complexity of the Tierwis Formation and its spatio-temporal relationship with the Schrattenkalk Formation. The new lithostratigraphic observations, backed by ammonites, shows that the Altmann type-section and the Tierwis paratype-section do not cover the same stratigraphic interval because of dynamic sedimentation processes as erosion and sedimentation in submarine channels. We suggest that a phosphatic conglomerate in the Dursberg Member of middle late Barremian age corresponds to the Chopf Bed, which we recognised for the first time in the Alsptein massif. The Drusberg Member strongly thickens toward the southeast and progressively covers an upward extended stratigraphic range. Furthermore, the new dating of the key-surfaces and beds highlight a dense succession of drowning phases which occurred through the latest Hauterivian to late Barremian time interval. The latest Hauterivian onset of the glauconite-rich sedimentation of the Altmann Member is associated with a first major drowning phase, followed by the Faraoni oceanic anoxic event. The change of sedimentation to a rhythmic marl-limestone alternation of the Drusberg Member takes place over a polyzonal phosphatic conglomerate. This conglomerate coincides with a second major drowning phase and the onset of the Mid-Barremian Event, which is calibrated on the Tethyan ammonite biozonation.

A condensed chalk–marl succession on an Early Cretaceous intrabasinal structural high, Danish Central Graben: Implications for the sequence stratigraphic interpretation of the Munk Marl Bed

The Deep Adda-1 well in the Danish Central Graben, North Sea, provides a record of mid-Cretaceous sedimentation on the eastern flank of the intrabasinal Adda–Tyra inversion high. An upper Hauterivian – lower Barremian core in the Tuxen Formation spans the lower boundary of the laminated organic-rich Munk Marl Bed (MMB), a key marker bed in North Sea Cretaceous stratigraphy. Multidisciplinary sedimentological–biostratigraphic–palaeoecological data document the abrupt environmental shift at this boundary. The upper Hauterivian – lowermost Barremian lower Tuxen Formation (nannozones BC10 – lowermost BC14), beneath the MMB, represents a well-ventilated, current-swept setting supporting a diverse benthic fauna and characterized by a condensed succession with hardgrounds, at one level defining a biostratigraphic hiatus, and stacked, thin shallowing-upward parasequences. The succeeding lower Barremian MMB (nannozone BC14) attests to poorly oxygenated bottom waters and a total lack of epi- and infauna; the calm, inhospitable sea floor was intermittently disturbed by muddy turbidity currents and debris flows. The base-MMB surface is a complex fractured hardground indicative of relative sea-level fall and protracted winnowing of the cemented sea floor. The Deep Adda-1 core thus records a sea-level excursion that accompanied the onset of early Barremian oxygen depletion in concert with additional potential forcing factors such as coeval volcanism and watermass warming.

Sequence stratigraphy and paleoenvironmental significance of the Neoproterozoic Bambui Group, Central Brazil

The Bambui Group is a ca. 2,000 m-thick mixed biochemical-siliciclastic sedimentary succession that accumulated in the Sao Francisco Basin, central Brazil. Its age is contentious, but deposition during the Cryogenian-Ediacaran (ca. 635–550 Ma) is consistent with a new sequence stratigraphic model presented herein and previously published geochemical and structural data. Six stratigraphic sequences are recognized that record depositional evolution from an epeiric sea to foreland basin during the Marinoan snowball glaciation (ca. 635 Ma) to shortly after the Gaskiers (ca. 580 Ma) glaciation. Although this stratigraphic architecture differs from the classic lithostratigraphic and more recent chemostratigraphic and sequence stratigraphic interpretations of the Bambui Group, it is supported by correlation to other Cryogenian-Ediacaran successions globally. Collectively, these six sequences preserve a robust record of seawater redox structure leading up to the oxygenation threshold that triggered the evolution of multicellular animals in the Ediacaran.Sequence 1 is composed of Marinoan glacial deposits and an overlying cap carbonate with aragonite fan pseudomorphs and barite. Sequence 2 is composed of lime-mudstone that changes laterally into loess-rich peritidal phosphorites, suggesting oxygen stratification in shallow paleoenvironments. Sequence 3 is a carbonate succession with abundant stromatolites implying widespread oxygen production. The top of Sequence 3 is marked by a regional subaerial unconformity developed during uplift associated with the onset of the Brasiliano-Pan African Orogeny. Sequence 4 signals a return to loess derived sedimentation. Distal deposits contain abundant organic matter with framboidal pyrite reflecting bacterial sulfate reduction in low sulfate Ediacaran seawater. Sequence 5 is composed of well-developed aggradational parasequences composed of alternating deep subtidal organic-rich siltstones and shallow-water carbonate grainstones. Their well-defined cyclicity is interpreted to record extreme sea level fluctuations during the onset of the Gaskiers glaciation. Stromatolites suggest an oxygen-stratified water column. Sequence 6 marks the change from an epeiric sea to a foreland basin. Interbedded basinal glauconitic siltstone and shale loess-rich are correlative with shoreface sandstones. Authigenic glauconite requires suboxic bottom waters to precipitate and thus reflects oxygenation of distal settings. Most of the deposition ended by ca. 560–550 Ma when the entire Bambui Group was deformed during the assembly of West Gondwana.In sequences 1, 2, 3, 4, 5, and the lower portion of 6, the lateral continuity of lithofacies, predictable layer-cake stratigraphy, and lack of fluvial coarse siliciclastic deposits are characteristics of epeiric sea successions. The appearance of immature sandstone, poorly sorted conglomerate, braided fluvial deposits, and synsedimentary deformation in the upper part of Sequence 6 are interpreted as the onset of foreland basin development. Stacking relationships of redox sensitive, authigenic lithofacies in both the epeiric sea and foreland phases support recent research suggesting seawater redox conditions were in a state of flux during the apex of the purported Neoproterozoic Oxygenation Event. Organic matter and authigenic pyrite in Sequences 4 and 5 reflect increased primary productivity in surface waters fertilized by nutrients such as P and Fe adsorbed on loess particles. This mechanism could be expanded to a global scale and have contributed to the increase in atmospheric and oceanic oxygen levels. These findings redefine and highlight the significance of the Bambui Group for Neoproterozoic Earth history.

Tectonostratigraphic evolution of the Sohar Basin, exploration concepts and emerging plays offshore on the UAE's east coast

The Sohar Basin, located offshore the East Coast of the Arabian Peninsula, is an area that has seldom been the focus of international oil companies. While all commercial discoveries in the UAE and Oman have been made to the west and south of the Oman Mountains, the Sohar Basin is underexplored and remains prospective. This lack of activity is primarily because the Thamama reservoir, which has yielded huge success to the west of the Oman Mountains is likely not present within the Sohar Basin. However, potential traps, reservoirs, and source rocks are present within the Tertiary succession. The Sohar Basin formed in the late Cretaceous after the obduction of the Semail Ophiolite onto the Arabian Plate. Following a period of tectonic quiescence, which lasted until the Early Eocene, the gravitational collapse occurred along the eastern edge of the present-day Oman Mountains.Interpretation of both 2D and 3D seismic reveals that the subsurface is comprised of three tectonostratigraphic units, defined as pre-, syn-, and post-tectonic packages. North-South trending thrust faults generate distinct mini-basins within the pre- and syn-tectonic sequences. The post-tectonic sequence is defined by the development of prograding systems in a tectonically quiescent margin formed in the Lower Miocene and which persists to the present day. Deep-water depositional systems dominate the undrilled far-east and south of the basin where the presence of submarine canyon systems and associated basin floor fans is supported by 3D seismic data. In recent times, the Sohar Basin has been subject to extensional faulting likely driven by sediment load caused by a prograding delta system.The results of this work reveal that multiple untested play types exist within the Sohar Basin, which are comprised of both structural and stratigraphic elements. This paper integrates both a structural and sequence stratigraphic interpretation to provide insights into the tectonostratigraphic evolution of the Sohar Basin. Consequently, the fundamentals of play fairway mapping have been employed to unlock the hydrocarbon prospectivity offshore the UAE's East Coast.

Verification and Application of Sequence Stratigraphy to Reservoir Characterization of Horn River Basin, Canada

Shale reservoirs, the most important unconventional resource, are difficult to characterize. Shale formations require detailed interpretation of geological, petrophysical, and geochemical analyses, and an integration of these disciplines. In terms of geological interpretation, the commonly used sequence stratigraphy analysis includes a lithofacies analysis. The application of sequence stratigraphy to shales facilitates the ability to relate between lithofacies and mineral composition, petrophysical parameters, and kerogen contents, which are affected by depositional setting. The classification of lithofacies is indispensable for reservoir quality prediction. In this study, porosity, permeability, and TOC content largely depend on lithofacies, and their correlation coefficient is relatively high. The sequence stratigraphic interpretation shows that organic carbon content usually increases within the maximum flooding surfaces and decreases stepwise. However, the relationship between total organic carbon contents and systems tract is less direct and redox dependent.

Depositional environment characterization in deltaic system by utilizing core, well log and seismic attribute data, Northern Malay Basin

The heterogeneity of shallow marine facies is quite hard to be solved. However, integrating all the data is essential to determine the details of facies distribution, whereas facies characterization is an important part that should be done before the petrophysical characterisation. This study aims to evaluate the reservoir depositional facies distribution using seismic well log and core data based on sequence stratigraphy concept, seismic interpretation, and seismic attributes, thus reducing the risk of oil exploration and improving the success rate. To ensure the objective is achieved, sequence stratigraphy interpretation has been done using integrating well log, core, and seismic data to identify the depositional environment. Distribution of coal helps to create well log correlation. Lithofacies and fluid have been interpreted according to core, test and log patterns, such as; gas sand, brine sand, shale, and coal. Based on the characteristic of logs, biostratigraphy and core data, five depositional environments have been interpreted in this study including in-distributary channel, delta plain (flood plain shale), crevasse splay, delta front shale (prodelta), and mouth bar. The interval of the target area is generally influenced by regression systems where on the lower formation is dominated by the transition depositional environment and changed to be dominant of delta plain.

Orbitally-paced climate change in the early Cambrian and its implications for the history of the Solar System

The early Cambrian was a critical period of Earth history, marked by explosive diversification of metazoans and several profound changes in Earth's surface environments and global climate. A valid temporal framework for the early Cambrian Period and across the major bio-events is poorly constrained, and the key underlying forcings of million-year (Myr) scale sea-level variations in a greenhouse lacking polar ice sheets are still disputed. Here, the high-resolution gamma-ray logs (GR) and Fe/Al records are utilized to conduct cyclostratigraphic analysis of the early Cambrian Stage 2-Stage 3 Qiongzhusi Formation of the Well A borehole in South China. A ∼10.8 Myr-long high-resolution astronomical time scale across Stage 2-Stage 3 is developed by astronomical tuning of gamma-ray logs and Fe/Al series to the stable 405-kyr long-eccentricity cycles. The Myr-scale sea-level fluctuations in the early Cambrian greenhouse are reconstructed through the sedimentary noise modeling of the 405-tuned GR series and agree with sequence stratigraphic interpretations, lithofacies stacking patterns, and depositional environment changes. The antiphase correlation of filtered ∼1.5 Myr cycles of sedimentary noise curves with the ∼1.5 Myr obliquity modulation cycles demonstrates that the variations in land-ocean water exchange linked to changes in poleward moisture and heat transport dominated by ∼1.5 Myr obliquity modulation cycles may be a primary driver for eustasy during non-glacial early Cambrian period. A new resonance state in the early Cambrian Stage 2-Stage 3, manifested by ∼1.5 Myr eccentricity: ∼1.5 Myr obliquity, could be mainly associated with the Earth-Mars secular resonance and constrain the chaotic evolution of the Solar System in deep time. Two independent approaches are employed to reconstruct the history of Earth-Moon system characteristics, including the precession constant, the Earth-Moon distance and the day length. Our results enhance knowledge of the connection of Myr-scale sea-level variations to astronomically induced climate change under greenhouse climate and provide new empirical constraints on the chaotic motion of the Solar System and the evolution of the Earth-Moon System in deep time.

Spatial skill predicts success on sequence stratigraphic interpretation

AbstractSequence stratigraphic interpretation and three-dimensional spatial and spatiotemporal skills are considered important for the petroleum industry. However, little is known about the relationship between the two. This study begins to fill this gap by testing whether spatial skills predict success on a sequence stratigraphic interpretation task. Students in this study (N = 78) were enrolled in undergraduate or graduate stratigraphy-focused courses at three U.S. state universities. Students completed (1) a sequence stratigraphic interpretation task with a sequence stratigraphic diagram and Wheeler diagram and (2) two spatial skills tests. Findings of simple linear regressions show that both disembedding (extracting or finding a pattern among other features, which is typically assessed by the hidden-figures test) and mental folding and unfolding (as assessed by the surface development test) are predictive of student success on the full sequence stratigraphic interpretation task. A nested regression, entering mental folding as the initial variable and disembedding as the secondary variable, showed that mental folding and unfolding accounted for almost all of the variance accounted for by disembedding in the simple regression. This may reflect the need to employ disembedding for the test of mental folding. Because the test of disembedding and the test of mental folding and unfolding were correlated, the distinct role of disembedding in stratigraphy remains unclear. However, the results clearly show that mental folding and unfolding is related to student success in sequence stratigraphic interpretation. Future studies should characterize how students utilize these skills, try to determine the causal direction of this effect, and identify good practices for supporting students in the classroom.

Lithofacies classification and sequence stratigraphic description as a guide for the prediction and distribution of carbonate reservoir quality: A case study of the Upper Cretaceous Khasib Formation (East Baghdad oilfield, central Iraq)

In the East Baghdad oilfield of central Iraq, the Upper Cretaceous Khasib Formation is the largest producing carbonate reservoir. The basic architecture of the “Khasib reservoirs” is heavily impacted by the lithofacies classification and sequence stratigraphic interpretation at East Baghdad field. A description of the Khasib Formation in terms of depositional systems, environments and lithofacies was obtained by studying the vertical distribution of the lithofacies, defined using well-log values and their mutual statistical relationships and constrained by the core data (thin sections). In this paper, a lithofacies classification and sequence stratigraphic interpretation are used to define reservoir geometry, lateral continuity and petrophysical characteristics of the Khasib reservoirs. The “Khasib” carbonate ramp stratigraphy exhibits different lithofacies in different depositional systems at the East Baghdad field. In addition, the log signatures and petrophysical properties of the same lithofacies vary. Multivariate statistical methods (principal component analysis and clustering) have been applied to separate stratigraphic packages and different carbonate lithofacies. Khasib carbonate lithofacies can be better discriminated from cluster analysis based on principal components rather than from arbitrary well log observations alone. The geometry of the strata in the ‘Khasib’ outer ramp is retrogradational, becoming aggradational during the lowstand systems tract (LST) and transgressive systems tract (TST). The highstand systems tract (HST) of the Khasib Formation is typically distinguished by a relatively thick aggradational-to-progradational geometry. This parasequence set is widespread upon the “Khasib” mid- and inner ramp. Petrographic analysis reveals the relative abundance and distribution of characteristic porosity types of the “Khasib” reservoirs. The HST (mid- and inner ramp) of “Khasib” Formation reveals the best reservoir characteristics (thickness, porosity, and hydrocarbon saturation) in the East Baghdad field. This enhancement of the knowledge of the lithofacies within a sequence stratigraphic framework through the application of principal component and cluster analysis is especially important to -optimally delineate and define the potential reservoir intervals of Iraq's Khasib Formation. Such multivariate statistical techniques may have similar beneficial application to carbonate reservoirs elsewhere in the world.

Sequence stratigraphic interpretation in marginal marine settings by the approach of parasequence-thickness-to-sandstone-fraction ratio: Case studies of the Gallup and Ferron outcrops in the Western Interior Basin, U.S.A.

ABSTRACTThe parasequence-thickness-to-sandstone-fraction ratio (TSF) is a simple but potentially powerful tool to identify stratal stacking patterns and associated systems tracts in siliciclastic paralic sequences. Parasequence thickness (T) reflects accommodation, and the sandstone fraction (SF) may serve as a proxy for the rate of sediment supply. Although previous research shows the effectiveness of applying TSF techniques to sequence stratigraphic analyses in siliciclastic depositional environments, constraints and analytical procedures of the technique are still not clearly illustrated. The paleogeography and sequence stratigraphy of the Cretaceous Gallup system and Ferron Notom deltaic complex in the Western Interior Basin have been extensively studied, providing an opportunity to explore the applicability and detailed workflow of the TSF method. TSF analyses are conducted first on two representative measured sections from the Gallup and the Ferron outcrops, respectively. The TSF analyses of the two one-dimensional (1D) sections are capable of identifying transgressive–regressive (T-R) cycles of the two deltas. However, the 1D sections can reflect the accommodation and sediment supply change only at single locations, which limits their usefulness in identification of stratigraphically consecutive parasequences and composite bounding surfaces. The utility of TSF analyses on cross sections is then tested on a depositional-dip transect of the Gallup delta, as well as dip-oblique and strike-oblique cross sections of the Ferron Notom delta. Parameters of T and SF are acquired from measured sections as well as interpolated virtual sections. For both the Gallup system and the Ferron Notom deltaic complex, the cross-sectional TSF analyses are more effective in recognizing systems tracts and associated bounding surfaces than the TSF analyses in 1D sections. Because dip-oriented cross sections usually encompass both proximal and distal parts of parasequences, and preserve stratigraphically continuous successions, they can provide more complete information for TSF interpretations than strike-oriented cross sections. Above all, TSF analysis via both measured and virtual sections along a depositional-dip profile is inclined to result in sequence stratigraphic categories that best match those based on the full set of geological observations.TSF analyses can also be used to identify general direction of shoreline trajectories in both the Gallup and Ferron Notom deltas. Parameterization of T, SF, and maximum progradation distance for both the Gallup and Ferron Notom parasequences are used to indicate shelf gradients throughout their deposition. The prominent differences of the shelf gradients between different deltaic parasequences indicate differences in allogenic and autogenic controls on the development of T-R sequences in these two deltas.

Biostratigraphy and sequence stratigraphy of the deep offshore Niger Delta and implications for future hydrocarbon exploration in the Paleogene sequences

The Niger Delta Deep Offshore Basin has been the latest frontier within the Niger Delta for hydrocarbon exploration and production activities. Until the onset of drilling activities in the early 1990s, little was known about the biostratigraphy of this frontier setting. Thus, uncertainties existed in the ages ascribed to the various stratigraphic sequences. Presently, several wells have been drilled in the deep offshore Niger Delta and three-pronged biostratigraphic data and some sequence stratigraphic interpretations have become available. We have integrated biostratigraphic data comprising palynological, foraminiferal, and calcareous nannoplankton zonation and biofacies, wireline log information, and regional-3D seismic data, with simple basic sequence stratigraphic analysis for a detailed well correlation, regional mapping of hydrocarbon-bearing intervals, and evaluation of the thicknesses of promising stratigraphic intervals. Results indicate a significant effect on not only the earlier conceptualized age of the deepest prospective reservoirs, but also the ages of possible source rocks in the Paleogene of the Niger Delta offshore sequence. Our findings will help in the realistic assumptions of source rock characterization, charge modeling/prediction, and the stratigraphic thicknesses of the offshore sequences and their reservoirs. We examine the specific derivable age interpretations from the integration of at least six wells, biostratigraphic data with seismic, and the implications of these interpretations for hydrocarbon prospectivity of the Paleogene sequences. Furthermore, we document the biostratigraphy work done in the area, in all the structural belts where drilling has occurred including the deepest well drilled in the deep offshore Niger Delta, Bosi-006. The results of this study have a critical impact on the current regional geologic understanding of the deep offshore Niger Delta.

Geological and Seismic Delineation of D2000 and D4000 Petroleum Reservoirs Within Afenmai Field, Niger Delta Basin, Nigeria

Afenmai Field is an old petroleum discovery within Central Swamp Depobelt on the eastern part of the Nigerian Niger Delta basin. Eight wells were drilled within the field, and Wells 006 and 008 respectively encountered petroleum in the D2000 Sand and D4000 Sands. This study was conducted to firm up the reservoirs’ spatial extent as well as their pore fluid distribution, which has hitherto been unattended to. To achieve this, the reservoirs were correlated within a framework of parasequence sets that was created from sequence stratigraphic interpretation of combined lithologic, geophysical and foraminiferal data. The reservoir tops were then tied at Well 008 to seismic reflections within a 3D seismic volume, and mapped through the volume. Root mean square (RMS) amplitude was extracted from the seismic volume along mapped reservoir surface. The depth maps produced reveal a southward concave, major synthetic structure building fault across the entire field. A series of en echelon synthetic faults are revealed on the NW and SE of the upthrown block of the major synthetic structure building fault. The D2000 Sand constitutes a rollover anticline with entrapped saturated petroleum in a four-way anticlinal closure that lacks fault support. The western, eastern and northern parts of the reservoir are yet to be penetrated by a well. Petroleum is entrapped in a three-way closure within the D4000 Sand in the western part of the upthrown block of the major synthetic structure building fault. Average RMS amplitude characterise the petroleum reservoir at Well 008 location and immediate vicinity. The values become low westward within the delineated reservoir. Opportunity for productive drilling exists within the undrilled part of the delineated reservoir, westward of Well 008. Opportunities for productive drilling also exist within the undrilled western, eastern and northern parts of the delineated D2000 reservoir sand.

Impact of sedimentation, climate and sea level on marine sedimentary pyrite sulfur isotopes: Insights from the Valle di Manche section (Lower-Middle Pleistocene, southern Italy)

Variations in the isotopic composition of sulfur in sedimentary pyrite (δ34Spyr) are often used to reconstruct global sulfur biogeochemical cycling and Earth's surface oxidation state over Earth history. Recent work, however, has shown that δ34Spyr is strongly impacted by local depositional conditions, which both confounds attempts to reconstruct global sulfur cycling and provides a new proxy for investigating local paleoenvironmental conditions. We present a chemostratigraphic record of δ34Spyr in a detailed paleoenvironmental and sequence stratigraphic framework spanning the Early-Mid Pleistocene transition from the Crotone Basin (Calabria, Southern Italy). Bulk δ34Spyr data parallel transgressive-regressive cycles and enable the differentiation of progradational and retrogradational stacking patterns, where δ34Spyr values decrease toward −50‰ in outer shelf deposits during late transgression and become more positive and variable in prograding sedimentary units accumulated in shallower environments. In parallel with the observed variations in δ34Spyr, complementary data on organic carbon abundance and isotopic composition and total nitrogen abundance document changes in terrestrial delivery of sediment to the basin, as a consequence of glacial-interglacial eustatic sea-level changes, regional climate and local tectonics. The chemostratigraphic record of δ34Spyr appears sensitive to variations in sedimentation rates and local erosional surfaces and therefore may represent a powerful tool to assess stratal stacking patterns for sequence stratigraphic interpretations of siliciclastic successions. These results highlight the role of local conditions in controlling sedimentary δ34Spyr records and illustrate how isotopic data can be used to reconstruct key parameters of the sedimentary environment that might not otherwise be accessible, especially in deep-time strata.