Seismic-driven geocellular modeling of fluvial meander-belt reservoirs using a rule-based method

A novel workflow is presented for building static models of fluvial reservoirs composed of large point-bar architectural elements, based on the application of a specialized forward stratigraphic model, named ‘Point-Bar Sedimentary Architecture Numerical Deduction’ (PB-SAND). The approach uses interpreted horizontal slices from 3D seismic datasets to reconstruct the planform evolution of meander loops, on which basis the geometry of point-bar deposits and associated accretion units can be simulated deterministically. The resulting meander-belt geometry is then populated with different types of facies, through a rule-based algorithm that generates facies architectures that reflect geologic understanding, enabling users to establish linkages between styles of meander evolution (e.g., meander growth via expansion, translation, rotation) and facies distributions. Input parameters define the proportions, geometries and distributions of types of point-bar deposits, as captured from subsurface data and/or from geologic analogs. Multiple stochastic realizations of facies architecture can be generated. To demonstrate the application of this modeling approach, the workflow has been applied to a meander-belt reservoir where large point-bar and channel-fill elements are imaged in seismic. A detailed example is used to illustrate workflows that can be used to build high-resolution sector models in pre-drill contexts, suitable for guiding development plans. An additional example is used to show how to achieve well match for densely drilled sectors, by means of a hybrid approach that combines the new algorithm with traditional geostatistical techniques. It is shown how the workflow allows consideration of point-bar growth styles, as inferred from seismic data, on distribution and geometry of heterogeneities, and how this facilitates the reproduction of geologic features that are important controls on the static connectivity of point-bar reservoirs (e.g., distribution and characteristics of bar-front mud drapes, and of mud-prone packages arising from progressive meander-bend tightening or from downstream fining of deposits beyond the apex of a meander bend). A comparison with traditional variogram-based methods is undertaken to compare metrics that describe intra-point-bar static connectivity and that represent proxies for the degree of compartmentalization of upper-bar sands by mud drapes

Controls on shallow plumbing systems inferred from the spatial analysis of pockmark arrays

In marine geological settings, pockmarks are evidence of highly focused fluid expulsion at the seabed. The modern seafloor of the Lower Congo Basin (LCB, offshore West Africa) is covered by densely packed arrays of thousands of pockmarks, whose distribution reflects in part the spatial organization of underlying seal bypass features. This study describes and analyses the variable distributions of seabed pockmarks using 3D seismic and spatial statistics, in order to infer subsurface processes that control the fluid migration routes and understand the overall shallow plumbing system of the area. The 3D seismic visualization of feeding conduits (pipes) allowed the identification of the source interval for the fluids expelled during pockmark formation. Pockmark formation may be linked to gas hydrate dissociation and/or expulsion of free gas beneath the GHSZ. Spatial statistics were used to show the relationship between underlying discontinuities and seabed pockmarks distributions, and revealed that pockmark occurrence is not considered to be random. Several different types of geo-mechanical controls were recognised and divided into 1) stratigraphic or depositional controls, 2) strati-structural controls, and 3) structural controls, corresponding to increasing stages of deformation affecting basin sediments. Furthermore, from the wide variability of pockmark sizes present in the area and the local geomorphology, it is possible to conclude that pockmark size is related 1) to the sub-surface depth at which the fluid source interval occurs and 2) to lateral variations of the degree of overpressure. The results of this study are relevant for the understanding of shallow fluid plumbing systems in offshore settings, with implications on our current knowledge of overall fluid flow systems in hydrocarbon-rich continental margins. This is relevant for the understanding of shallow fluid plumbing systems in offshore settings and overall fluid flow systems in hydrocarbon-rich continental margins

Seismic characterisation of fluid leakage in marine sediments

Hydrocarbon migration is one of the key processes that takes place in the development of a successful petroleum system. Understanding when fluids migrated, how they migrated and which routes they took either into primary reservoirs or via transient seals into shallow reservoirs is paramount for successful extraction. Fluids in the sub-surface can be imaged in seismic reflection data as anomalously high reflection amplitudes owing to their contrasting acoustic properties (density and p-wave velocity) with sedimentary rocks. This thesis uses 3-Dimensional seismic reflection data from the Møre Basin, offshore mid-Norway and the Lower Congo Basin, West African margin to investigate the migration of fluids from primary reservoir intervals through overlying successions of fine-grained sediments. These shallow intervals are typically considered as regional seal layers and understanding how seals were breached and fluids migrate through them is vital to risking exploration targets. The Møre Basin case study investigates a gas-associated amplitude anomaly at the crest of a domal structure cored with alternating fine-grained biosiliceous and calcareous ooze sediments. The anomaly has a rather unique convex-upward basal contact which is explained by the superposition of lateral velocity variations through the gas-filled dome. The centre of the dome has more gas than the flanks resulting in a lower velocity which pushes the basal contact to deeper positions with respect to the flanks. The domal trap was charged from gas migrating from depth via capillary entry pressure and possibly via sub-vertical pathways created by compaction-derived polygonal faults which pervasively deform the host stratigraphy. The Lower Congo case study investigates a range of high-amplitude seismic amplitude anomalies in a thick sequence of hemipelagites (the waste zone) above a deep-seated turbidite reservoir. Anomalies take many forms and include; Linear anomalies, Sub-circular anomalies, Patchy anomalies at which finger-shaped anomalies emanate from their lateral edges, and Discrete filamental anomalies. The Sub-circular and Patchy anomalies were interpreted as being related to the presence of hydrocarbons. Detailed analysis of a sub-set of the hydrocarbon-bearing amplitude anomalies suggest leakage occurred through two means; 1) vertical leakage through feeders and 2) via deep-seated extensional faults formed during gravity-driven gliding of vi an underlying salt detachment. Vertical leakage is expressed in the form of Vertical Anomaly Clusters which comprise vertically stacked assemblages of high-amplitude anomalies. A common aspect of the two case studies are that high-amplitude anomalies within fine-grained sedimentary successions are linked to vertical or sub-vertical migration pathways provided either by faults or pipe-like structures formed during overpressure. These results have implications for our understanding of how seals are breached when reservoirs are overpressured

STRUCTURE AND STRATIGRAPHY OF THE AJDABIYA TROUGH AREA, EAST SIRT BASIN - LIBYA

The structural style within the deepest parts of the Ajdabiya Trough is defined by a system of Early - Late Cretaceous syn-depositional fault blocks bound by normal faults and basement highs devoid of syn-rift sediments, which are buried under a thick succession of Cenozoic post-rift deposits. The range of fault orientations likely reflects the conflicting influences of the ~NE-SW regional extension direction and the dominant ~N-S trending basement fabric. Mainly NW-trending normal faults dissecting Cretaceous and older rocks have been inferred from 2D seismic reflection and potential field data. Other faults trend NE-SW and E-W, and mainly cut Miocene and older strata. Some of these faults have both sinistral and dextral strike slip components and are possibly linked to on-going seismicity in the Sirt Basin and the Cyrenaica Platform. Vertical displacements on these faults are several hundred meters and are defined by large throws on Cretaceous and underlying horizons. Structural mapping confirms the presence of relay ramps associated with overlapping faults developed in the hangingwalls adjacent to west downthrowing normal faults along the eastern margin of the Ajdabiya Trough. The seismic stratigraphic framework is organised into six mega-sequences that correlate to variations in relative sea-level and/or sediment supply during Late Mesozoic and Cenozoic times. The stratigraphic architecture of the trough is largely influenced by relative sea level changes and minimal tectonic effects during the Cenozoic; observed progradation of the Paleocene, Early and Middle Eocene sequences along the trough margin is attributed to relatively rapid sedimentation rates and relatively slow rates of increase in accommodation space. Depositional environments are interpreted using the resultant facies analysis and the characterisation of the seismic reflections indicated that the geological units were deposited in marginal marine, shallow shelf and moderately deep marine environments. Special consideration is given to the principle of seismic sequence stratigraphy analysis of carbonate depositional systems where the facies group took initially place on a homoclinal ramp which later developed into a rimmed platform. This analysis additionally reveals that similar depositional architectures can be divided into systems tracts. The earliest systems tract of the Lower Eocene sequence is interpreted as lowstand prograding wedge distinguished on the basis of the component facies that indicate the dominant depositional regime. Localized debris flow or mass transport complex formed during early highstand systems tract deposition began during the Middle Eocene. The tectono‐stratigraphic analysis of the Ajdabiya Trough reveals that two major extensional pulses controlled the architecture of the trough during continental rifting with crustal stretching (β) factor ranging from 1.3 to 1.5 consistent with subsidence in the Ajdabiya Trough having been controlled by thermal cooling and isostatic adjustments of the crust beneath the trough. Growth strata within grabens and half-grabens denote persistent tectonic subsidence and demonstrate the progressive depocenter locus migration towards the north. In such a context, the current geometry of the Ajdabiya Trough is interpreted to have resulted mainly from rifting cycles and possible renewed continental extension. The investigations of the tectono‐stratigraphic controls reveal that after a period of relative tectonic quiescence, post‐rift tectonic reactivation affected the Ajdabiya Trough almost continuously since the latest Cretaceous to the Miocene. Burial history curves correlated with one-dimensional back-stripping assuming Airy isostasy shows that Cenozoic subsidence in the Ajdabiya Trough can be divided into three episodes of post-rift subsidence characterized by short and long-lived subsidence pulses and rapid sedimentation rates that may lead to development of overpressure by disequilibrium compaction

The marine geology of the Aliwal Shoal, Scottburgh, South Africa.

Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2012.This study represents the first detailed geological, geophysical and geochronological investigation of the continental shelf surrounding the Aliwal Shoal, ~5 km offshore of Scottburgh, in southern KwaZulu-Natal. Mapping of the seafloor geology using geophysics and direct observations from SCUBA diving transects were integrated with the seismic stratigraphy and constrained by new geochronological data. Four seismic stratigraphic units (A to D) were identified and interpreted with the subsequent sequence stratigraphic model consisting of four incompletely preserved stratigraphic sequences separated by three sequence boundaries (SB1 - SB3) comprising complex reworked subaerial unconformity surfaces. Sequence 1 is the deepest, subdivided by a basin-wide marine flooding surface (MFS1) into a lower Campanian (and possible Santonian) TST and an upper Maastrichtian combined regressive systems tract comprising HST/FRST deposits. SB1 follows Sequence 1 and spans most of the Tertiary representing multiple erosional events. Shelf sedimentation resumed during the Late Pliocene to early Pleistocene with deposition of Sequence 2, the shelf-edge wedge, which again was followed by erosion and non-deposition during the high frequency and amplitude Early to Middle Pleistocene sea-level fluctuations resulting in the formation of SB2. Sequence 3 consists of coast-parallel, carbonate cemented aeolianite palaeo-shoreline ridges of various ages overlying Sequence 1 and 2. Sequence 4 unconformably overlies all the earlier sequences and comprises a lower TST component displaying characteristic retrogradational stacking patterns and an upper local HST clinoform component showing progradation and downlapping. Inner and middle shelf TST units constrained between Sequence 3 ridges form thick sediment deposits showing a progression from lagoonal and lower fluvial-estuarine deposits, overlain by foreshore and shoreface sands, documenting the changing depositional environments in response to a sea-level transgression. Laterally, in the absence if Sequence 3 ridges, TST sediments comprise only a thin transgressive sand sheet. The upper HST component comprises a prograding shore-attached subaqueous-delta clinoform sediment deposit, the Mkomazi Subaqueous-Delta Clinoform (MSDC) which evolved in four stages. An initialization and progradation stage (Stage 1) (9.5 to 8.4 ka cal. B.P.) was interrupted by retrogradation (Stage 2) and backstepping of the system due to rapid sea-level rise between 8.4 to 8.2 ka cal B.P. Stage 2 backstepping of the clinoform controlled the subsequent overlying topset morphologies resulting in later stages inheriting a stepped appearance upon which shoreface-connected ridges (SCR’s) are developed. Stages 3 (8.2 to 7.5 ka cal. B.P.) and 4 (7.5 to 0 ka cal. B.P) show a change from ‘proximal’ topset aggradation to ‘distal’ foreset progradational downlap, linked to a change in the dominant sedimentary transport mechanism from aggradational alongshore to progradational cross-shore related to variations in accommodation space and the rate of sediment supply. Morphologically the MSDC is characteristic of sediment input onto a high energy storm-dominated continental shelf where oceanographic processes are responsible for its northward directed asymmetry in plan-view, for the lack of a well defined bottomset and for the re-organisation of its topset into very large SCR’s. The SCR’s are 1 - 6 m in height, spaced 500 to >1350 m apart and vary from 3 km to >8 km in length, attached on their shoreward portions to the shoreface between depths of -10 m to -15 m (average at -13 m) and traceable to depths exceeding -50 m, although the majority occur on the inner shelf between -20 m to -30 m. Several individual crests can be identified forming a giant shoreface-connected sand ridge field with a sigmoidal pattern in plan-view postulated to be a surficial expression of the subjacent retrogradational phase (MSDC Stage 2). SCR’s development occurred in two stages. Stage 1 involved deposition of sediment on the shoreface and ridge initiation during the MSDC Stage 2 retrogradational event. Sediment was reworked during sea-level rise generating clinoforms with proximal along-shore aggradation and distal across-shore progradation. This occurred during the last post-glacial sea-level rise from ca. 8.4 ka cal. B.P. SCR Stage 2 represents modern maintenance of the SCR system which is continually modified and maintained by shelf processes and consists of two physical states. State 1 considers SCR maintenance during fair-weather conditions when transverse ridge migration is dominant and driven by the north-easterly flowing counter current shelf circulation. State 2 considers SCR development during storm conditions when longitudinal ridge growth is suggested to occur as a result of storm return flows. Following the storm, the regional coast-parallel current system is restored and the fair-weather state then moulds the SCRs into a transverse bedform. Deposition on the MSDC is ongoing on a continental shelf that is still in a transgressive regime. The exposed seafloor geology comprises late Pleistocene to Holocene aeolianite and beachrock lithologies, deposited as coastal barrier and transgressive shoreface depositional systems. Extensive seafloor sampling was combined with a multi-method geochronological programme, involving the U-series, C14 and optically stimulated luminescence (OSL) to constrain the evolution of the aeolianite and beachrock complex. The Aliwal Shoal Sequence 3 ridge comprises three distinct aeolianite units (A1 to A3) which represent different types of dune morphologies deposited during the climatic and associated sea-level fluctuations of MIS 5. Units A1 and A2 deposited during the MIS 6/5e (~134 to ~127 ka cal. B.P.) transgression represent contemporaneous evolution of a coastal barrier system which consisted of two different dune forms associated with a back-barrier estuarine or lagoonal system. Unit A1 most likely originated as a longitudinal coastal dune whilst Unit A2 comprised a compound parabolic dune system that migrated into the back-barrier area across an estuary mouth/tidal inlet of the back-barrier system. The coastal barrier-dune configuration established by Unit A1 and A2 was most likely re-established during similar subsequent MIS 5 sea-level stands which during MIS 5c/b resulting in the formation of the back-barrier dune system of Unit A3. Palaeoclimatic inferences from Units A1 and A2 aeolianite wind vectors indicate a change from cooler post-glacial climates (lower Unit A1) to warmer interglacial-like conditions more similar to the present (upper Unit A1 and Unit A2). Unit A3 palaeowind vector data show variability interpreted to be related to global MIS 5c climatic instability and fluctuations. For Units A1, A2 and A3 pervasive early meteoric low-magnesium calcite (LMC) cementation followed shortly after deposition protecting the dune cores from erosion during subsequent sea-level fluctuations. Sea-spray induced vadose cementation in Units A1 and A2 may have been a key factor in stabilising dune sediment before later phreatic meteoric cementation. The final preserved Late Pleistocene depositional event in the study area was that of the storm deposit of beachrock Unit B5. Induration followed shortly after deposition by marine vadose high-magnesium calcite (HMC) cementation. Following deposition and lithification, Units A1, A2, A3 and B5 underwent a period of cement erosion associated with decementation and increased porosity due to either 1) groundwater table fluctuations related to the high frequency MIS 5 sea-level fluctuations and/or 2) carbonate solution due to complete subaerial exposure related to the overall MIS 4 - 2 sea-level depression towards the LGM lowstand. In addition to the decementation and porosity development Unit B5 also experienced inversion of the original unstable HMC cement to LMC. During MIS 4 to 2 the Aliwal shelf comprised an interfluve area which was characterised by subaerial exposure, fluvial incision of coast-parallel tributary river systems and general sediment starvation. Beachrock Units B1 to B4 were deposited in the intertidal to back-beach environments and subsequently rapidly cemented by marine phreatic carbonate cements comprising either aragonite or HMC. Unit B1 was most likely deposited at 10.8 ka cal. B.P., B2 at 10.2 ka cal. B.P, B3 at 9.8 ka cal. B.P and B4 <9.8 ka cal. B.P. thereby indicating sequential formation during the meltwater pulse 1b (MWP-1b) interval of the last deglacial sea-level rise. Unit B3 marks the change from a log-spiral bay coastal configuration established by Units B1 and B2 to a linear coastline orientation controlled by the trend of the pre-existing aeolianite units. This change in the morphology of the coastline is also documented by the shape of the underlying transgressive ravinement surface (reflector TRS, Sequence 4) which again was controlled by the subjacent sedimentary basin fill architecture and subsequent transgressive shoreline trajectory (Sequence 4). Sea-level rose at an average rate of 67 cm/100 years from B1 to B2 and 86 cm/100 years from B2 to B2 indicating an acceleration in the rate of sea-level rise supporting enhanced rates of sea-level rise during the MWP-1b interval which also seemed to have altered the coastal configuration and resulted in the closure of the southern outlet of the back-barrier estuarine system. Two cycles of initial aragonite followed by later HMC cement are tentatively linked to two marine flooding events related to different pulses of enhanced rates of sea-level rise during MWP-1b which are considered responsible for significant changes in the marine carbon reservoir ages. Comparisons of the U-series, C14 and optically stimulated luminescence (OSL) methods have shown OSL to be the most reliable method applied to dating submerged aeolianites and beachrocks. OSL not only provides the depositional age of the sediment but also does not suffer from open system behaviour, such as marine reservoir changes and contamination. Acoustic classification of the unconsolidated sediment samples resulted in the demarcation of 3 major acoustic facies, C to E, interpreted with sample analyses as quartzose shelf sand (C), reef-associated bioclastic-rich sand (D) and an unconsolidated lag and debris deposit (E). Grain size distribution patterns of the unconsolidated seafloor sediments indicate that the SCR system delivers fine and medium sand to the inner and middle shelf and imparts a general N-S trending pattern to the gravel and sand fractions. In addition grain size distributions support selective erosion of the seaward flank of the Sandridge with the remobilised sediment deposited in the Basin as low amplitude bedforms over the Facies E lag and debris pavement. The mud fraction is interpreted to be deposited by gravity settling from buoyant mud-rich plumes generated by river discharge. Integration of acoustic mapping, field observations and sample analyses indicate that the present distribution of the unconsolidated sediment is the result of a highly variable distribution of modern and palimpsest sediments which are continually redistributed and reworked by a complex pattern of bottom currents generated by the interaction of opposing oceanographic and swell driven circulation patterns

The kinematics of intra-salt layers during salt tectonics

The structures and dynamics of intra-salt layers have received limited study in comparison with the external shape of salt structures. Our limited understanding of the behaviour of intra-salt layers generally comes from salt mines, outcrops, analogue, and numerical modelling where the full three-dimensionality of intra-salt layers is barely observed. To understand the internal dynamics of giant salt structures and the response of their intra-salt layers during regional tectonics, this thesis provides detailed interpretation and analysis of intra-salt layers from the Silverpit Basin, in the Southern North Sea Basin, and the Birba Area, in the South Oman Salt Basin. These two locations provide unique natural laboratories where driver mechanisms for salt tectonics are investigated using high-resolution, high-quality three-dimensional (3D) seismic reflection data. The Silverpit Basin is a buckled basin formed during the Mid Eocene to Late Oligocene, while the Birba Basin was affected by massive sediment loading, which generated differential loading from the Early Cambrian to the Late Permian. Differential loading of the basin caused down-building and influenced the growth of diapirs and minibasins, which later led to intense deformation and fragmentation of the intra-salt carbonate stringers. In the Silverpit Basin, regional salt anticlines encapsulated a 23–63 m-thick intra-salt layer known as the Z3 Stringer. Lithologically, the Z3 Stringer is composed of anhydrite, and it represents a strong seismic marker across the Southern North Sea Basin. Relative to regional anticlines and synclines at the Top Salt level, the Z3 Stringer deformed in a ductile manner comparable in geometry and attitude to the regional salt structure. Non-cylindrical stringer folds, which vary from gentle to isoclinal, are related to the intensity of the regional-scale structure, whereby tighter vii stringer folds are observed under well-developed Top Salt anticlines and synclines. Synclines at the Top Salt level include long-wavelength gentle folds. Extreme thinning of the Zechstein by the downward displacement of the Top Salt causes the stringers to extend and finally break laterally in a mode-1 tensile fracture mechanism. This thesis highlights the complexity of intra-salt deformation and forms a good large-scale case study for the analysis of the kinematics and rheology of competent material enclosed within an incompetent medium. Understanding the complexities and attitudes of intra-salt layers and their encasing salt structures has broader implications for regional tectonic history, hydrocarbon prospectivity, and industrial applications

Natural and anthropogenic fluid migration pathways in marine sediments

Fluids are an important agent in nearly all geologic processes that shape the planet Earth. Fluid abundance and composition are governed by flow along permeable beds or natural and anthropogenic structures in the subsurface including faults, wells, and chimneys/pipes. Spatial and temporal variations in fluid flow activity modify total fluxes between geosphere, cryosphere, hydrosphere, and atmosphere. These fluxes have broad implications for geological processes including the formation of natural resources or the occurrence of geohazards including landslides, earthquakes and blowouts. They further play a crucial role for the global carbon cycles and the climate system. A qualitative and quantitative understanding of fluid flow in the subsurface is therefore important to assess the role of fluids in the Earth system and to quantify fluxes from the geosphere into the hydro- and atmosphere. In this Ph.D. thesis I use an integrated, interdisciplinary approach to study natural and anthropogenic fluid migration pathways in marine sediments in the North Sea, the convergent Hikurangi margin, and a section of the ancient Tethys margin which is now exposed near Varna, Bulgaria. The applied methods include conventional 3D seismic, high-resolution 3D seismic, and 2D seismic data as well as hydroacoustic, sedimentological, unmanned aerial vehicle-based photogrammetric and geochemical data. In each of the studied systems, natural and/or anthropogenic fluid migration pathways allow the transport of significant amounts of fluids through marine sediments towards the seafloor. Often the co-existence of multiple pathways enables the fluids to bypass permeability barriers within the Earth’s crust resulting in the formation of structurally complex flow systems. Focused fluid flow along normal faults in the Hikurangi margin likely plays an active role in the subduction drainage system, influences the slope stability and the morphotectonic evolution of the margin. Results from the Eocene Tethys margin show that focused fluid flow in marine sediments is possible in unconsolidated sands if seepage is focused at the top of faulted units and the flux rate is high enough. This stands in contrast to the general assumption that focused fluid flow in marine sediments is limited to low-permeable sediments. In the marine environment the term fluid flow is often used to exclusively refer to the flow of hydrocarbons. However, geochemical data from the North Sea and the Tethys margin indicate that the involved fluids are of different origin including compaction-related dehydration and submarine groundwater discharge. In each of the investigated cases, the temporal and spatial evolution of fluid flow is not fully addressed yet, especially with regard to vertical fluid conduits or the safety of subsurface drilling and storage operations. The results of my thesis highlight that the investigation of fluid migration pathways requires an interdisciplinary approach which may indicate the origin of the fluids, help understand the fluxes of fluids from the geosphere into the hydrosphere and atmosphere of the past, present and future and reveal the resulting consequences for the global carbon cycles and the climate system