Showing papers in "Petroleum Geoscience in 2010"
TL;DR: In this article, the authors present two multi-channel seismic sections across the Angola and conjugate Brazilian rifted margins that they consider as the type of sections for hyper-extended magma-poor rifted margin in the South Atlantic.
Abstract: The discovery of giant hydrocarbon reservoirs in the pre-salt sequence of the deep-water Brazilian rifted margin together with the new acquisition of high-quality reflection and refraction seismic surveys across many rifted margins worldwide has attracted the interest of industry and researchers to deep-water rifted margins. For the first time, the new data sets enable the imaging and description of the pre-salt structures, which indicate that deep-water rifted margins are very different from what classical models had predicted thus far. Instead of the expected fault-bounded basins and a sharp ocean–continent boundary, the new data suggest the existence of a sag basin lying on hyper-extended crust with little indication for brittle high-angle faulting, a transitional domain between continental and oceanic crust showing neither characteristics of oceanic nor continental material, and very asymmetrical distal conjugate rifted margins. These observations raise significant doubts on the validity of the classical concepts used in rheology, mechanics and isostasy to explain extensional systems leading to seafloor spreading. They also require new concepts and more data in order to understand how these rifted margins evolved in time and space. This has important implications for the exploration and evaluation of petroleum systems in the frontier areas of hydrocarbon exploration. In this study we publish two multi-channel seismic sections across the Angola and conjugate Brazilian rifted margins that we consider as ‘type’ sections for hyper-extended magma-poor rifted margins in the South Atlantic. The aim of this study is to discuss various possible interpretations and models to explain the high-resolution seismic images presented in this paper.
198 citations
TL;DR: This paper treated the South China Sea region as a large multi-phase continental rift basin and made a series of observations and possible interpretations to advance the notion that a hybrid tectonic models need to be proposed and tested.
Abstract: The Cenozoic evolution of SE Asia records a diverse array of tectonic processes with rifting, subduction, terrane collision and large-scale continental strike-slip faulting occurring in spatially and temporally complex relations. Oligocene seafloor spreading and rift propagation in the South China Sea are critical tectonic events that overprint an earlier phase of regional extension. Two end-member models proposed to explain the opening of the South China Sea differ in the relative importance of extrusion versus subduction as the driving mechanism. This paper treats the South China Sea region as a large multi-phase continental rift basin. Synthesizing recently published studies and using filtered Bouguer gravity data, we make a series of observations and possible interpretations to advance the notion that a hybrid tectonic models need to be proposed and tested. We present an example from the Phu Khanh Basin where flexural backstripping supports our interpretation that an ‘out-of-sequence’ rifting event was of sufficient magnitude to completely attenuate the continental crust in the ultra deep water part of the basin. The complex rift history of the region leads us to believe that future frontier hydrocarbon exploration will carry large uncertainties from basin to basin.
198 citations
TL;DR: In this article, the authors define the characteristics of these continental blocks and to describe their relationship and position within the rifted margins, and to understand the underlying processes that govern their formation.
Abstract: The conjunction of high-quality seismic surveys, deep sea drilling, and progress in numerical modelling has changed the way of thinking about how continents rift and oceans form. In particular the discovery of exhumed continental mantle and hyper-extended crust in deep-water rifted margins has led to a paradigm shift in research into the evolution of rifted margins. Although rifted margins now appear to be more complex and their architecture more diverse than previously thought, their study worldwide shows that there are in fact a limited number of structures observed in seismic images that characterize their architecture. These ‘building stones’ include crustal blocks of various sizes, often referred to as microcontinents, continental ribbons, H-blocks, extensional allochthons and outer highs. The aim of this paper is to define the characteristics of these continental blocks and to describe their relationship and position within the rifted margins, and to understand the underlying processes that govern their formation. We propose, using the example of the North Atlantic, that these crustal blocks are the result of specific rift processes that correspond to the sequential evolution from stretching, to thinning and exhumation of the continental lithosphere. We show that the relationship between the various rift structures provides fundamental insights into the controlling processes that thin and finally rupture continental lithosphere.
182 citations
TL;DR: In this article, a hybrid model for the Baram-Balabac Basin is proposed in which the basin post-dates the Sarawak Orogeny, which appears to be more regionally extensive than previously believed, is attributed to Eocene to Early Oligocene collision of the Dangerous Grounds and Reed Bank with Sabah and Palawan.
Abstract: The West Baram Line separates NW Borneo’s two petroleum systems. Oligocene sandstone and Lower Miocene carbonate reservoirs of the gas-prone Luconia system lie SW of that line. Northeast of the West Baram Line, the oil-rich Baram-Balabac Basin produces from Middle Miocene to Early Pliocene sandstones
deposited in a foreland basin on the western side of the mountainous interior of
Borneo. On the present-day shelf exploration efforts spanning nearly four decades have focused on the Champion and Baram deltas and associated extensional growth fault structures. Many of these structures have experienced youthful inversion owing to ongoing tectonic shortening. Recent discoveries prove this petroleum system extends into deep water beyond the modern shelf edge where an active fold-thrust belt has formed above autochthonous rifted continental crust of the Dangerous Grounds province in the South China Sea. New regional mapping (c. 100 000 km2) integrating seismic, borehole and gravity data shows that the Baram-Balabac Basin is segmented into four structural domains whose boundary zones trend NW-SE similar to the strike of the West Baram Line. Domain boundaries appear to control the position of the basin’s palaeoshelf edges, turbidite depositional systems, major unconformities and the position of the basin’s largest fields. These observations suggest that the domain boundaries are the expression of deep structures, probably within the underlying rifted continental crust. Two of the domain boundaries can be projected onshore to align with fault systems separating contrasting geological
elements indicating they represent features of tectonic interest. The onshore geology of NW Borneo represents the early history of the Baram-Balabac Basin. The onshore geology is reviewed and new outcrop, biostratigraphic and palaeomagnetic data from Sabah are discussed in light of current models for the region’s tectonic evolution. A hybrid model is proposed in which the Baram-Balabac Basin post-dates the Sarawak Orogeny. The Sarawak Orogeny, which appears to be more regionally extensive than previously believed, is attributed to Eocene to Early Oligocene collision of the Dangerous Grounds and Reed Bank with Sabah and Palawan. In the proposed model Oligo-Miocene subduction of oceanic crust under NW Borneo is minimal. The Sabah Orogeny and younger inversion events are related to underthrusting of the Dangerous Grounds driven by both the opening of the South China Sea and by NW-directed subduction beneath SE Sabah in the Semporna-Dent Peninsula. In this context the structural complexity of the Baram-Balabac foreland basin reflects the adjustment of deep crustal blocks to far-field tectonic stress.
153 citations
TL;DR: In this paper, the authors analyzed potential field data (gravity and magnetic) from the Argentine to the Brazilian oceanic basins and extending over the Pelotas, Santos and Campos basins.
Abstract: This paper discusses the geological and geophysical interpretation of rift structures in the region extending from the Rio Grande Rise, in the Southeastern Brazilian margin, towards the Cabo Frio High, which separates the Campos and Santos basins. We have analysed potential field data (gravity and magnetic) from the Argentine to the Brazilian oceanic basins and extending over the Pelotas, Santos and Campos basins. The Rio Grande Rise shows a relatively negative Bouguer anomaly in an area that corresponds to a major positive bathymetric feature between the Argentine and Brazil basins. North–south propagators related to the early spreading centres of the Atlantic Ocean are observed from Argentina towards the southern Santos Basin, which is characterized by an elevated basement topography relative to the Pelotas Basin. The region adjacent to the Florianopolis Fracture Zone between the Santos and Pelotas basins is also characterized by an elevated basement region aligned in an east–west direction, and locally it is marked by rift structures aligned along a NW–SE direction, forming a lineament or shear zone (Cruzeiro do Sul lineament) that extends from the Cabo Frio High towards the Rio Grande Rise, thus involving both continental and oceanic crusts. The Rio Grande Rise is associated with the east–west-trending fracture zones, which are characterized by several aligned magnetic anomalies in the southern Santos Basin. The Rio Grande Fracture Zone continues landward as the Sao Paulo Ridge, and extends towards the platform as the Florianopolis High. Oceanic propagators are identified from Argentina towards the Pelotas and Santos basins, and locally we observe rupturing of the salt layer by igneous intrusions or possibly by mantle exhumation. The Florianopolis (or Rio Grande) Fracture Zone is marked by an abrupt topographic offset separating the Pelotas Basin from the southern Santos Basin, and the associated volcanic belts limit the southernmost occurrence of the late Aptian evaporite sequence. The evaporite sequence in this segment of the continental margin shows remarkable layering of halite, anhydrite and carnalite. Conjugate to the Rio Grande Rise, the Walvis Ridge, offshore Namibia, is similarly a topographic high, but rift structures as observed in the Brazilian side are apparently unique in the South Atlantic. Alternative interpretations for the origin of the Rio Grande Rise include: a volcanic edifice or plateau rooted in the mantle; an intraplate shear zone affecting both continental and oceanic crust; an oceanic area of igneous over-productivity caused by a hotspot or a thermal anomaly in the mantle; a palaeo-spreading centre in the Cretaceous Atlantic Ocean; an area of excessive volcanic activity resulting from mantle differentiation due to adiabatic decompression; or perhaps an isolated remnant of continental crust left outboard of the Brazilian continental margin during the drifting process.
97 citations
TL;DR: In this paper, a margin-opposed rift basin subsidence characteristics, reconcile important facies relationships and consider the associated processes responsible for contrasting basinward transitions to oceanic crust.
Abstract: Using a new margin restoration model for the central South Atlantic we highlight margin-opposed rift basin subsidence characteristics, reconcile important facies relationships and consider the associated processes responsible for contrasting basinward transitions to oceanic crust. We interpret these parameters as diagnostic of strain evolution during rifting. Pre-salt subsidence patterns and sequence isopachs of late syn-rift and early post-rift sequences are symptomatic of these complex extension behaviours. Inherited basement fault trends partition extensional strain during stretching with resultant rift-related structural styles recording the heterogeneity of the pre-rift crust. Correspondingly the seismic and gravity expression of the transition from continental crust to oceanic crust differs systematically along the length of the 'conjugate' basins. The presence of a conspicuous coast parallel positive linear gravity anomaly, referred to as the 'terminal horst', defines the basinward extent of attenuated continental crust in the more symmetrical Campos Basin and its interpreted conjugate. In the 'asymmetrical' Santos and Benguela Basins the ocean-continent transition at the present-day OCB (ocean-continent boundary) becomes equivocal with little gravity expression. Consequently the relationship of primary salt basin edges to the basinward extent of crustal extension appears more complex. Pre-existing crustal and lithospheric mantle scale heterogeneities are considered to impart a first-order control on whole crust deformation and ultimately 'sag' basin development. These structurally defined heterogeneities are believed to partition crustal strain and juxtapose end-member mechanisms of pure-shear and simple shear deformation styles as recorded by the complex distribution of syn-rift subsidence patterns within the pre-salt basins of the central South Atlantic. These conclusions challenge the necessity for invoking dominantly depth-dependent processes as a mechanism to explain apparent pre-salt, syn-rift subsidence anomalies. The complex interplay of structure and stratigraphy with an overlay of palaeoclimatic models provides important insights into hydrocarbon play fairways along the margin. In particular the impact on reservoir and source rock distribution and heat flow are discussed.
93 citations
TL;DR: In this paper, an accurate description of the workflow followed in the geocellular modelling of a fluvial-dominated continental formation is presented. But the work also qualitatively discusses the uncertainties identified during the workflow and proposes methods to reduce them.
Abstract: Outcrop analogue studies provide key information for reservoir modelling which is difficult to obtain from traditional subsurface datasets (i.e. seismic data, well data). Terrestrial laser scanners or LiDAR (light detection and ranging), combined with digital photography, provide a new technique to create high resolution 3D digital outcrop models (DOMs). These DOMs generate exhaustive information which is used to build more realistic three-dimensional facies-based geocellular models and populate the different model zones. This paper documents the use of an extensive dataset, which combines high resolution traditional field data and DOMs. We provide an accurate description of the workflow followed in the geocellular modelling of a fluvial-dominated continental formation. Geocellular facies association and connectivity models are well constrained by outcrop observations and a number of different techniques are used in the quality control of the final model. The study also qualitatively discusses the uncertainties identified during the workflow and proposes methods to reduce them. The workflow and results shown in this paper can be applied in similar analogue systems in order to help improve model building for subsurface reservoirs.
71 citations
TL;DR: In this paper, an interpretation of a volcanic margin structure based on regional mapping of high quality 2D seismic data in conjunction with regional satellite derived gravity data and selected subsidence analyses is presented.
Abstract: In this case study of the West Indian Continental Margin we present an interpretation of a volcanic margin structure based on regional mapping of high quality 2D seismic data in conjunction with regional satellite derived gravity data and selected subsidence analyses. The area shows many classic characteristics of a volcanic-type margin. Volcanic facies identified and mapped along the margin include seaward-dipping reflectors (SDRs), sub-aerial seamounts, clinoform packages interpreted as lava and volcaniclastic delta systems and thick, seismically layered sequences interpreted as volcanically derived sediment deposited in both fluvial and marine environments. The results show major variation in the overall thickness and style of volcanism across the margin both in dip and strike directions which may be related to variability in influence of the Deccan Plume in addition to localization along structural features inherited from older tectonic events. Our interpretation of rapid lateral variation in the thickness of extrusive volcanism has important implications for the distribution, preservation and hydrocarbon potential of the pre-rift sequence across the margin. The interpreted crustal structure also has a major impact on predictions of the historical and present-day heat-flow into the post-rift section. Our interpretation of the timing and distribution of volcanism is consistent with the presence of a broad region of elevated mantle potential temperatures at the time of the final break-up event on the West Indian Continental Margin, commonly attributed to the Deccan/Reunion Plume. Pre-existing structural heterogeneities appear to have played an important part in controlling the distribution of volcanism. Interpreted tectonic subsidence, based on backstripping analysis of the post-break-up interval, is also shown to be consistent with post-break-up thermal subsidence in combination with dynamic support associated with the elevated mantle temperatures into the Early Eocene.
54 citations
TL;DR: In this article, a workflow described in this paper combines digital structural restoration and extrapolation of major stratigraphic surfaces, grouped palaeocurrents and architectural geometries observed at outcrop to create 3D digital models.
Abstract: Submarine slope channel-fills form complicated stratigraphy and lithofacies distributions through repeated phases of erosion and deposition. This provides a challenge to accurate 3D modelling, particularly in representing lithofacies transitions within sand-poor areas. In this paper, traditional (sedimentary logs, palaeocurrent measurements, architectural panels) and non-conventional technologies (Light Detection and Ranging; Ground Penetrating Radar) were integrated to quantitatively describe lithofacies distributions and sedimentary architectures from two large-scale outcrops, one base of slope, high sandstone content system (Unit B) and one from a mid-slope, more mixed lithology system (Unit C), in the Laingsburg Formation, Karoo Basin, South Africa. The workflow described in this study combines digital structural restoration and extrapolation of major stratigraphic surfaces, grouped palaeocurrents and architectural geometries observed at outcrop to create 3D digital models. The model was divided into zones along major stratigraphic discontinuities and populated using lithofacies associations that were adjusted for outcrop rugosity and palaeodispersal direction. Observed channel margin asymmetries, distribution of lithofacies and stacking patterns were all honoured in the digital models. The Unit C slope-channel system differs from many exposed submarine channels due to the low proportion of sandstone present within the infill. Thin-bedded channel margin lithofacies are preserved through the lateral stepping of channels and allow the correlation of stratigraphy from channel axis to margin and on to overbank areas. In the older, sandier Unit B base-of-slope system, the stratigraphic change in stacking pattern, channel aspect ratio, lithofacies of channel-fills and stratigraphic hierarchy were all captured. This research captured the architectural complexity observed at outcrop to generate more realistic models than could be constructed normally using limited subsurface data.
47 citations
TL;DR: In this article, the rifted margins of the central segment of the South Atlantic are considered in terms of magmatism, symmetry and the amount and timing of extension, in comparison to the margins of North Atlantic, and it is suggested that the south Atlantic margins are probably moderately magmatic, with most of the magmatisms occurring in the Hauterivian-Barremian, after the onset of rifting in the Berriasian.
Abstract: The rifted margins of the central segment of the South Atlantic are considered in terms of magmatism, symmetry and the amount and timing of extension, in comparison to the margins of the North Atlantic. It is suggested that the South Atlantic margins are probably moderately magmatic, with most of the magmatism occurring in the Hauterivian–Barremian, after the onset of rifting in the Berriasian. Most of the crustal extension took place between the Berriasian and early Barremian, before focusing along a narrow line of breakup in the Aptian. The changing locus of the rifting with time led to the development of considerable late asymmetry, and the stranding of the large part of the salt basins on variously the African and South American margin. The amount of Berriasian–Barremian extension may have been underestimated and may be sufficient to explain most of the observed crustal thinning. The apparent limited subsidence by the time of deposition of the Aptian evaporites may result from either delayed thermal subsidence due to the influx of hot asthenosphere or indicate that Early Cretaceous global sea level was well above the local water level within this isolated, partly empty basin.
44 citations
TL;DR: The Peciko field contains gas in multiple stacked reservoirs within a Miocene deltaic sequence, and gas is trapped hydrodynamically by high lateral overpressure gradients as mentioned in this paper.
Abstract: The Peciko Field contains gas in multiple stacked reservoirs within a Miocene deltaic sequence. In the deeper reservoirs, gas is trapped hydrodynamically by high lateral overpressure gradients. We have analysed overpressure and compaction in this field by using wireline log, pressure, temperature and vitrinite reflectance data. The top of the overpressure is located below 3 km burial depth, below the depth range for transformation of discrete smectite to mixed-layer illite/smectite. Density-sonic and density-resistivity crossplots for mudrocks show reversals within the transition zone into hard overpressure below 3.5 km depth. Vitrinite reflectance measurements indicate that the start of unloading coincides with the onset of gas generation. Moreover, mudrock density continues to increase with depth in the overpressured section to values above 2.6 g cm –3 . We conclude that gas generation and chemical compaction are responsible for overpressure generation, contradicting previous interpretations that disequilibrium compaction is the principal mechanism for generating overpressure in the Lower Kutai Basin. The particular circumstances which make our radical interpretation plausible are that it is a warm basin with lateral reservoir drainage, so the overpressured mudrocks are probably overcompacted as a result of diagenesis.
TL;DR: In this article, the authors used two orthogonal sets of 2D seismic sections to demonstrate that the Arabian folds in the study area are due to folding during the Late Cretaceous and to additional folding during Late Cenozoic times.
Abstract: The study area is located in the foreland and in the frontal part of the Zagros Range (Persian Gulf), which contains very large oil and gas fields. The data consist of two orthogonal sets of 2D seismic sections. Folds trending NNE–SSW and N–S (Arabian trend) and NW–SE (Zagros trend) are documented. These folds developed during two folding phases: an older Late Cretaceous phase and a younger Late Cenozoic one. Folds with N–S and NNE–SSW trends formed during the older phase. During the younger phase the older, deep-seated, NNE–SSW Arabian folds were reactivated and tightened. These folds affected beds younger than the Cretaceous, forming open folds with the Arabian trend. At the same time, other folds with the NW–SE Zagros trend developed and these have been active until recent. Some giant and super-giant oil and gas fields are found in broad Arabian-trending folds in the Zagros foreland. This paper demonstrates that the Arabian folds in the study area are due to folding during the Late Cretaceous and to additional folding during Late Cenozoic times.
TL;DR: In this paper, 3D seismic and well data are integrated to determine the tectono-stratigraphic evolution of the SW margin of the Utsira High, northern North Sea rift system.
Abstract: 3D seismic and well data are integrated to determine the tectono-stratigraphic evolution of the SW margin of the Utsira High, northern North Sea rift system. During the Triassic, a series of minibasins formed due to passive diapirism of the evaporite-bearing, Upper Permian, Zechstein Supergroup. Subsequently, during the Jurassic, a series of secondary minibasins developed as the underlying salt walls collapsed. These minibasins were a few hundred metres deep, bound by sub-circular to elongate salt-cored structural highs and caused the development of complex subaerial topography and submarine bathymetry on the SW margin of the Utsira High. Salt withdrawal may have been related to: (i) partial dissolution of salt; (ii) differential erosion of the salt walls and adjacent Triassic-filled minibasins; or (iii) salt remigration caused by sub- or supra-salt extension or sediment loading. This study provides insights into the tectono-stratigraphic evolution of the SW margin of the Utsira High and has implications for (i) facies distribution of the Zechstein Supergroup within the northern North Sea rift system; and (ii) depositional system development, and thus reservoir distribution, within the Jurassic sedimentary succession.
TL;DR: In this paper, the structural and thermal evolution of the Ghana transform margin has been investigated using TECMOD2D, a suite of 2D realizations performed with TEC MOD2D modeling software for automated basin reconstructions.
Abstract: This study explores the structural and thermal evolution of the Ghana transform margin. The main objective is to explore how the opening of the Atlantic Ocean and subsequent interaction with the Mid-Atlantic Ridge (MAR) has affected the margin9s structural and thermal evolution. Two representative evolution scenarios are described: a reference case that neglects the influence of continental breakup and a second scenario that accounts for a possible heat influx during the passage of the MAR as well as magmatic underplating. These two scenarios have further been analysed for the implications for the hydrocarbon potential of the region. The scenario analysis builds on a suite of 2D realizations performed with TECMOD2D, modelling software for automated basin reconstructions. As the observed stratigraphy is input, the structural and thermal evolution of the basin is automatically reconstructed. This is achieved through the coupling of a lithosphere scale forward model with an inverse algorithm for model parameter optimization. We find that lateral heat transport from the passing MAR in combination with flexure of the lithosphere can explain the observed uplift of the margin. These results were obtained for a broken plate elasticity solution with a relative large value for the effective elastic thickness (Te=15) and necking level (15 km). Lateral heat flow from oceanic lithosphere is clearly visible in elevated basement heat flow values up to 50 km away from the ocean–continent transition (OCT). This influx of heat does not seem to have affected the maturation history along the margin significantly. Only the deepest sediments close to the OCT show slightly elevated vitrinite reflectance in simulations that account for the passage of the MAR. In conclusion, it appears that that lateral heat transport from the oceanic lithosphere is instrumental in shaping the Ghana transform margin but seems to have only limited control on the maturation history.
TL;DR: In this paper, the initial source potential of the Jurassic Khatatba Formation source rock as well as its kinetic parameters for hydrocarbon generation were evaluated within an exploration framework, and it was shown that maturation of the source rock does not occur below 140°C or a vitrinite reflectance of around 0.9%.
Abstract: The South Alamein study area is located in the Western desert of Egypt between the prolific petroleum trends of Razzak-Alamein and Baharriya. The petroleum system of the area has been evaluated within an exploration framework. Potential source rocks in the Cretaceous Alam El Bueib and Abu Roash Formations and the Cenozoic Apollonia Formation display good petroleum potential; however, they are thermally immature over a large part of the studied area. Compilation of existing data and new measurements have been carried out to determine the initial source potential of the Jurassic Khatatba Formation source rock as well as its kinetic parameters for hydrocarbon generation. Results show that maturation of the Jurassic source rock does not occur below 140 °C or a vitrinite reflectance of around 0.9%. This combination of temperature and vitrinite reflectance is not unusual for a mixed type II-type III source rock, but has crucial implications for exploration in the area. The main hydrocarbon kitchens are located northward of the study area and migration to the south began around 40 Ma ago. The hydrocarbons are primarily trapped in the crest of the Mesozoic tilted blocks. It is believed that both migration and trapping have been influenced by fault hydraulic behaviour. Since this behaviour is complex, a sensitivity analysis was carried out on the migration pathway compared with the fault sealing factor to understand the filling history of the structures.
TL;DR: In this paper, a large, well-calibrated 3D seismic volume, electrical well-log data and core samples from the Quadrant 53 area of the Southern North Sea provides a new-found basis for understanding the controls on gas production in the Wissey Field.
Abstract: Interpretation of a large, well-calibrated 3D seismic volume, electrical well-log data and core samples from the Quadrant 53 area of the Southern North Sea provides a new-found basis for understanding the controls on gas production in the Wissey Field, a successful test of the Upper Permian (Zechstein Group; Z3 Cycle) Plattendolomit Formation carbonate play in the Southern Permian Basin (SPB). The petrophysical assessment of wells combined with facies analysis demonstrates that the Plattendolomit Formation represents the northward progradation of an important upward-shoaling carbonate platform. The results show that the highest primary porosity values, lie within the uppermost reservoir sub-units of the Plattendolomit Formation, which consist of brecciated packstones and overlying oolitic grainstones developed at the shelf edge of the Z3 carbonate ramp and sealed by back-barrier lagoonal anhydritic mudstones (ascribed to the Scolt Head Formation). It can now be shown that the play fairway is enhanced where two (WSW–ESE and NNW–SSE-striking) fault trends intersect and cause fracturing of the reservoir interval, providing the additional permeability critical for reservoir production. Seismic stratigraphic studies demonstrate that early (pre-Cretaceous) normal fault sets were formed on both trends and were locally affected by contractional deformation (structural inversion) during the Late Cretaceous. As well as explaining the key geological factors that contribute to production success in the Wissey Field itself, the results provide a foundation for evaluating the prospective potential that the Plattendolomit Formation has elsewhere in the SPB.
TL;DR: The field of rock physics represents the link between qualitative geological parameters and quantitative geophysical measurements and has become an integral part of quantitative seismic interpretation and stands out as a key technology in petroleum geophysics.
Abstract: The field of rock physics represents the link between qualitative geological parameters and quantitative geophysical measurements. Increasingly over the last decade, rock physics has become an integral part of quantitative seismic interpretation and stands out as a key technology in petroleum geophysics. Ultimately, the application of rock physics tools can reduce exploration risk and improve reservoir forecasting in the petroleum industry.
TL;DR: In this paper, the proportionate distribution of surficial facies of three representative basins at different stages of rift evolution was mapped and calculated to show that long-range, axial fluvial deposits should not automatically be included as a significant part of dryland rift sedimentary facies models.
Abstract: Facies models for continental rift basins suggest longitudinal (axis-parallel) fluvial activity dominates sediment transport and deposition. In contrast, modern basins in the arid to semi-arid northern Basin and Range Province, USA, show axial drainage development to be characterized by short endorheic systems that contribute little to the basin fill. Mapping and calculation of the proportionate distribution of surficial facies of three representative basins at different stages of rift evolution show that basin fill is dominated by the deposits of transverse catchments, and axial fluvial deposits are restricted to a narrow corridor by the progradation of lateral systems. Drainage integration in these dryland rifts is limited by the moisture-stressed climate, with a resultant reduction in stream power, and the complex, tectonically-induced physiography, which limits potential drainage pathways. River systems that flow through multiple structural basins are rare, restricted to those systems with catchment headwaters lying outside the dryland climatic regime. These data imply that long-range, axial fluvial deposits should not automatically be included as a significant part of dryland rift sedimentary facies models. Sediment routing pathways in ancient dryland rift systems may be much shorter than commonly predicted, so affecting the spatial distribution of lithofacies. Climate, therefore, has a much stronger control on drainage and lithofacies at all stages of rift development than is generally stated.
TL;DR: In this paper, a petrographic and micro-thermometric study of fluid inclusions in Jurassic and Cretaceous sandstones from the Porcupine Basin, offshore Ireland was integrated with innovative fluorescence lifetime measurements of hydrocarbon-bearing fluids inclusions to determine the compositions of the fluids associated with diagenesis and post-diagenetic fluid migration.
Abstract: A petrographic and microthermometric study of fluid inclusions in Jurassic and Cretaceous sandstones from the Porcupine Basin, offshore Ireland was integrated with innovative fluorescence lifetime measurements of hydrocarbon-bearing fluid inclusions to determine the compositions of the fluids associated with diagenesis and post-diagenetic fluid migration and the extent of hydrocarbon and aqueous fluid migration pathways. Petrographic analyses indicate that Jurassic strata were the main fluid migration pathways for hydrocarbon fluids and that hydrocarbon migration occurred relatively late in the diagenetic history of these sandstones. UV fluorescence and fluorescence lifetime measurements have recognized at least two chemically distinct hydrocarbon groups (Types 1a and 1b) with dissimilar lifetime-wavelength (τ-λ) profiles, consistent with at least two petroleum charges derived from different sources. Primary aqueous inclusions in authigenic cements show that cementation in Cretaceous sandstones occurred at relatively shallow levels at low temperatures (
TL;DR: In this article, a workflow for the evaluation of gas hydrates is proposed, guided by field examples, set against the backdrop of a hydrate categorization scheme, which brings together the type mode of hydrate formation at the pore scale, the type class of hyrate occurrence at the reservoir zonal scale, and the resolvability of mesoscale hydrate-bearing layers by contemporary logging tools.
Abstract: Gas hydrates are recognized as a massive source of fossil fuel that could be far in excess of conventional hydrocarbon resources. The evaluation of formations that contain gas hydrates is therefore receiving renewed emphasis through contemporary petrophysical technology. A key factor is the use of logging-while-drilling (LWD) to sense hydrate-bearing intervals before drilling-induced thermal invasion and thence hydrate dissociation take hold. Recent advances in LWD technology have brought most of the potentially diagnostic tools onto the drill string, so there is little disadvantage in not having a wireline database. Moreover, modern tools have a sharper spatial resolution and a greater capability for differential depths of investigation. Petrophysical models have to be capable of distinguishing hydrates from ice in permafrost regions: this complication does not exist in the subsea environment. In general, pristine hydrates are characterized by high resistivity, low sonic transit time, and low density, possibly in conjunction with gas shows from mud logs. High neutron porosity can also be diagnostic away from permafrost. Other tools with a role to play include dielectric logs, for distinguishing ice from methane hydrate; electrical imagers, for identifying laminated hydrate formations; and magnetic resonance logs, for contributing to estimates of hydrate volume by difference, because of hydrate invisibility to these tools. The mode of hydrate formation is especially important, because a hydrate-supported structure will not produce as well as a framework-supported structure due to pore collapse with dissociation. A proposed workflow for the petrophysical evaluation of gas hydrates is guided by field examples. This is set against the backdrop of a hydrate categorization scheme, which brings together the type mode of hydrate formation at the pore scale, the type class of hydrate occurrence at the reservoir zonal scale, and the resolvability of mesoscale hydrate-bearing layers by contemporary logging tools. Although the formation evaluation of gas hydrates remains largely semi-quantitative, current interests are driving towards data-driven interpretation protocols that target estimates of producibility. Indicators are provided as to how this objective might be best approached.
TL;DR: Two linked asymmetrical half-grabens have been identified in the Cuyo basin: Cacheuta in the south and Las Penas-Tamberias in the north as discussed by the authors, showing significant lateral thickness variations that reflect the contrasting subsidence rates on the fault and flexural margins.
Abstract: Extensional relaxation due to the collapse of the active margin of Gondwanaland during the Triassic led to rapidly subsiding, fault-bounded half-grabens in west Argentina. The Cuyo rift basin was the largest of these fault-bounded troughs. Two linked asymmetrical half-grabens have been identified in the Cuyo basin: Cacheuta in the south and Las Penas-Tamberias in the north. Their stratigraphy exhibits a classic tripartite internal organization with a basal alluvial and fluvial section followed by a lacustrine interval which in turn is overlain by fluvial deposits. The basin fill in both half-grabens shows significant lateral thickness variations that reflect the contrasting subsidence rates on the fault and flexural margins. The lacustrine shales in the Cacheuta half-graben have an average total organic carbon (TOC) content of 4%, locally reaching 20%, dominated by type I, amorphous, algal organic matter and high hydrogen index (HI) values. The shales are associated with parasequences in river-dominated deltas. Oils derived from these source rocks are waxy and with low sulphur content. The oil shales are associated with sandstones arranged in parasequences deposited in river-dominated Gilbert-type deltas. This interval in the Cacheuta half-graben can be assigned to a slightly overfilled to balanced-fill lake type. In Las Penas-Tamberias, the dominant source rock facies in the lacustrine section is made up of calcareous shales with oil-prone, type I(II) kerogen and TOC values up to 13% and high HI values. The presence of gammacerane and b-carotane, common in saline conditions, is conspicuous. The presence of oolitic and bioclastic grainstones and microstromatolitic limestones on the ramp margin and clastic facies on the border fault suggests a slightly underbalanced to balanced lake type. The Cuyo rift basin branches to the NE into the Ischigualasto-Villa Union half-graben. Lacustrine shales along the fault margin of this half-graben are dominated by type III, gas-prone organic matter with TOC values up to 4% and low HI values. Parasequences with a strong progradational stacking pattern and steep front slopes are interpreted as mouth bars in a Gilbert-type delta. These characteristics are consistent with an overfilled lake basin type where sedimentation rate exceeds subsidence rate. The Triassic rift system of west Argentina shows the gamut of lacustrine source rocks that, combined with the analysis of diagnostic associated facies, allow the discrimination of lake basin types and their influence in the resulting hydrocarbon phase.
TL;DR: In this paper, the authors presented a mapping and seismic sequence stratigraphic interpretation of parts of a c. 450 m thick, mid-Eocene delta extending over some 3500 km 2 in the Faroe-Shetland Basin.
Abstract: During the last decade several 3D digital reflection seismic datasets have been acquired in the Faroese sector of the Faroe–Shetland Basin which allow detailed seismic interpretation and mapping of parts of the area. This study presents mapping and seismic sequence stratigraphic interpretation of parts of a c . 450 m thick, mid-Eocene delta extending over some 3500 km 2 in the Faroe–Shetland Basin. The delta built out into the basin from the Munkagrunnur Ridge and has been divided into nine seismic units. Isochore maps of these seismic units reveal a NW–SE depositional maximum trend, parallel to the slope front of the delta in most units. The dip of the slope front of these units is about 2.3–3.8°. The lowest unit has the outer form of a thin sheet, whereas the upper unit, a basinward divergent wedge, is mostly constrained to the shelf area. Using the outer form of the seismic units and reflector terminations against the bounding horizons, the surfaces separating the nine mapped units are interpreted as either maximum regression surfaces (MRS) or surfaces representing the start of base level fall (BSFR) or a combination of these two types of surface. Surfaces of regression and erosion are observed within several units and, with additional information regarding the seismic character and nature of internal unconformities and systems tracts, the nine seismic units are divided into systems tracts deposited during eight cycles of relative sea level change. Within the study area falling stage and lowstand systems tracts (FSST and LSST) are generally better preserved than transgressive and highstand systems tracts (TSST and HSST). Maximum flooding surfaces separating a TSST from the overlying HSST are not recognized, and in several sequences the TSST and HSST are either absent or so thin that they cannot be resolved in the seismic data. The slope front of the delta was first developed in the lower, aggradational part of the delta and retained during the upper, progradational part, where the delta evolved as a small shelf margin delta. Adding realistic thermal subsidence to the global sea level curve indicates that the Faroe–Shetland Basin was characterized by almost continuous relative sea level rise in the mid Eocene. Multiple stages of uplift of the Munkagrunnur Ridge during the mid-Eocene are considered to be the most likely explanation for the multiple cycles of sea level change inferred in the delta.
TL;DR: In this paper, a series of tests were carried out employing a modified triaxial cell able to measure both elastic and electric properties simultaneously at reservoir conditions, and a good correlation was obtained in most cases.
Abstract: A series of tests were carried out employing a modified triaxial cell able to measure both elastic and electric properties simultaneously at reservoir conditions. Measured quantities such as porosity, permeability, resistivity and P- and S-wave velocities were cross plotted, and possible correlations were investigated. Identified experimental trends were then tested against a variety of rock physics models. A good correlation was obtained in most cases. Such results together with petrophysical logging data can be used to constrain possible joint inversion of controlled source electromagnetic data and seismic data. The simultaneous use of resistivity and acoustic measurements to monitor a water-substituting-oil process at reservoir conditions was also investigated. This study indicated that the electrical properties are much more sensitive to changes in saturation in the case of water flooding for enhanced oil recovery than the elastic properties.
TL;DR: In this article, the authors show that Miocene graben fill accumulated in the hanging wall of the east-west-trending first fault system, which has a well-developed wedge geometry.
Abstract: The east–west-trending Alasehir graben is one of the prominent structures in western Turkey proved to have hydrocarbon potential. Previously published seismic profiles have been re-interpreted and they show that Miocene graben fill accumulated in the hanging wall of the east–west-trending first fault system, which has a well-developed wedge geometry. It is also clearly seen in the seismic profiles that the relatively younger, second and third fault systems are nicely merged with the first fault system, indicating that a rolling hinge mechanism was working in the Alasehir graben.
TL;DR: In this article, 676 fine-grained rocks from 30 wells were analyzed for bulk parameters (total organic carbon (TOC), carbonate, sulphur, RockEval), and a subset of samples were selected for maceral and biomarker analysis, pyrolysis-gas chromatography and kinetic investigations.
Abstract: The Dniepr–Donets Basin (DDB) is a major petroleum province in Eastern Europe. In order to understand the regional and stratigraphic distribution of source rocks for the dominantly gas-prone petroleum system, 676 fine-grained rocks from 30 wells were analysed for bulk parameters (total organic carbon (TOC), carbonate, sulphur, RockEval). A subset of samples was selected for maceral and biomarker analysis, pyrolysis-gas chromatography and kinetic investigations. Organic-rich sediments occur in different intervals within the basin fill. Maximum TOC contents (5.0 ± 1.9%) occur in the Rudov Beds, several tens of metres thick. The oil-prone rocks (Type III–II kerogen) were deposited in basinal settings above an unconformity separating Lower and Upper Visean sections. While maximum TOC contents occur in the Rudov Beds, high TOC contents are observed in the entire Tournaisian and Visean section. However, these rocks are mainly gas condensate-prone. Highly oil-prone black shales with up to 16% TOC and hydrogen index values up to 550 mgHC g –1 TOC occur in Serpukhovian intervals in the northwestern part of the DDB. Oil-prone Lower Serpukhovian and gas condensate-prone Middle Carboniferous coal is widespread in the southern and southeastern part of the basin. Although no source rocks with a Devonian age were detected, their presence cannot be excluded.
TL;DR: The Molve Field is the most important gas-condensate reservoir in Croatia as mentioned in this paper, which includes cataclased granite, gneiss, schists, quartzites, dolomites, limstones and grainstones.
Abstract: The Molve Field is the most important gas-condensate reservoir in Croatia. This petroleum system is not typical for the Pannonian System, because it comprises several reservoir lithologies, relatively high structural closure and significant tectonic influence on the field9s compartmentalization. Strike-slip extension in the Middle Miocene and younger Late Miocene and Pliocene tectonics formed the present-day tectonic setting. Reservoir stratigraphy includes four lithofacies (from Devonian to Neogene) with a unique gas-water contact. The lithologies encompass cataclased granite, gneiss, schists, quartzites, dolomites, limestones and grainstones. Source rocks were generated in lacustrine organic facies and migration occurred in the Late Miocene to Pliocene. Reservoir gas includes 4.5–15.7% C 2+ , but also non-hydrocarbon components. Analysed porosity data were approximated with a normal-distribution curve in lithofacies I, II and III, making it possible to calculate mean and variance easily by descriptive statistics. Moreover, gas production and effective thicknesses generally can be linked through a linear trend. However, significant deviations in the expected increased production rate with regard to greater reservoir thickness are observed for particular wells. This is a result of locally abrupt changes in effective porosities and permeabilities, and the size of the drainage area along the main fault zones. These faults resulted in significant compartmentalization of the field. Furthermore, owing to significant facies variations, permeability and porosity gradually change, especially in the vertical direction. Significant reserves of condensate (3 × 10 6 m 3 ) and gas (43 500 × 10 6 m 3 ) with a high recovery rate of 71% make this field significant for geological reservoir models. The well-established geological model for this field and its stable high pressure have maintained production rates at a present level of approximately 2900 m 3 gas and 165 m 3 condensate per day, thus providing a valuable example for other large heterogeneous reservoirs in the Pannonian Basin.
TL;DR: In this article, the potential use of the Hurst test as a statistical tool to discriminate sub-environments within geologically complex turbiditic units that fill a confined basin with well-exposed facies transitions and onlaps, at the scale of several stacked reservoirs (Cengio and Bric la Croce-Castelnuovo Turbidite Systems in the Tertiary Piedmont Basin, Oligocene, northern Italy).
Abstract: Turbidite sequences within confined basins constitute important hydrocarbon reservoirs world-wide and, for this reason, the discrimination of sedimentary sub-environments based on an objective statistical method is of interest for pure and applied science. We investigated the potential use of the Hurst test as a statistical tool to discriminate sub-environments within geologically complex turbiditic units that fill a confined basin with well-exposed facies transitions and onlaps, at the scale of several stacked reservoirs (Cengio and Bric la Croce–Castelnuovo Turbidite Systems in the Tertiary Piedmont Basin, Oligocene, northern Italy). In vertical stratigraphic sections, the Hurst test determines the degree of clustering of low and high values of sedimentological variables, such as bed thickness, grain size and sand/mud ratio, which are dependent on sub-environments of deposition. We applied the Hurst test to depocentral and marginal sub-areas across the basin (parallel and perpendicular to the main palaeocurrent direction), documenting a different clustering of thick and thin beds, and of high and low values of the sand/mud ratio, in the depocentre–distal sector with respect to the onlap areas. A new field (onlap sub-environment) could thus be added to the classification diagram of turbidite settings based on the Hurst index. The Hurst phenomenon (clustering of high and low values of the selected variables) was also able to distinguish between proximal and distal (depocentral) lobe settings, and to recognize the fingerprint of the different depositional lobes (fully confined aggrading, prograding, backstepping). The map of turbidite sub-environments obtained by interpolation of the Hurst index is quite comparable to the field-observed facies map, providing impressive robust validation of the Hurst statistics. This method seems to represent a very promising predictive tool for subsurface studies of turbiditic oil fields based on core and log analyses.
TL;DR: In this paper, the authors propose a new approach to determine reasonable fluvial channel sizes with indicator variograms, based on the relationship between the channel geometry and the variogram.
Abstract: Geological models of fluvial reservoirs are often constructed with object-based modellling techniques. This approach requires a specification of the distribution of channel sizes determined traditionally from core and well log data. The problem with these data is that they only show the apparent thicknesses of the channel objects: sometimes the thicknesses are too small because the well intersects the channel at the edge and sometimes the thicknesses are too large because channels are stacked. This paper proposes a new approach to determine reasonable fluvial channel sizes with indicator variograms. The relationship between the channel geometry and the variogram is established. The relationship is verified with synthetic examples. The calculated indicator variogram range is shown to be insensitive to channel width and width/thickness ratio. Two real reservoir examples show how the approach works in practice.
TL;DR: In this paper, the authors integrate densely sampled reflection data with deep velocities from OBS data to give a common velocity model in a new, iterated, pre-stack depth-migration workflow using complementary constraints from the two datasets.
Abstract: Stacked basalt flows cover much of the NW European continental margin, including potentially prospective sediments of the Faroes shelf. Such flows attenuate seismic energy, hindering sub-basalt structural imaging which is critical for both exploration and tectonic studies. Low-frequency, long-offset reflection surveys have yielded improved images below top basalt, while coincident ocean-bottom seismometer (OBS) data have mapped low velocity zones (LVZ) from tomographic inversion. Image correlation in a common depth domain is challenged by low spatial resolution of the tomographic image, the absence of turning rays in a LVZ and the lack of wide-angle arrivals in the reflection data. We integrate densely sampled reflection data with deep velocities from OBS data to give a common velocity model in a new, iterated, pre-stack depth-migration workflow using complementary constraints from the two datasets. The matched velocity model and depth image enable interpretation of (i) a uniform flood basalt sequence, 2–4 km thick beneath the Fugloy Ridge, with velocities correlating with those in the Lopra 1/1A borehole and (ii) a sub-basalt LVZ with chaotic reflections corresponding to sill-intruded, probably syn-rift sediments. Such a workflow could be extended to targets beneath high velocity salt or basalt and could provide constraints for 3D datasets.
TL;DR: In this article, an artificial neural network was used to predict the saturation in the complex Gharif Formation in Oman, where different neural network models were developed using different input variables, including resistivity, cation exchange capacity and porosity.
Abstract: A well-known method to determine the hydrocarbon saturation distribution in a reservoir model is by using a saturation-height function derived from capillary pressure measured on core samples. This approach fails, however, in complex formations and does not use information from wireline logs. In this paper we use an artificial neural network to develop a saturation-height function for the complex Gharif Formation in Oman to predict the hydrocarbon saturation. Different neural network models were developed using different input variables. The optimal model was able to generate the saturation-height function with an error of 0.046 (fraction of pore volume, PV) using wireline logs, including the logarithm of resistivity, cation exchange capacity and porosity. This is a considerable improvement over conventional methods based on capillary pressure. The neural network model was then used to predict the saturation in the formation as a function of depth, and robust results were obtained.