bluuebunny/crossref_metadata_2025 huggingface dataset:
Petrophysical characterization of a heterolithic tidal reservoir interval using a process-based modelling tool
Authors: Nordahl Kjetil , Ringrose Philip S. , Wen Renjun
Published: October 2008

Heterolithic lithofacies in the Jurassic Tilje Formation, offshore mid-Norway, consist of three components – sand, silt and mud intercalated at the centimetre scale – and are generally difficult to characterize petrophysically with core and wireline data. A near-wellbore model of the lower part of the Tilje Formation in the Heidrun Field is constructed to illustrate the application of these results to formation evaluation studies. The sedimentological model is developed by detailed parameterization of a cored well interval and the petrophysical properties are based on core plug data, taking into account sampling bias and length scale. The variation in petrophysical properties as a function of sample volume is examined by calculating the representative elementary volume. The sensitivity of the representative permeability values to the contrast between the three components is studied and gives a better understanding of the flow behaviour of this system. These results are used to rescale the core plug data to a representative value and thereby quantify the uncertainty associated with the wireline-based estimates of porosity and horizontal permeability and to give an improved estimate of the
k
v
/
k
h
ratio.
Heat flow in the Vøring Basin, Mid-Norwegian Shelf
Authors: Ritter Ulrich , Zielinski Gary W. , Weiss Hermann M. , Zielinski Robyn L. B. , Sættem Joar
Published: October 2008

In situ
temperature and heat flow were determined in 1994 at 159 sites, grouped into 66 clusters between latitude 65° N and 67°30’ N at water depths from 669 m to 1464 m. The mean of all cluster heat-flow measurements conducted in this survey was 58.5 mW m
−2
, with a standard error of ±4.40 mW m
−2
. The mean heat flow from IKU well data for the Trøndelag Platform is 56.2±6.65 mW m
−2
. Shorter wavelength heat-flow variations appear to be controlled structurally and can be explained by sedimentation and thermal refraction effects. High heat flow associated with faulted structural highs such as the Nyk High and Vema Dome–Rym Fault Zone may also result from hydrothermal convection. Relatively isolated high (106.6 mW m
−2
) heat flow observed at 846 m water depth may be an artefact of bottom water disturbances; however, virtually identical deep-water heat-flow anomalies, believed to be of hydrothermal origin, also exist. While heat-flow measurements made at water depths less than 1000 m should be regarded with caution, there is presently no justification for eliminating those exhibiting linear heat flow with depth. Submarine avalanches seem unimportant in the survey area. Neither crustal thinning, underplating nor sill intrusion, within the last 50 Ma, would have a measurable effect on present-day heat flow. The net effect of crustal thinning may be a reduction of the crustal heat generation potential, depending on the degree of thinning of the upper crust, since the accumulating sediments cannot compensate fully for the lost heat generation from a crystalline basement.
3D high-resolution digital models of outcrop analogue study sites to constrain reservoir model uncertainty: an example from Alport Castles, Derbyshire, UK
Authors: Pringle J. K. , Westerman A. R. , Clark J. D. , Drinkwater N. J. , Gardiner A. R.
Published: October 2008

Advances in data capture and computer technology have made possible the collection of 3D high-resolution surface and subsurface digital geological data from outcrop analogues. This paper describes research to obtain the 3D distribution and internal sedimentary architecture of turbidite channels and associated sediments at a study site in the Peak District National Park, Derbyshire, UK. The 1D, 2D and 3D digital datasets included Total Station survey, terrestrial photogrammetry and remote sensing, sedimentary logs and a Ground Penetrating Radar (GPR) dataset. A grid of 2D GPR profiles was acquired behind a cliff outcrop; electromagnetic reflection events correlated with cliff face sedimentary horizons logged by Vertical Radar Profiling. All data were combined into a Digital Solid Model (DSM) dataset of the site within reservoir modelling software.

The DSM was analysed to extract 3D architectural geometries for petroleum reservoir models. A deterministic base model was created using all information, along with a suite of heterogeneous turbidite reservoir models, using 1D, 2D or 3D information. The model suite shows significant variation from the deterministic model. Models built from 2D information underestimated connectivity and the continuity of geobodies, but overestimated channel sinuosity. Advantages of using 3D digital outcrop analogue data for 3D reservoir models are detailed.
Thermal history and oil charge on the UK Atlantic margin
Authors: Parnell J. , Green P.F. , Watt G. , Middleton D.
Published: October 2008

Integrated apatite fission track analysis and vitrinite reflectance data show that well 204/19-1 in the West of Shetland region, UK Atlantic margin, has experienced only limited additional burial beyond present-day depths. Uplift and cooling to present-day levels probably occurred during late Cenozoic (Eocene to Miocene) basin inversion. Fluid inclusion data indicate that Paleocene–Eocene sandstones have experienced temperatures much higher than can be explained by burial alone. Temperatures up to 200±°C indicate the passage of hot fluid through Cenozoic sandstones, which by-passed the pre-Cenozoic section in this and other wells. The hot fluid event must have been of very brief duration (up to 100 years) to show no record in the fission track and reflectance data, implying that the fluids migrated through fracture systems.

Oil inclusions in the Cretaceous of well 204/19-1 have a chemistry that suggests derivation from a Kimmeridgian-aged source rock. They occur in cements that show no evidence for the hot fluid event and it is concluded that the cements pre-date the event. Oil inclusions in Cenozoic sandstones have a heavy, degraded character and were trapped at high temperature, suggesting that degradation was related to the hot fluid event. Present-day oils in the West of Shetland region are mixtures, which could reflect components from the two charges distinguished by the integrated thermal and geochemical histories. The inference of fracture-bound flow is consistent with existing models of overpressure development and hydrofracturing.
Reserve growth in oil fields of the North Sea
Authors: Klett T. R. , Gautier Donald L.
Published: October 2008

The assessment of petroleum resources of the North Sea, as well as other areas of the world, requires a viable means to forecast the amount of growth of reserve estimates (reserve growth) for discovered fields and to predict the potential fully developed sizes of undiscovered fields. This study investigates the utility of North Sea oil field data to construct reserve-growth models. Oil fields of the North Sea provide an excellent dataset in which to examine the mechanisms, characteristics, rates and quantities of reserve growth because of the high level of capital investments, implementation of sophisticated technologies and careful data collection. Additionally, these field data are well reported and available publicly.

Increases in successive annual estimates of recoverable crude oil volumes indicate that oil fields in the North Sea, collectively and in each country, experience reserve growth. Specific patterns of reserve growth are observed among countries and primary producing reservoir-rock types. Since 1985, Norwegian oil fields had the greatest volume increase; Danish oil fields increased by the greatest percentage relative to 1985 estimates; and British oil fields experienced an increase in recoverable oil estimates for the first ten years since 1985, followed by a slight reduction. Fields producing primarily from clastic reservoirs account for the majority of the estimated recoverable oil and, therefore, these fields had the largest volumetric increase. Fields producing primarily from chalk (limestone) reservoirs increased by a greater percentage relative to 1985 estimates than did fields producing primarily from clastic reservoirs. Additionally, the largest oil fields had the greatest volumetric increases. Although different reserve-growth patterns are observed among oil fields located in different countries, the small number of fields in Denmark precludes construction of reserve-growth models for that country. However, differences in reserve-growth patterns among oil fields that produce from primarily clastic and primarily chalk reservoirs, in addition to a greater number of fields in each of the two categories, allow separate reserve-growth models to be constructed based on reservoir-rock type.

Reserve-growth models referenced to the date of discovery and to the date of first production may be constructed from North Sea field data. Years since discovery or years since first production are used as surrogates for, or measures of, field-development effort that is applied to promote reserve growth. Better estimates of recoverable oil are made as fields are developed. Because much of the field development occurs some time later than the field discovery date, reserve-growth models referenced to the date of first production may provide a more appropriate measure of development than does date of discovery.
Quantification of exhumation in the Eromanga Basin and its implications for hydrocarbon exploration
Authors: Mavromatidis Angelos , Hillis Richard
Published: October 2008

Exhumation in the Eromanga Basin of South Australia and Queensland has been quantified using compaction methodology. All methods of estimating exhumation utilize rock properties that are affected by, and retain a memory of, burial in excess of that presently observed. The tool used for estimating the exhumation in this study is analysis of the degree of overcompaction of rock units. Since porosity describes compaction state, the sonic log, controlled strongly by the amount of porosity, is an appropriate indicator of compaction and, hence, is used for quantifying exhumation from compaction. The standard method of estimating exhumation based on the degree of overcompaction of a single shale unit has been modified, and seven units, predominantly shales ranging in age from the Cretaceous to the Jurassic, have been analysed. All units yield similar results. Burial at depth greater than currently observed is the most likely cause of overcompaction since it is unlikely that sedimentological and/or diagenetic processes are responsible for similar amounts of overcompaction in different lithologies. The results of the compaction analysis reveal that Late Cretaceous–Tertiary exhumation increases eastwards from the Patchawarra Trough, through the Gidgealpa–Merrimelia–Innamincka Trend and Nappamerri Trough into the Queensland sector of the basins. This study has major implications for hydrocarbon exploration. Predicted maturation of source rocks will be greater for any given geothermal history if exhumation is incorporated in maturation modelling. The exhumation study helps to quantify velocity anomalies associated with overcompaction. Exhumation values can also be used to improve porosity predictions of reservoir units in undrilled targets.
Ekofisk Field: fracture permeability evaluation and implementation in the flow model
Authors: Toublanc A. , Renaud S. , Sylte J. E. , Clausen C. K. , Eiben T. , Nådland G.
Published: October 2008

The Ekofisk Field is a naturally fractured chalk reservoir located in the Norwegian sector of the North Sea. The natural fractures clearly control the permeability distribution, as the effective permeability can reach 50 mD whereas the matrix permeability only ranges between 0.1 mD and 10 mD. Permeability mapping in this field has been challenging due to the structural, stratigraphic and mineralogical complexity, tectonic history and non-negligible matrix permeability. A detailed fault interpretation has resulted in a complex fault pattern. A fault intensity (P21) parameter calculated from the fault pattern has proved to be the critical component for permeability mapping. Correlations were found between the fault intensity (P21) values and (1) the fracture distributions from cores and logs at individual wells, and (2) the fracture component of the well test permeability (
K
frac
– total permeability less matrix permeability). These relationships allowed field-wide fracture permeability maps to be developed based on the P21 results. Total permeabilities were obtained by summing the matrix permeability and the calculated fracture permeability. These permeability maps were introduced in the Ekofisk Flow Model 2002 and refined to match the rate performance of the 50 initial wells run in prediction mode (well head pressure constraint). The runs in prediction mode have proved to be very effective for calibrating the permeability distributions, based on initial well performance above the bubble point. This simulation technique was extended to cover all producers (262 wells) during the entire history of the field to refine the maps further. After calibration with the performance data, a satisfactory history match was obtained by making minor changes to permeability and other dynamic parameters. Additionally, the running of the model of the mature Ekofisk Field in prediction mode for its full field life has provided a robust tool for calibrating field performance.
Sub-basalt structures east of the Faroe Islands revealed from wide-angle seismic and gravity data
Authors: Raum T. , Mjelde R. , Berge A. M. , Paulsen J. T. , Digranes P. , Shimamura H. , Shiobara H. , Kodaira S. , Larsen V. B. , Fredsted R. , Harrison D. J. , Johnson M.
Published: October 2008

Two semi-regional wide-angle Ocean Bottom Seismograph (OBS) profiles, acquired east of the Faroe Islands, have been analysed by use of forward and inverse modelling to map the crustal structures. In the present wide-angle data, the Tertiary basalt shows a maximum thickness of 3 km under the Faroe Islands, decreasing towards the Faroe–Shetland Channel where it terminates. Sedimentary rocks are present below the basalts and vary in thickness from 2 km to a maximum of 8 km towards the Faroe–Shetland Channel. These sedimentary rocks appear mainly as a low-velocity zone, and the presence of high-velocity intrusions in these layers generate several step-back features in the wide-angle refraction data. Pre-Cretaceous sedimentary rocks are only inferred north of the Clair fracture zone, while Cretaceous rocks dominate southeast of the Westray fracture zone. The crystalline basement is divided into an upper granitic and a lower granodioritic part. P-wave velocities around 7.0 km s
−1
are modelled in the lowermost part of the crust, indicating that magmatic underplating is not present below the Faroe Islands. The depth to the Moho is modelled with a maximum depth of 29–30 km below the northern part of the Faroe Islands, decreasing both southeastwards and southwestwards to 25 km and 17 km, respectively.
Reservoir challenges of heterolithic tidal sandstone reservoirs in the Halten Terrace, mid-Norway
Authors: Martinius A. W. , Ringrose P. S. , Brostrøm C. , Elfenbein C. , Næss A. , Ringås J. E.
Published: October 2008

Production from the Halten Terrace hydrocarbon province (Mid-Norwegian shelf) is mainly from heterolithic siliciclastic successions as well as diagenetically altered sandstones. Eight hydrocarbon fields are currently in production, which produce
c
. 840 000 BBL oil equivalent per day, with several new fields expected to come on stream in the next decade. This paper is an introduction to a thematic set on the characterization and modelling of heterolithic reservoirs and focuses on the three main types of heterogeneity: (1) heterolithic facies, (2) faulting and (3) diagenesis. Challenges vary according to field setting: shallow (1–3 km burial depth), deep (3–5 km) or very deep (currently up to 5.6 km). Water depths vary from 200 m to 500 m. Heterolithic sedimentary packages are composed of shale or siltstone layers intercalated with clean, but often thin, sandstone layers of varying lateral extent. These were deposited in Lower Jurassic tide-influenced or tide-dominated deltaic and estuarine environments along the margin of a shallow seaway. Hydrocarbon traps are formed by faulted and rotated fault blocks created during rifting. Faulting of these heterolithic facies is a critical parameter for fluid flow, with fault transmissibility and fault position often difficult to determine. Complex patterns of diagenetic cementation are an additional aspect of heterogeneity in the deeply buried reservoirs, such as the Smørbukk and Kristin fields. However, grain coatings of chlorite, illite/chlorite and illite have prevented or hindered the development of quartz overgrowths and allowed the preservation of anomalously high porosity and permeability. Modelling and assessing the impact of these reservoir uncertainties has included development of novel tools and methods, leading to a much-improved level of understanding, better prediction of recoverable reserves and significantly increased recovery factors.
Characterization of deeply buried heterolithic tidal reservoirs in the Smørbukk Field using inverted post-stack seismic acoustic impedance
Authors: Klefstad L. , Kvarsvik S. , Ringås J. E. , Stene J. J. , Sundsby O.
Published: October 2008

Deeply buried reservoirs in Smørbukk Field play an important role in oil production from the Halten Terrace hydrocarbon province (Mid-Norwegian shelf). These heterolithic reservoir sandstones are sedimentary systems for which estimation of reservoir properties is challenging. The main problem is to derive realistic models of the lateral changes of porosities and permeabilities in the inter-well space. There is a very significant upside potential in producible reserves if infill production wells can be placed and completed in an optimal position with respect to reservoir quality.

In this paper reservoir characterization work, based on good quality seismic data, has been performed in order to predict porosity, net: gross ratio and permeability both qualitatively and quantitatively supported by reliable geological models. Essentially, Acoustic Impedance (AI) is correlated to porosity and inverted seismic data have been used to predict reservoir properties. Two different porosity modelling methods in the Tilje Formation are used: (1) by means of simple regression; (2) using geostatistical methods.

Lateral changes in porosity follow a pattern that is sedimentologically plausible. A geostatistical workflow resulted in porosity maps that take into account both lack of vertical resolution in seismic data and poor lateral resolution in well data. This is considered to be the most optimal method for predicting geologically acceptable reservoir properties, because uncertainties in porosity modelling are measured. Porosity maps were used to update the reservoir simulation model and adjust targets for infill wells in the Smørbukk Field. Results were tested in two wells drilled after utilizing the seismic AI and showed an excellent match.
North Sea Quaternary morphology from seismic and magnetic data: indications for gas hydrates during glaciation?
Authors: Fichler Christine , Henriksen Sverre , Rueslaatten Haakon , Hovland Martin
Published: October 2008

Buried circular depressions and channels in Quaternary strata have been investigated by 3D seismic data and, less commonly, by high-resolution aeromagnetic data. Sub-glacial melt-water drainage channels of various dimensions are the most distinct morphological features. In the same strata, numerous crater-shaped depressions were found, some coinciding with the initiation of channels. The diameter ranges from 500 m to 3000 m and the depth from 20 to 300 m – appreciably larger than common pockmarks. It is argued here that the craters were generated by gas expulsion from melted gas hydrates, combined with melt-water expulsion and erosion. This is supported by: (1) a similarity with published seafloor craters that originate from melted gas hydrates; (2) appropriate physical conditions for the formation and melting of gas hydrates during glacial and interglacial periods; (3) correlation with shallow gas occurrences and seismic gas indications. An increased number of craters occurs above a major fault and some Tertiary hydrocarbon discoveries, which may indicate thermogenic gas. At such locations, an increase in high-frequency magnetic anomalies can be explained by deposition of sediments with contrasting magnetic susceptibilities. This is an alternative explanation for the occasionally increased shallow magnetic anomalies above hydrocarbon fields, otherwise attributed to secondary changes in the magnetic mineralogy.
Sandstone porosity related to vitrinite reflectance
Authors: Wangen Magnus
Published: October 2008

Vitrinite reflectance (VR) and the volume fraction of quartz cement in sandstones are both measures of thermal exposure. This paper shows how porosity reduction by quartz cementation can be related to VR for heating at a constant rate, using simple models for quartz cementation and VR. This relationship reveals that the heating rate is an explicit parameter, which means that there is no pure relationship between porosity (volume fraction quartz cement) and VR. However, the rate of quartz cementation and the rate of change of TTI (time–temperature integral) have a close exponential dependence on temperature. Porosity reduction by quartz cementation as a function of VR therefore nearly follows the same curve for very different temperature histories. This property of porosity reduction by quartz cementation is used to calibrate average specific surface parameters as a function of porosity. The calibration procedure is demonstrated on two datasets, where the average specific surface depends on porosity to the power of an exponent that is somewhat greater than 3.
Advances in seismic imaging through basalts: a case study from the Faroe–Shetland Basin
Authors: Spitzer Roman , White Robert S. ,   iSIMM Team
Published: October 2008

New seismic reflection data have been used to image intra- and sub-basalt features beneath the Faroe–Shetland Basin in the North Atlantic, where the highly reflective top and base boundaries of flood basalts and their complex internal structure make successful seismic imaging difficult. This study demonstrates that appropriate acquisition and processing of marine seismic data from hydrophone streamers and ocean-bottom seismometers (OBS) has the potential to enhance significantly imaging of intra-basalt and sub-basalt seismic reflections. The intersection of a new seismic reflection profile recorded in 2002 with a seismic profile recorded in 1998 allows a direct comparison of advances in sub-basalt imaging over this period and an interpretation of geological structures and seismic velocities at the intersection.

To achieve better resolution of sediments below basaltic layers, surface seismic reflection data using a broad-band, low-frequency source have been recorded. By using a source wavelet with a significantly enhanced frequency spectrum centred at 10 Hz, generated from a large (167 l) airgun array tuned to the first bubble pulse, a wavelet is produced that is capable of penetrating through thick basaltic sequences. Furthermore, recording the low-frequency reflections to large offsets along a 12 000 m long streamer with 3.125 m single sensor spacing allowed effective suppression of multiple reflections, enhanced the precision of velocity analyses and produced better migration results. Seismic data along the same profile were also recorded on a dense deployment of 85 four-component OBS at 2 km or 6 km station spacing. These complementary ocean-bottom data allowed the large-scale velocity variations within and beneath the basalt to be constrained by recording at large offsets the termination of the diving rays (
c
. 18 km) propagating through the basalt.

Integration of seismic reflection data and OBS recordings yields good seismic velocity estimates and, therefore, increased confidence in the interpretation of intra- and sub-basalt features and lithology. Four major stratigraphic units characterized by their seismic velocities and reflection characters were identified: (a) post-basalt sediments; (b) basalt sequence; (c) sub-basalt unit; and (d) the top of presumed Cretaceous basement. The top boundary of the basalt sequence is defined by a strong reflection with a steep velocity increase from 3.5 km s
−1
to 4.5 km s
−1
in the
c
. 200 m thick layer above it. The basalt sequence here is
c
. 2.5 km thick. Beneath the base of the basalt there is a velocity decrease from 5.8 km s
−1
to 5.2 km s
−1
. The sub-basalt unit is subdivided further into hyaloclastites mixed with sediments and sills, and sub-basalt sediments separated by another velocity inversion from 5.2 km s
−1
to 4.5 km s
−1
. The Cretaceous basement with velocities above 5.5 km s
−1
lies underneath these sub-basalt sediments.
The Deep-Water Architecture Knowledge Base: towards an objective comparison of deep-marine sedimentary systems
Authors: Baas Jaco H. , McCaffrey William D. , Knipe Robert J.
Published: October 2008

A quantitative method is proposed for the comparison of deep-marine clastic depositional systems and the analysis of their architectural properties. The method comprises a knowledge base of quantitative, literature-derived information from modern and ancient, surface and subsurface deep-water systems, implemented as a relational database management system and referred to as the Deep-Water Architecture Knowledge Base (DWAKB). The types of information contained within the knowledge base include: (1) internal and external parameters controlling system architecture, such as tectonic setting, grain size of available sediment, degree of basin confinement and number and distribution of sediment input points; (2) the dimensions of architectural elements, such as channels, levees, lobes, open slope and basin plain; (3) the spatial organization of the architectural elements; and (4) the bed thickness distribution and proportion of different lithologies within the architectural elements. The potential value of the DWAKB for comparative studies of deep-marine clastic systems is considerably higher than that of classification schemes and system analogue concepts presently available. Thus, in contrast to classification schemes, the knowledge base is not limited in the number of controlling parameters and it does not have a limited time span because of the flexibility to update existing records and add new datasets. Moreover, system analogues can be selected more objectively through the use of statistical methods, and the knowledge base allows unsurpassed integration of large-scale architectural data with bed- and facies-scale data. The expected value of the DWAKB is illustrated with several examples of quantitative data analysis. These include the determination of the frequency of occurrence of levees in systems of different grain size, the calculation of the dimensions of submarine channels as a function of grain size and proximity to the source area, and the construction of idealized models for sand-rich and mixed mud–sand systems based on probabilities of spatial transitions between architectural elements.
The geological and geochemical characteristics of source rocks in the Tertiary Nadu Formation of the Eastern Depression, Baise Basin, China
Authors: Luofu Liu , Hoiland Harald , Suping Zhao , Yongqiang Guo , Shuangwen Li , Yuanzhuang Chen
Published: October 2008

Dark mudstones are widely distributed in the Upper and Lower Sub-members of the Na1 Member and the Na2 Member of the Tertiary Nadu Formation in the Eastern Depression of the Baise Basin, with a cumulative thickness of 200–1000 m. There is more than 1% organic carbon in these three sets of dark mudstones in most areas of the Eastern Depression, indicating that they are good source rocks. The kerogen of these three sets of effective source rocks is mainly of Type II
A
, but also with Types II
B
and III. Distribution of the kerogen type was controlled by sedimentary environment. R
o
values of the Na1 Member mudstones are 0.4–0.65%, showing that the organic matter is at the immature to low-mature stage, while those of the Na2 Member, which are deeper, are 0.7–0.8% or even higher, showing that the organic matter is at the early mature stage. The facies of thicker dark mudstones in the Depression suggest a semi-deep lake environment, and the areas with higher organic carbon and R
o
values – the depositional centre and the subsiding centre of the basin – coincide with the hydrocarbon generation centre. The Na2 Member should be the most effective source rock in this area because of its maturity, thickness, the sedimentary environment, and the organic abundance and type. The Tiandong Sedimentary District, with its larger area, greater thickness of mudstone, wider range of semi-deep lacustrine environment, higher organic carbon amount, better organic matter type and higher maturity, has a greater potential for hydrocarbon generation and expulsion than the Tianyang Sedimentary District.
A regional chemostratigraphically-defined correlation framework for the late Triassic TAG-I Formation in Blocks 402 and 405a, Algeria
Authors: Ratcliffe K. T. , Martin J. , Pearce T. J. , Hughes A. D. , Lawton D. E. , Wray D. S. , Bessa F.
Published: October 2008

The Triassic Argilo-Gréseux Inférieur Formation (TAG-I) is one of the principal hydrocarbon reservoirs in the Berkine Basin of Algeria. Sedimentological studies have shown that it exhibits marked spatial and temporal facies variations on both a local field scale and a regional basinal scale. This variability, combined with a lack of diagnostic flora and fauna, makes regional correlation within the unit difficult. In turn, the lack of a consistent regional stratigraphic framework hampers the comparison of the various correlation schemes devised by operators in the basin.

Contrasting the TAG-I in Blocks 402 and 405a exemplifies the problems encountered when attempting regionally to define a correlation framework for the interval. Between these two blocks, a distance of approximately 200 km, there are marked changes in the style of deposition from sand-dominated, proximal fluvial systems in the SW (Block 405a, MLN, MLC, KMD and MLNW fields) to a more distal, more clay-prone system in the NE (Block 402, ROD/BRSE/BSFN, SFNE and BSF fields). A chemostratigraphic study of the TAG-I in these two blocks has allowed a four-fold correlation framework to be defined, where each chemostratigraphic package has distinctive geochemical features. Chemostratigraphic Package 10, the oldest unit, lies above the Hercynian Unconformity, but beneath a geochemically identifiable hiatal surface. Chemostratigraphic Package 20 lies above the hiatal surface but is separated from the overlying packages by a mineralogical change identifiable in both claystone and sandstone geochemistry. Chemostratigraphic Packages 30 and 40 are chemically somewhat similar, but are separated by a regional event interpreted as a period of dolocrete and lacustrine development. By combining the geochemical differentiation of the units and recognition of their stratal boundaries, it is possible to define a correlation for the TAG-I between Blocks 402 and 405a.

The proposed correlation between the two blocks suggests that the northern parts of Block 405a may have been occupied by a spur or subsidiary channel from the main SW–NE-trending fluvial system, resulting in one of the chemically defined packages being demonstrably absent in the MLNW, MLN, KMD and MLC fields when compared with the other areas of the study.
Sandstone compaction, grain packing and Critical State Theory
Authors: Pettersen Øystein
Published: October 2008

Based on the physics of grain packing in a granular material, this paper demonstrates that sands or sandstones are modelled most correctly by Critical State Theory, which can be used to define a consistent compaction relationship for use in rock mechanics or reservoir simulation. The theoretical model is compared with experimental data for volume and permeability variation during loading or unloading.
An atypical early mature oil in Block 35/1, Norwegian North Sea – hypersaline, carbonate Jurassic environment?
Authors: Ohm S. E. , Beeley H. , Karlsen D. A. , Hall P. B. , Foss A.
Published: October 2008

Block 35/1 with the dry Sturlason structure, is located on the northernmost part of the Marflo Ridge in the Norwegian part of the Northern North Sea. It is separated by deep faults from the Sogn Graben to the east and the Marulk Basin to the west. The 35/1-1 well proved only minor shows of gas and oil in the well.

The Sturlason structure comprises a series of upthrown fault blocks in a structurally complex area. The well-established Brent Formation carrier and reservoir sandstone has shaled out this far north and the stratigraphically deeper Lower Jurassic Statfjord Formation and Triassic Lunde Formation sandstones were, in the exploration model, suggested as both carrier beds and reservoirs. The prolific Upper Jurassic Draupne (Type II organic matter, OM) and Heather (Type II/III OM) Formation source rocks were, based on seismic data, interpreted to be absent or thin over the prospect, thus implying lateral migration for filling the structure with petroleum. Structural back-stripping suggests that part of Block 35/1 was sub-aerially exposed as an island during deposition of the Upper Jurassic source rocks. This may have impacted the quality and nature of the fringing organic material due to a more oxic environment and a greater influx of Type III organic matter.

The geochemical analyses were hampered by contamination from the use of oil-based mud (C
13–23
range hydrocarbons) while drilling. Despite this, traces of true indigenous C
4–10
and C
25+
range hydrocarbon are demonstrated. These results suggest presence of an evaporative condensate and heavy oil fraction originating from a source rock related to a hypersaline carbonate depositional environment.

A buoyancy-driven fluid flow study, without taking faults into account, shows the difficulty in charging the prospect and clearly suggests the presence of sealing faults. The latter are also substantiated by a separate fault-seal analysis. Traps in flanking areas could, however, receive petroleum. Gas is also interpreted to be present in shallower sediments over the eastern flank of the Sturlason structure.
Interaction of the Zagros Fold–Thrust Belt and the Arabian-type, deep-seated folds in the Abadan Plain and the Dezful Embayment,SW Iran
Authors: Fard Iraj Abdollahie , Braathen Alvar , Mokhtari Mohamad , Alavi Seyed Ahmad
Published: October 2008

The Dezful Embayment and Abadan Plain (SW Iran) contain major parts of the remaining Iranian oil reserves. These oil provinces are characterized by two types of structural closure: very gentle N–S- to NE–SW-trending basement-cored anticlines (Arabian-type highs) in the SE; and open to tight, NW–SE-trending thrust-related folds in the NE (Zagros Fold–Thrust Belt; ZFTB). Most deep-seated anticlines are upright and symmetrical in Cretaceous and older units. In some cases they reveal steep faults in their core which, in the light of regional observations, suggest that the basement is involved in the faulting. Untested plays around these anticlines include reefal build-ups, debris flows, truncated sedimentary sections and onlapping clastic units.

The ZFTB shows a classic structural style, with overall shortening reflected in thrust displacement declining from the Dezful Embayment towards the frontal zone in the Abadan Plain. The Early Cambrian Hormuz Salt represents the fundamental sole for the fold–thrust belt and locates major fault-propagation folds in the southwestern Dezful Embayment. These folds represent the main petroleum target of the area. Another important unit is the Mid-Miocene Gachsaran Formation. This detachment reveals both in-sequence and out-of-sequence thrusting. Interaction of deep-seated anticlines and fold–thrust structures results in thrust imbrications and formation of duplexes within the Gachsaran Formation when thrusts abut deep-seated anticlines. Above the crest of the anticlines, thrusts are forced up-section into syn-tectonic deposits, whereas the forelimb reveals out-of-the-syncline thrusts. Several petroleum plays are identified in such zones of structural interaction, including anticlines above buttress-related duplexes, out-of-sequence imbricate thrust fans with associated folds above major anticlines, truncation of footwall layers below potentially sealing thrusts, and sub-thrust anticlines.
Assessment of the petroleum generation potential of Lower Carboniferous coals, North Sea: evidence for inherently gas-pronesource rocks
Authors: Petersen Henrik I. , Nytoft Hans P.
Published: October 2008

The petroleum generation potential of Lower Carboniferous coastal plain coals in the Danish Gert-2 and Norwegian 2/11-9 wells in the Central Graben, North Sea, has been assessed in order to determine the source-rock quality of the coals in connection with deeper plays. The coals are vitrinite-rich, have H/C ratios from 0.75–0.79, and hydrogen index (HI) values from 171–219, which are very close to the HI
max
values attained during thermal maturation. The thermal maturity of the Gert-2 coals is close to the start of the effective oil window (oil expulsion),
c.
0.90–0.95% vitrinite reflectance. The proportion of aliphatic chains >C
18
in the coals is negligible or very low: the C
19–35
aliphatic chains constitute only
c.
6% of the total amount of C
12–35
aliphatic chains in the coal structure. Aliphatic moieties are characterized by shorter-chain aliphatics and isolated CH
2
. The generated liquid petroleum is trapped in the coals and the saturated hydrocarbon fraction is dominated by shorter-chain aliphatics. Upon solvent extraction, the measured HI values of the Gert-2 coals can, on average, be reduced by 30%, indicating that the trapped petroleum contributes to the measured generation potential. The Lower Carboniferous coals are thus inherently gas-prone, a feature they have in common with Carboniferous coals in general and which is not maturity related. Inefficient expulsion of the saturated hydrocarbons may enhance the gas-proneness by intra-source rock oil-to-gas cracking. In a deep exploration context in the North Sea area, Lower Carboniferous coal/coaly sections can therefore only constitute a gas source rock.
Geological modelling and finite difference forward realization of a regional section from the Zagros fold-and-thrust belt
Authors: Alaei Behzad , Petersen Steen A.
Published: October 2008

This paper provides detailed geological modelling and finite difference realization results of a 2D regional section from the Zagros fold-and-thrust belt, Iran. Different scale geological detail has been included in the model. The modelling approach is based on a hierarchical procedure and the model is made up of cells representing the geometry and properties (such as seismic P-wave velocity) of parts of the model. By using a hierarchical procedure for building the model, the time and space consistency of the geological model is preserved. The constructed model is 81 × 17 km and crosses several drilled and undrilled structures. The model geometry is controlled by surface, subsurface and seismic interpretation results along the model. The model building blocks were chosen based on the deformation history of the area and the modelling was carried out in two stages. First, a regional model was constructed including the major structural elements. At the second stage (fine tuning of the regional model), structural and stratigraphic details, such as onlap, truncations, pinch-outs and thickening/thinning of salt beds, were added to the regional model. The velocity and density models were extracted from well logs, check shots and seismic processing-derived velocity data. A few shots were generated using an acoustic finite difference method to see the effect of the inclusion of detailed internal geometry in the model and the seismic wave complexity around three main anticlines of the model. The seismic response shows that the detailed modelling of stratigraphy and structure gives results that simulate the real data better. It shows the existence of complex diffracted and non-hyperbolic events, suggesting the poor image quality of real data is not related entirely to acquisition problems caused by irregular topography. Advanced depth-imaging methods (e.g. reflection tomography and wave-equation-based extrapolation or migration algorithms) are required to get better images. The sharp lateral velocity changes associated with major thrust faults cause the generation of diffractions that can compromise reliable velocity analysis, especially at conventional offsets. This effect could be considered as one of the causes of the poor seismic response observed on real data. The use of larger offsets, therefore, can distinguish between diffractions and primary reflections during velocity analysis in time and depth.
Faulting and fault sealing in production simulation models: Brent Province, northern North Sea
Authors: Jolley S. J. , Dijk H. , Lamens J. H. , Fisher Q. J. , Manzocchi T. , Eikmans H. , Huang Y.
Published: October 2008

Faults can severely compartmentalize pressures and fluids in producing reservoirs, and it is therefore important to take these effects into account when modelling field production characteristics. The Brent Group fields, northern North Sea, contain a complex arrangement of fault juxtapositions of a well-layered sand-shale reservoir stratigraphy, and fault zones containing a variety of fluid flow-retarding fault rock products. It has been our experience that these fault juxtapositions impact the ‘plumbing’ of the faulted layering system in the reservoirs and the models that are built to mimic them – and are, in fact, a first-order sensitivity on compartmentalization of pressures and fluid flow during production simulation. It is important, therefore, to capture and validate the geological feasibility of fault- horizon geometries, from the seismic interpretation through to the static geocellular model, by model building in conjunction with the interpretation. It is then equally important to preserve this geometrical information during geocellular transfer to the simulation model, where it is critical input data used for calculation of fault zone properties and fault transmissibility multipliers, used to mimic the flow-retarding effects of faults. Application of these multipliers to geometrically weak models tends to produce ambiguous or otherwise potentially misleading simulation results. We have systematically modelled transmissibility multipliers from the upscaled cellular structure and property grids of geometrically robust models – with reference to data on clay content and permeability of fault rocks present within drill core from the particular reservoir under study, or from similar nearby reservoirs within the same stratigraphy. Where these transmissibility multipliers have been incorporated into the production simulation models, the resulting history matches are far better and quicker than had been achieved previously. The results are particularly enhanced where the fault rock data are drawn from rocks that have experienced a similar burial–strain history to the reservoir under study.
Cenozoic evolution of the Lariang and Karama regions, North Makassar Basin, western Sulawesi, Indonesia
Authors: Calvert Stephen J. , Hall Robert
Published: October 2008

The Lariang and Karama regions of western Sulawesi, an area of approximately 10 000 km
2
, were the subject of a field-based investigation with the aim of understanding the Cenozoic evolution of the North Makassar Basin. Western Sulawesi was influenced by the development of the Makassar Straits to the west and the collision of continental, ophiolitic and island-arc fragments to the east. The timing of these events has been the subject of considerable debate and it has been suggested that Neogene collisions in Sulawesi caused inversion in Borneo. A new stratigraphy for the Lariang and Karama regions of western Sulawesi, based on fieldwork, provides new and significant insights into the evolution of the Makassar Straits region. The oldest sediments are non-marine and could be as old as Paleocene; they include coals, sandstones and mudstones. Rifting had started by the Middle Eocene and continued into the Late Eocene. Syn-rift Eocene sediments were deposited in graben and half-graben in both marine and marginal marine environments. The Eocene Makassar Straits rift was highly asymmetrical; the Kalimantan margin was approximately twice the width of the Sulawesi margin. Thermal subsidence had started by the latest Eocene and by the end of the Oligocene most of western Sulawesi was an area of post-rift shelf carbonate and mudstone deposition. This shallow-marine depositional environment persisted throughout the Early Miocene and, in places, until the Middle or Late Miocene. In the Pliocene the character of sedimentation changed significantly. Uplift and erosion was followed by the deposition of coarse clastics derived from an orogenic belt to the east of the study area. The Palaeogene half-graben were inverted, there was localized detachment folding and the overlying Neogene section was folded, faulted and eroded in places. Contractional deformation in western Sulawesi dates from the Pliocene, whereas in eastern Kalimantan it dates from the Early Miocene.
Mesozoic Gulf of Mexico basin evolution from a planetary perspective and petroleum system implications
Authors: Fillon Richard H.
Published: October 2008

The lack of a sharp boundary between North and South Atlantic stress fields recorded in the arcs of transform faults and ridge segments suggests a gradual merging of mantle stresses within a broad Central Atlantic plate boundary zone. The nature of descending slabs deep in the mantle beneath North and South America suggests that this intra-American plate boundary zone has existed since the Early Cretaceous, at which time it was located beneath the Gulf of Mexico Basin. Simple Euler sums of North and South America to Africa rotation poles validate the concept of merging stress fields, providing a geologically reasonable trajectory and rotation data for the Yucatan microplate with respect to Africa. The new Yucatan rotation geometry is consistent with initiation of back-arc spreading in the western Gulf of Mexico Basin during the Late Berriasian or Early Valanginian,
c.
140 Ma, triggered by a strengthening South Atlantic stress field. Continued spreading and rotation of Yucatan likely persisted through the Late Albian,
c.
100 Ma. These findings are supported by Early Cretaceous deposystem architecture, basin margin reef trends and source-rock distribution. Kinematic analysis predicts that most Gulf of Mexico seafloor (
c.
60%) was created during the Cretaceous period of stable normal geomagnetic polarity,
c.
125–83.5 Ma (the ‘Cretaceous Quiet Zone’). Salt-lubricated detachment faulting in the young Gulf of Mexico likely covered newly formed oceanic crust with large allochthons of Oxfordian–Valanginian strata.