Papers by Madeleine Raven
Carbon and oxygen isotope characteristics of the Clydach Valley Subgroup, Courceyan, South Wales‐Mendip shelf, UK
The Depositional Record
Late Aptian incised valleys and siliciclastic infill at the top of the Shu’aiba Formation (Block 5, offshore Qatar)
Late Aptian long-lived glacio-eustatic lowstand recorded on the
ABSTRACT Terra Nova, 25, 87–94, 2013 AbstractCompelling physical evidence for a Late Aptian lowst... more ABSTRACT Terra Nova, 25, 87–94, 2013 AbstractCompelling physical evidence for a Late Aptian lowstand with an amplitude of at least 50 m is presented in subsurface seismic and core data from the Arabian Plate. Biostratigraphic dating indicates that the fall and rise bracketing this lowstand were rapid, and that the lowstand lasted for around 5 ma with distinctly cyclic sedimentation at the 0.4–0.5 ma scale (eccentricity band). A glacio-eustatic mechanism is invoked as the most likely cause, which is supported by cooling indicated in oxygen isotope shifts and by evidence for a global expression of this lowstand from a number of locations at the mid and high latitudes. Hence, the Late Aptian data presented here document the longest Cretaceous sea-level lowstand, interpreted as the longest cooling phase during the Cretaceous greenhouse.
Microbial Carbonate Reservoirs of the Argyll and Auk Fields Reinterpreted in a Sequence Stratigraphic Context*
The shift of focus to basin margin areas, prompted by the UK 29th Licensing Round, has led to re-... more The shift of focus to basin margin areas, prompted by the UK 29th Licensing Round, has led to re-evaluation of the Palaeozoic, including the Zechstein. The Zechstein reservoir interval in the Northern Permian Basin (NPB) is encompassed by the Halibut Carbonate Formation, equivalent to the entire Z1 (highstand and lowstand systems tracts) and lower part of the Z2 (highstand systems tract) cycles of the Southern Permian Basin (SPB). However, previously, little attempt has been made to extend the SPB sequence stratigraphic understanding into the NPB. The Zechstein is a major reservoir in the Argyll and Auk fields. Historically, opinions have varied as to the interpretation of the Zechstein reservoir facies in these fields. Some former interpretations suggest deposition on a shallow carbonate shelf, with the main reservoir consisting of peritidal stromatolites. However, after re-examined of the core from these fields within the context of a regional study, it has been concluded that dep...
The diagenetic history of the Oolite Group from Pwll du to Blaen Onneu has been unravelled. Cemen... more The diagenetic history of the Oolite Group from Pwll du to Blaen Onneu has been unravelled. Cement types, solution events, dolomitisation, concretions and other authigenic minerals are described. The calcite cement crystals show chemical zonation through staining but cathodoluminescence (CL) has proved the most useful method for displaying internal features of calcite crystals. CL makes it possible to trace the growth of crystals and monitor changes in crystallographic form during growth. Seven cement zones are identified using stained specimens. Each zone has a characteristic luminescence. The age of these zones relative to the exposure of the top of the Oolite Group is established. Two distinct diagenetic areas are recognised on the basis of the distribution of the cement zones. Area 1 : the Pwll-y-cwm and Blaen Onneu Oolites in the Clydach area, in which Zones 1, 2b and 6 are present, Area 2 : the rest of the Oolite Group, in which Zones 2a, 3, 4, 5 and 6 are present. It is possi...
GeoArabia, 2007
Evidence has been found for the involvement of bacterial methanogenic processes in the diagenesis... more Evidence has been found for the involvement of bacterial methanogenic processes in the diagenesis of late Early Aptian Shu’aiba Formation sedimentary rocks from Block 5, offshore Qatar. This is in the form of δ13C values significantly higher than the presumed late Early Aptian marine signal. High δ13C values were recorded in early-formed inclusion-rich calcite that represents neomorphosed aragonite/marine cement (+6.6‰) and the latest precipitated blocky non-ferroan calcite cement (+8.9‰). Oxygen-isotope data suggest that the later methanogenic episode occurred during burial. In bulk rock samples from another well, δ13C values of +6.5 to +11.5‰ were determined from a 10 ft thick zone located 4 ft below the top of the Shu’aiba. In other wells for which data were available the equivalent interval of the Shu’aiba was characterised by expected marine δ13C values. Finally, δ13C values up to +9.1‰ were recorded in a localised area of dolomite replacement immediately beneath the Top Shu’ai...
Evidence for top Shu’aiba exposure and incision in Block 5, offshore Qatar: Regional consequences
GEO 2008, 2008
Detection of Facies Changes in Horizontal Carbonate Wells using LWD NMR
Detection of facies changes along horizontal wells in a Middle East carbonate reservoir, known to... more Detection of facies changes along horizontal wells in a Middle East carbonate reservoir, known to comprise several different facies, is not always possible with conventional LWD logs because the different facies have similar porosity responses. Drill-pipe-conveyed wireline nuclear magnetic resonance (NMR) logs have proved to be an effective means of facies identification. Changes in facies are most often coupled with changes in the pore system / pore geometry, which shows up as a change in the NMR T2 distributions. However, pipe-conveyed wireline logging in horizontal wells is time consuming and only the section where sliding of drill pipe is feasible can be logged. Hence, the ability of the wireline NMR to identify variations in facies types is limited to the proximal part of extended-reach horizontal wells and the data will only be available after the well has been drilled. This paper addresses the results of what was the first application of a new logging-while-drilling (LWD) NMR tool in carbonates. The test demonstrated that changes in facies and associated permeability can be detected along the entire length of the horizontal well trajectory, even at high drilling rates. These valuable data can be used to position the well in the most productive part of the reservoir. The LWD-NMR was run in combination with the conventional porosity/resistivity LWD tool string. As this was the first application in carbonates, pipe-conveyed wireline NMR was acquired over selected intervals to provide quality control for the LWD-NMR data. The extended LWD-NMR data set also improved the understanding of the geological reservoir model.
Carbonate Rock Typing Using NMR Data: A Case Study from Al Shaheen Field, Offshore Qatar
International Petroleum Technology Conference, 2005
Complex pore size distributions encountered in carbonate rocks have a large impact on the fluid f... more Complex pore size distributions encountered in carbonate rocks have a large impact on the fluid flow characteristics of carbonate reservoirs. Consequently, although nuclear magnetic resonance (NMR) has been frequently used for characterization of clastic reservoirs, it has not been widely applied to carbonates. This paper describes a case study from the Shuaiba carbonate reservoir of the Al Shaheen field, offshore Qatar. Core data from a study well were used to establish an NMR carbonate rock-typing model for permeability estimation. The rock-typing model was verified in the study well with a wireline NMR logging tool. Core analysis included thin-section petrography, NMR surface relaxivity, mercury injection, porosity and permeability measurements. NMR distributions determined on core were partitioned and linked to pore body size and pore throat size distributions. Several rock types were also defined based on their NMR and petrographic characteristics. To improve permeability prediction in the cored interval of the study well, an NMR-based permeability equation was derived. The core-calibrated NMR carbonate rock typing model was applied to noncored sections of horizontal wells drilled in the Shuaiba formation having logging-while-drilling (LWD) NMR data to improve rock typing and permeability estimation, thus, providing valuable input data for reservoir modeling. Introduction Permeability estimation and rock typing of the complex Shuaiba carbonate reservoir in the Al Shaheen Field is an important element in the development of reliable petrophysical and geological models. Rose et al. demonstrated that facies changes along horizontal wells in the Shuaiba reservoir were captured by the T2 distribution from an LWD NMR tool. The corresponding permeability variation was linked to the facies, and a qualitative determination of the lateral permeability distribution was achieved.
1st International Petroleum Conference and Exhibition Shiraz 2009, 2009
An integrated subsurface data set of core, vertical and horizontal wells and seismic has provided... more An integrated subsurface data set of core, vertical and horizontal wells and seismic has provided evidence for the presence of karstification and quartz sand filled platform incision at the top of the Shuaiba Formation in offshore Qatar. These observations have significant implications for the late Aptian palaeogeography and related understanding of the distribution of late Aptian/early Albian siliciclastics in the region. In addition, it provides a Late Aptian reference curve for the Arabian plate.
Late Aptian long-lived glacio-eustatic lowstand recorded on the Arabian Plate
Terra Nova, 2012
ABSTRACT Terra Nova, 25, 87–94, 2013 AbstractCompelling physical evidence for a Late Aptian lowst... more ABSTRACT Terra Nova, 25, 87–94, 2013 AbstractCompelling physical evidence for a Late Aptian lowstand with an amplitude of at least 50 m is presented in subsurface seismic and core data from the Arabian Plate. Biostratigraphic dating indicates that the fall and rise bracketing this lowstand were rapid, and that the lowstand lasted for around 5 ma with distinctly cyclic sedimentation at the 0.4–0.5 ma scale (eccentricity band). A glacio-eustatic mechanism is invoked as the most likely cause, which is supported by cooling indicated in oxygen isotope shifts and by evidence for a global expression of this lowstand from a number of locations at the mid and high latitudes. Hence, the Late Aptian data presented here document the longest Cretaceous sea-level lowstand, interpreted as the longest cooling phase during the Cretaceous greenhouse.
Rifting, subsidence and continental break-up above a mantle plume in the central South Atlantic
Geological Society, London, Special Publications, 2013
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Papers by Madeleine Raven