Seismic characterization of carbonate platforms and reservoirs: an introduction and review
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Seismic characterization of carbonate platforms and
reservoirs: an introduction and review
James Hendry1,2*, Peter Burgess3, David Hunt4, Xavier Janson5 and
Valentina Zampetti6
1
Iapetus Geoscience Ltd, Unit 7, Watson & Johnson Centre, Mill Road, Greystones,
Co. Wicklow, Ireland
2
School of Earth Sciences, University College Dublin, Bellfield, Dublin 4, Ireland
3
School of Environmental Sciences, University of Liverpool, Brownlow Street, Liverpool
L69 3GP, UK
4
Equinor, Sandsliveien 90, Sandsli, N-5234, Norway
5
Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at
Austin, Austin, TX 78713, USA
6
Shell Global Solutions, Lange Kleiweg 40, 2288 ER Rijswijk, The Netherlands
JH, 0000-0002-8448-6057; DH, 0000-0003-1911-9389
*Correspondence: info@iapetusgeo.com
Abstract: Improved seismic data quality in the last 10–15 years, innovative use of seismic attribute combina-
tions, extraction of geomorphological data and new quantitative techniques have significantly enhanced under-
standing of ancient carbonate platforms and processes. 3D data have become a fundamental toolkit for mapping
carbonate depositional and diagenetic facies, and associated flow units and barriers, giving a unique perspective
on how their relationships changed through time in response to tectonic, oceanographic and climatic forcing.
Sophisticated predictions of lithology and porosity are being made from seismic data in reservoirs with good
borehole log and core calibration for detailed integration with structural, palaeoenvironmental and sequence
stratigraphic interpretations. Geologists can now characterize entire carbonate platform systems and their
large-scale evolution in time and space, including systems with few outcrop analogues such as the Lower Cre-
taceous Central Atlantic ‘pre-salt’ carbonates. The papers introduced in this review illustrate opportunities,
workflows and potential pitfalls of modern carbonate seismic interpretation. They demonstrate advances in
knowledge of carbonate systems achieved when geologists and geophysicists collaborate and innovate to max-
imize the value of seismic data from acquisition, through processing to interpretation. Future trends and devel-
opments, including machine learning and the significance of the energy transition, are briefly discussed.
Fundamental advances in seismic imaging of car- can now be determined from seismic images, their
bonate strata over the past decade have revolution- attributes interpreted using modern software, and
ized scientific understanding of carbonate sediment correlated to information from boreholes or other
accumulation and modification, with implications geophysical surveys. Seismic attributes, singly and
from hydrocarbon exploration to geomorphology in combination, determined by the physics of
and palaeoclimate change. These advances have acoustic-wave interaction with solid-rock matrix
been driven by oil and gas exploration and produc- and its fluid-filled porosity are providing a novel
tion, given the fact that carbonates are believed to toolbox with enormous potential to map heterogene-
host more than 60% of the world’s oil reserves and ity within carbonate systems, and to improve under-
40% of the gas reserves (e.g. https://www.slb. standing of depositional and diagenetic processes at
com/technical-challenges/carbonates), but the data scales rarely achievable from surface exposures.
obtained are also increasingly being used to funda- Whilst 2D seismic provides key evidence of car-
mentally reassess paradigms of carbonate platform bonate platform development and morphology in
development, architecture and post-depositional previously unexplored basins and stratigraphic inter-
modification. Study of present-day or outcrop ‘time- vals, improvements in acquisition, processing and
snapshots’ do not reveal how carbonate platform filtering of 3D seismic data in basins with higher
development on millennial timescales and kilometric data density, as well as improved user-friendly inter-
length scales was forced by long-term eustatic, pretation software, have led to significant advances
oceanographic or tectonic factors, yet this forcing in carbonate geology. For example, seismic
From: Hendry, J., Burgess, P., Hunt, D., Janson, X. and Zampetti, V. (eds) Seismic Characterization of Carbonate
Platforms and Reservoirs. Geological Society, London, Special Publications, 509,
https://doi.org/10.1144/SP509-2021-51
© 2021 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution
License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London.
Publishing disclaimer: www.geolsoc.org.uk/pub_ethics
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J. Hendry et al.
geomorphology, a concept first employed with spec- from exhausted easy-to-find large fossil fuel reserves
tacular results in passive-margin siliciclastic subma- towards a sustainable energy source future. As a log-
rine fan settings with strong sand–shale impedance ical extension, 4D seismic data will be essential to
contrasts and simple overburden geology (Posa- monitor CO2 fronts in sequestration projects build-
mentier et al. 2007), is now applicable to carbonate ing on technology that has already been demon-
platform systems in a variety of basin contexts strated for enhanced oil recovery (EOR) projects
(e.g. Ahlborn et al. 2014; Saqab and Bourget 2016; (e.g. Wang et al. 1998; Li 2003; Raef et al. 2005),
Wang et al. 2016; Paumard et al. 2017; Rankey as well as more recent research (e.g. Yenugu et al.
2017; Rinke-Hardkopf et al. 2018; Grant et al. 2015; Nuwara 2020).
2019). Indeed, 3D seismic has become a fundamen- This Special Publication illustrates some recent
tal toolkit for mapping carbonate depositional and advances in the seismic characterization of carbonate
diagenetic facies and associated flow units and barri- platforms and reservoirs, and demonstrates substan-
ers: for example, reticulate reef networks, lagoons, tial advances in data quality and visualization made
shoals or beach ridges, slope-collapse scars and toe since the last compilation of seismic imaging and
of slope mass-flow deposits. interpretation in carbonate systems (Eberli et al.
At a reservoir scale, the heterogeneity of carbon- 2004). That previous collection of papers was pub-
ate pore systems, which result from complex deposi- lished when 3D seismic data on carbonate platforms
tional fabrics and their susceptibility to significant and reservoirs were becoming more widely available
diagenetic modification, has long been recognized but before many of the major improvements in
as a challenge for successfully appraising, develop- seismic acquisition (e.g. increased bandwidth and
ing and managing hydrocarbon fields. Accurate illumination), processing (noise, ghost and multiple
extrapolation of flow units between widely spaced removal, velocity modelling), resolution and visual-
wells has usually relied on conceptual models and ization (attribute extraction and co-rendering, colour
analogues, or purely stochastic models that assume blends), and interpretation (autotracking, geobody
the geology is too complex to predict in a more deter- extraction, stratal slicing and machine learning
ministic manner. In a standout paper, Yose et al. methods such as neural networks) that have featured
(2006) demonstrated the potential for 3D seismic in the past decade (e.g. Chandoola et al. 2020). Many
data to discriminate platform facies and flow units, more carbonate platforms of diverse ages have been
and to populate them into a sequence stratigraphic imaged globally, including some shallow carbonate
framework for reservoir model building. Similar systems spectacularly imaged above deeper reservoir
workflows have since become established practice. targets, such as the Neogene carbonates above the
Inversion of seismic data, relationships of rock typ- regional Cretaceous seal in the Browse and Carnar-
ing to rock physics and seismic resolution of deposi- von basins (discussed below). There have also
tional geometries now permit a more deterministic been substantial advances in quantitative seismic
and iterative approach to reservoir modelling that is interpretation, including inversion, fluid detection
geologically conditioned and reduces uncertainty, and rock-physics modelling (e.g. see Avseth et al.
allowing more accurate prediction of the inter-well 2014). These constitute a technical discipline in
volume (e.g. Rodrigues et al. 2016; Liu and Wang their own right but still pose some of the greatest
2017; Ferreira and Lupinacci 2018; Lupinacci challenges in predicting reservoir properties from
et al. 2020; Ghon et al. 2021). Further insights can seismic data, especially where well data for model
be drawn in producing fields from integrating calibration are lacking. Because seismic interpre-
dynamic data with seismic characteristics and bore- tation is only the first stage in understanding car-
hole log and core information (e.g. Warrlich et al. bonate reservoir potential in the subsurface, this
2019). This approach can be augmented by cross- contribution complements and should be considered
well seismic, especially where vertical imaging is alongside Geological Society, London, Special Pub-
complicated by overburden effects (Beavington- lications, Volume 406 (Agar and Geiger 2015).
Penney et al. 2019). Major diagenetic porosity–per- As the ability to extract additional quantitative
meability modifiers, such as fracture networks, information from seismic data has grown, seismic
palaeokarst cave systems and gas chimneys, can be interpretation of carbonates has moved out of the
identified and extracted from 3D seismic attributes, arena of specialist geophysics to become an essential
allowing well locations not only to target the best geological activity in exploration and field develop-
potential reservoir facies, but also to avoid critical ment. Geological knowledge of carbonate systems,
drilling hazards (e.g. Zeng et al. 2011; Sun et al. how they are deposited and modified, and how
2013; Burberry et al. 2016; Tian et al. 2019; Aboaba they develop characteristic geometries with distinct
and Liner 2020; Yan et al. 2020). In addition, the role rock properties (geobodies) is critically important
of seismic interpretation in improving recovery from for making robust and reliable seismic facies inter-
carbonate fields is increasingly important for security pretations (e.g. Burchette 2012; Lanteaume et al.
of energy supply during the critical transition period 2018) (e.g. Fig. 1). Whilst seismic geophysicists
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Seismic characterization of carbonates
are actively collaborating with and learning from car- stack to form progradational, aggradational and ret-
bonate sedimentologists, so the latter are becoming rogradational patterns that record a deep-time history
more attuned to the importance of understanding of the accommodation and sediment production and
key aspects of seismic data acquisition, processing transport variations (e.g. Dujoncquaoy et al. 2018;
and quantitative interpretation through rock-physics Tesch et al. 2018; Rankey et al. 2019), and how
modelling. The cross-disciplinary approach means karst systems develop in 3D on carbonate platforms
that new fundamental geological knowledge of car- repeatedly exposed to subaerial weathering by rela-
bonate depositional and post-depositional processes tive sea-level falls (Carrillat et al. 2005; Hunt et al.
is generated even when the primary aim of seismic 2010, Sayago et al. 2012). Both of these are illus-
campaigns is the discovery and extraction of hydro- trated in Figure 2. Subsurface 3D imaging has sub-
carbon resources (e.g. Purkis et al. 2015; Esestime stantially impacted understanding of depositional
et al. 2016; Sanford et al. 2016; Wu et al. 2016; and post-depositional processes in basin-floor and
Courgeon et al. 2017; Rankey 2017; Sayago et al. pelagic carbonates, showing them to be more
2018; Smit et al. 2018; Tesch et al. 2018; Jackson dynamic on a large scale than many outcrop studies
et al. 2019). have been able to capture (e.g. Back et al. 2011; Smit
For example, from analysis of 3D seismic data et al. 2018) (Fig. 3). The possible roles of tectonics
collected for hydrocarbon exploration, we now bet- and fluid migration v. oceanographic and eustatic
ter understand how carbonate platform margin strata controls on the initiation of carbonate platforms are
Fig. 1. (a) Example of carbonate seismic facies analysis.
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J. Hendry et al.
Fig. 1. Continued. (b) Interpretation with respect to carbonate depositional environments and their typical lithofacies
associations. Because carbonate platforms vary in composition and depositional architecture through time and in
response to tectonic, environmental and eustatic controls, geological knowledge is of paramount importance in the
interpretation process. From Paumard et al. (2017, figs 3 and 13). TWT, two-way time.
also being reassessed with the aid of high-quality 3D recently also in the South China Sea. This ability
seismic attributes and frequency blends (e.g. Saqab to integrate modern process sedimentology and
and Bourget 2018; Oppo and Hovland 2019). A fur- quantitative physical oceanography with imaged
ther recent example that has dramatically increased depositional geometries and platform architectures
interest in seismically imaged carbonates are the extending into the recent geological past (when cli-
studies of the supergiant and giant non-marine car- mate and eustatic conditions are better known) pro-
bonate reservoirs of the South Atlantic pre-salt play vides a unique 4D perspective for investigating
that are characterized by seismic and well data but carbonate platform dynamics that would otherwise
have few direct analogues at outcrop (e.g. Buckley be impossible or extremely difficult to capture (e.g.
et al. 2015; Wright and Rodriguez 2018; Ferreira Betzler et al. 2016; Lüdmann et al. 2016; Principaud
et al. 2019, 2021). As more seismic datasets are et al. 2017; Huang et al. 2020a). For example, these
released into the public domain, usage to address studies are providing a new understanding of the
fundamental questions of carbonate production, dep- dynamic interaction of slope to slope-apron sedi-
osition and diagenetic modification, and how these mentation with subsurface contour currents (Lüd-
processes record deep-time environmental change mann et al. 2013; Tournadour et al. 2015; Wunsch
will undoubtedly increase (see the Discussion). et al. 2018). In turn, these studies have implications
In addition to knowledge of deep-time carbonate for exploration and field development in ancient set-
systems acquired as a by-product of industry seismic tings, where currents have impacted the growth and
campaigns, significant advances have been made flank facies distributions of isolated platform clusters
using multichannel seismic data to study the Neo- (e.g. Ting et al. 2021).
gene–Quaternary history recorded in the flanks of The papers in this volume arose from an interna-
modern carbonate platforms. This is commonly tional gathering of more than 170 geoscientists from
done in parallel with seabed imaging techniques industry and academia at the Geological Society of
such as multibeam bathymetry and sidescan sonar, London in October 2018 to discuss the seismic char-
plus borehole data. Attention has been focused on acterization of carbonate platforms and reservoirs.
the Bahamas and the Maldives archipelago, and Fifty-six talks and posters addressed advances in
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Seismic characterization of carbonates
Fig. 2. Example of the sophisticated interpretations of carbonate platform architecture, depositional processes and
history, sequence stratigraphy, and diagenesis that are possible with high-quality 3D seismic data. (a) Interpreted 2D
seismic section of a Miocene isolated platform from the Browse Basin, offshore NW Australia. (b) Variance
(coherency) attribute extraction showing karst sinkholes on a sequence boundary. (c) Two-way time structure map
showing the platform geomorphology and its influence by ocean currents. Modified from Van Tuyl et al. (2019, fig. 6).
seismic imaging and modelling of carbonate strata, • Seismic characterization of fluid flow and diagen-
illustrating the variety of geological and reservoir esis in carbonates.
characteristics than can be extracted from seismic
data. The programme also explored the breadth of
seismic technology applications in carbonates, Leveraging seismic data in exploration and
from de-risking exploration plays and optimizing development of carbonate plays
production strategies to understanding long-term
and large-scale geological forcing of platform devel- Since the early days of frontier exploration driven
opment. The volume likewise exemplifies both the primarily by 2D seismic surveys there has been a rec-
substantial scientific and commercial value gener- ognition that the unique combinations of depositio-
ated from careful integration of seismic geophysical nal geometries (especially synoptic relief ) and
and carbonate geological expertise. To be accessible acoustic impedances (typically higher than sur-
to a broad geoscience readership the emphasis is on rounding shales + sandstones) in carbonates can
practical characterization and geological interpreta- create distinctive responses. Coupled with a prevail-
tion workflows rather than detailed theoretical treat- ing favourable view of prospectivity in shallow-
ments of rock physics and seismic inversion, marine carbonate platforms, driven no doubt by
although the latter are referenced where appropriate. many exploration successes and giant fields in the
The contributions are arranged into four themes, Middle East, as noted by Jiménez Berrocoso et al.
although there is necessarily some overlap: (2021), there has been a historical predilection for
interpreting carbonate build-ups where seismic
• Leveraging seismic data in exploration and devel- geometries and amplitudes do not conform to well-
opment of carbonate plays. known siliciclastic systems. Nonetheless, many car-
• The unique challenge of the South Atlantic pre- bonate prospects identified on seismic data have
salt carbonate reservoirs. been drilled and found to consist of basement
• Novel developments in seismic modelling of highs, volcanics or erosional remnants of siliciclastic
carbonates. systems (Greenlee and Lehmann 1993). Still others
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J. Hendry et al.
Fig. 3. Recent 3D seismic data from a Caribbean basin showing pipe-like structures with apparent collapse features
and adjacent amplitude anomalies in Neogene post-rift basin-floor carbonates (above the purple horizon marker), plus
submarine channels and slump features in the shallowest part of the section related to gravity-flow processes on an
adjacent platform flank. (a) Arbitrary line through several pipes. (b) Depth slice through a variance attribute cube
showing clusters of individual pipes (green arrow) and similar features aligned along fault trends (red arrow). (c) Box
probe in a variance attribute cube showing pipe features. (d) Spectral decomposition extraction showing submarine
channels and slope talus. (e) Arbitrary line showing a basin-floor slide block with internal deformation; inset is a
variance slice through the same feature. Images courtesy of United Oil and Gas plc.
have been verified as carbonates but with insufficient are being published, with some regional biases that
reservoir quality or hydrocarbon column to be viable reflect the commercial sensitivities associated with
producing fields. Although such ‘false positive’ the plays and the confidentiality thresholds of the
examples could provide valuable knowledge and operators. In some cases, the seismic data have been
data archives, very few of them get published as shared with academic researchers, allowing more
exploration companies are quick to move onto new in-depth interpretation and analysis of depositional,
opportunities and teams are redeployed. The situation structural and diagenetic topics that advance knowl-
is better where discoveries are made, and associated edge of carbonate systems but may not be business
appraisal and development drilling ensues, with the priorities for exploration and development teams.
benefit that the seismic interpretations of the The huge increase in both the use and quality of
carbonates are validated and refined with additional 3D seismic data, and in the visualization and inter-
sedimentological, biostratigraphic, petrophysical pretation software, over the past decade has made
and rock-physics analyses. An increasing number it easier to recognize and characterize some carbon-
of seismically based carbonate exploration successes ate platforms. This is due to the obvious benefit of
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Seismic characterization of carbonates
3D imaging, increased resolution and reduction in viewed as providing competitive advantage, there
migration artefacts caused by out-of-plane reflec- is hope that a greater sharing of data and workflows
tions, and through the variety of seismic surface will become possible, perhaps following the model
and volume attributes that are available to tease out of the Virtual Seismic Atlas (https://seismicatlas.
geomorphological details and map facies, and struc- org/). Additionally, large-scale seismic datasets are
tural and diagenetic trends (e.g. Yose et al. 2006; beginning to be available to academic users by
Rafaelsen et al. 2008, Yin et al. 2010; Janson et al. some regulatory authorities, a key example being
2011; Wang et al. 2016; Di Lucia et al. 2017; Pau- 2D and 3D datasets on the NW continental shelf of
mard et al. 2017; Makhankova et al. 2020; Embry Australia (Browse and Carnarvon basins) held by
et al. 2021). However, in frontier and emerging Geoscience Australia. This has led to several signifi-
plays it is still the norm that a potential carbonate tar- cant papers and theses documenting the stratigraphic
get is first identified on 2D data, risked and econom- and geomorphological development of Neogene car-
ically evaluated, prior to making a decision for bonate platforms, and their tectonic and eustatic con-
investment in 3D data. Moreover, the greater cost text (e.g. Rosleff-Soerensen et al. 2016; Saqab and
and complexity of shooting 3D surveys onshore, Bourget 2016; Belde et al. 2017; Rankey 2017;
especially in relation to well cost differentials, Tesch et al. 2018; Van Tuyl et al. 2019). There are
means that relatively few onshore carbonate pros- also exiting new possibilities for employing multi-
pects benefit from 3D seismic. Selection of survey attribute machine-learning processes for seismic rec-
type and acquisition parameters for the 3D surveys ognition and facies characterization of carbonate
are fundamental to successfully imaging and charac- prospects (Roden and Sacrey 2017; Chopra et al.
terizing of the platform, and the authors’ experience 2018; Pattnaik et al. 2020; Wu et al. 2020), although
is that companies increasingly appreciate the impor- again this is most likely to be trialled in large compa-
tance of geological specialist involvement at the nies with access to multiple datasets. It should
planning stage. The same applies for other play be added that this kind of application has been
types that have to be imaged beneath carbonate over- available for many years (e.g. Baaske et al. 2007).
burden, because the relatively high but heteroge- However, the rapid increase in computing power
neous impedance and high-velocity properties of plus the range of new seismic attributes from modern
carbonates can have a major impact on acoustic surveys will be likely to produce a paradigm shift in
energy transmission and its spatial variability. This how carbonates are routinely interpreted from 3D
is exemplified by Embry et al. (2021), where a car- seismic data.
bonate target was only identified following pre-stack Of the many published seismic images and stud-
depth migration due to complex velocity contrasts in ies of carbonate platforms in the past c. 10–15 years,
the overburden. many have been focused on the Neogene of SE Asia,
Addressing the challenges in confidently recog- and include some excellent detailed analyses of seis-
nizing carbonate platforms from seismic data, Bur- mic geomorphology (e.g. Posamentier et al. 2010;
gess et al. (2013) published a set of objective Kŏsa et al. 2015; Paumard et al. 2017), internal
criteria for identification of isolated carbonate build- and external seismic geometries, faulting and facies
ups. These were accompanied with a simple scoring (e.g. Zampetti 2010; Fyhn et al. 2013; Widarmayana
scheme based on the application of these criteria to et al. 2014; Jamaludin et al. 2018; Rankey et al.
234 published examples, to de-risk interpretations 2019; Makhankova et al. 2020), and detail of rela-
and determine which features have the greatest dis- tionships between contemporaneous carbonate and
criminatory power when alternatives such as volca- siliciclastic depositional systems (e.g. Tcherepanov
nics, tectonic features and basement highs are et al. 2008, 2010; Saller et al. 2010; Kŏsa 2015).
considered. Whilst the authors candidly acknowl- The reason for this geographical skewing of pub-
edged the limitations and remaining challenges in lished studies partly reflects a prevalence of high-
the approach, and notwithstanding its focus on a par- relief isolated reefal carbonate platforms, commonly
ticular type of isolated high-relief carbonate plat- developed in late synrift to post-rift tectonic regimes
form, it provides a systematic and geologically and buried beneath acoustically soft distal siliciclas-
grounded approach that has been adopted, expanded tics when the platforms drowned owing to various
and refined for internal use by several major explora- environmental or relative sea-level changes. The
tion companies such as Repsol and Equinor. This characteristic shapes and high impedance of these
relies on expansion of the image database through platforms makes them relatively easier to identify
incorporation of more examples, tackling other car- (see above) than platforms with less steeply inclined
bonate platform types that are less morphologically flanks, so proportionately more of them have been
striking, and considering combinations of criteria drilled and more discoveries made. Reservoir quality
or more granularity in the weightings (Burgess within them is often good but typically distributed in
et al. 2013). Whilst internal databases are likely to complex ways, in response to variably interdepen-
contain commercially sensitive examples and be dent depositional and diagenetic processes, so data-
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J. Hendry et al.
rich appraisal wells are commonly required and used improved reservoir characterization and geomodel-
for further calibration of the seismic. In this volume, ling than towards identifying new plays, leads or
Ting et al. (2021) illustrate the value of a high- prospects. Geopolitical factors also likely play a
quality regional 3D mega-survey in the Central role in the extent to which data and interpretations
Luconia province (South China Sea) for understand- reach the international/western scientific media,
ing environmental and tectonic controls on the flank and it is more common to find integrated reservoir
geometries and reservoir facies development of mul- characterization studies involving seismic attributes
tiple isolated platforms previously studied in isola- and inversion workflows reported in technical con-
tion or on a loose grid of 2D lines and with a few ference proceedings than in ‘mainstream’ geoscience
small discrete 3D surveys. Using seismic geometries journals. On the African Atlantic margin, carbonate
and attribute combinations, the impacts of mon- exploration is still at an early stage with few com-
soonal wind-driven submarine currents, nearshore mercial discoveries other than in the Albian (post-
tidal currents, antecedent topography and syndeposi- salt) carbonate and mixed carbonate–siliciclastic
tional tectonics are shown to have varied in relative shelf systems of Angola and Gabon; and although
importance across the province in a consistent and an extensive Jurassic–Neocomian carbonate shelf
predictable manner. However, clusters of large plat- is present from Morocco to Guinea, it is little tested
forms are also shown to have modified the current and consequently few seismic images currently exist
circulation and resulted in atypical facies distribu- in the public domain. The same is true of the conju-
tions on adjacent or intervening smaller platforms. gate margin, except for the Jurassic Abenaki plat-
Late Paleozoic platforms of the Norwegian form on the Scotian Shelf, which is relatively well
Barents Sea represent another carbonate mega-play documented (Kidston et al. 2005; Harvey and Mac-
with much published seismic data and interpretation, donald 2013). There are also a growing number of
facilitated by a culture of information sharing, cen- integrated carbonate seismic and well studies from
tralized oversight of well and seismic data at the Nor- Chinese onshore basins, although typically with a
wegian Petroleum Directorate, and national policies development rather than exploration focus, and
setting out time limits on data confidentiality. Pub- employing seismic attributes for lithological and dia-
lished studies have provided a unique window on genetic studies, especially of karstification (see the
3D carbonate geomorphology, including the first ‘Seismic characterization of fluid flow and diagene-
evidence of large-scale reticulate build-ups that are sis in carbonates’ section later in this chapter).
impossible to appreciate from size-limited 2D out- The Caribbean is a region of active frontier and
crops (Elvebakk et al. 2002; Rafaelsen et al. 2008; emerging basins where relatively few ancient car-
Colpaert et al. 2010; Purkis et al. 2015). Seismic bonate platforms have published 3D seismic data
data have also been generating important knowledge (e.g. Bunge et al. 2017), and it was also a region
on the 2D (surface) morphology and 3D distribution characterized by localized but frequent tectonic and
of palaeokarst networks (Carrillat et al. 2005; Hunt volcanic activity that can provide alternatives to car-
et al. 2010, Sayago et al. 2012), as well as on the spa- bonate platform interpretations. Jiménez Berrocoso
tial and temporal evolution of carbonate–evaporite et al. (2021) discuss an undrilled putative Miocene
relationships (Ahlborn et al. 2014). Other notable carbonate platform from the Lesser Antilles, using
carbonate plays with recently published seismic a modern 3D seismic survey carefully integrated
images and interpretations are the Permian platforms with regional geology, structural restoration and
flanking the Zechstein salt basin (Patruno et al. 2018; potential field data, and applying the Burgess et al.
Grant et al. 2019), the Cretaceous–Neogene post-salt (2013) scorecard. They persuasively show that the
platforms offshore Brazil (Bueno et al. 2014; Buar- seismic geometries, facies and ancillary data com-
que et al. 2017; Cruz et al. 2019), Paleogene–Neo- bine to support a carbonate platform interpretation,
gene isolated platforms in the Indian Ocean possibly analogous to the Perla Field in the Gulf of
(Shazad et al. 2018, 2019) and Early Paleozoic car- Venezuela (Castillo et al. 2017), but that the data
bonate platforms from onshore China basins (e.g. fail to negate alternatives such as an eroded siliciclas-
Huang et al. 2020b). tic deposit. The study demonstrates the importance
Relatively less seismic data has been published of mitigating exploration risk by integrating seismic
since the mid–late 2000s from some of the ‘classic’ interpretation with all the geological and exploration
petroliferous carbonate systems, such as the Juras- data available. It also makes the important point that
sic–Cretaceous of the Middle East, the Gulf of frontier carbonate prospects may carry a significant
Mexico, the Paleozoic intracratonic basins of North reservoir risk that can only be addressed by drilling,
America and the Western Canada Sedimentary provided other risk elements of the petroleum system
Basin. These are ‘mature’ provinces and/or onshore are favourable.
basins where drilling is relatively cheap and Whilst modern 2D and 3D seismic data are
targeted seismic campaigns directed more towards unlocking new knowledge about carbonate platform
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Seismic characterization of carbonates
intervals and length scales that are not otherwise They are now known to be associated with
accessible, challenges remain in characterizing the significant depositional, diagenetic and structural
rock–fluid properties and reservoir potential of heterogeneities, with corresponding challenges for
these carbonates at an exploration stage where there reservoir-quality prediction, dynamic modelling
are few or no data-rich wells that penetrate the plat- and reserve determinations. To unlock the explora-
forms themselves. Although carbonates are mineral- tion potential, assess the value of existing assets
ogically simple, their elastic properties are very and define the field development strategy of these
difficult to predict because of the variety in size, lacustrine carbonate reservoirs requires a multidisci-
shape and connectivity of pore systems, the large plinary approach that integrates detailed understand-
impedance contrast between this heterogeneous ing of the tectonic setting, stratigraphic evolution
fluid-filled porosity and the surrounding matrix, and impact of diagenesis. A fundamental objective
and the lack of simple VP–VS relationships (dis- is to determine how lacustrine carbonate heterogene-
cussed further below). This means that generic ity is reflected by changes in seismic geometries and
rock-physics attributes are lacking, other than for seismic facies.
texturally simple and homogeneous carbonates, Since 2018, in parallel with rapid field-develop-
and it can even impact identification of carbonates ment strategies and with most attractive acreage
where other geometric attributes and contextual positions being taken, there has been a significant
data are equivocal. For example, a stratabound high- increase in pre-salt carbonate publications, primarily
porosity unit in a carbonate platform could have the with examples of seismic expressions of the lacus-
same impedance as a transgressive shale, so careful trine carbonate systems in the field (e.g. Barnett
geologically guided seismic interpretation is always et al. 2018; Correa et al. 2019; Ferreira et al. 2019;
fundamental. Menezes de Jesus et al. 2019; Penna et al. 2019; Oli-
Because of the aforementioned challenges, vito and Souza 2020; Ferreira et al. 2021). In this
reservoir-quality assessments involving quantitative volume, two key articles from Barnett et al. (2020)
interrogation or inversion of the seismic data from and Minzoni et al. (2021) make an important contri-
undrilled carbonate prospects typically has to be bution to the study of pre-salt lacustrine reservoirs
model driven, and conditioned by data from wells from both a field-development and regional/explo-
in analogue fields that are considered to be geologi- ration context, respectively. To place these in con-
cally comparable. Embry et al. (2021) combined text, it is important to appreciate both the range of
seismic geometries, seismic stratigraphy and multi- pre-salt reservoir types and the ongoing debate asso-
attribute analysis to record the depositional history, ciated with their occurrence and origin.
geomorphology and facies architecture of a Creta- As summarized by Saller et al. (2016), Sabato
ceous carbonate platform margin prospect in the Ceraldi and Green (2017) and Wright (2021), the
Gulf of Mexico. The use of frequency decomposition South Atlantic pre-salt lacustrine carbonates com-
and colour blending techniques on the high-quality prise three very different primary reservoir types
3D survey generated spectacular images of facies (Fig. 4a). These are associated with major temporal
belts, reef morphologies and palaeokarst sinkholes. changes in lake-water chemistries and structural
The study discusses an iterative workflow employed positions on the basin margins (Fig. 4b). The oldest
to interpret seismic facies in terms of rock properties lacustrine carbonate reservoirs are biogenic (BIO),
using analogue long-offset well data and seismic dominated by accumulations or ‘coquinas’ of
forward modelling, leading to optimal placement of bivalve molluscs (Fig. 4a), and are interpreted as
an exploration well and subsequent post-drill valida- having been deposited in relatively freshwater to
tion of the conceptual model. saline waters (i.e. Thompson et al. 2015; Muniz and
Bosence 2018; Pietzsch et al. 2018; de Oliveira et al.
2019; Olivito and Souza 2020). In the Santos Basin,
The unique challenge of the South Atlantic these belong to the Itapema Formation. The low-
pre-salt carbonate reservoirs diversity fauna and the general absence of ‘normal’
lacustrine carbonate producers (such as charophytes)
Following the 2006 Tupi discovery (now known as points toward environmentally stressed conditions
the Lula Field) in the Lower Cretaceous lacustrine (e.g. Muniz and Bosence 2018; Olivito and Souza
carbonates of the Santos Basin, Brazil, the pre-salt 2020). Evidence from studies of faunal diversity is
carbonate systems have become the focus of inten- also supported by geochemical data (Pietzsch et al.
sive interest both from the petroleum industries and 2018). Although there are excellent analogues for
academia. Little was published on the prolific petro- these deposits at outcrop (e.g. Carvalho et al. 2000;
leum system of these lacustrine carbonates in the Favoreto et al. 2021), uncertainties have persisted
South American and conjugate African margin rift over the geometries of large-scale deposits and the
basins for over 10 years, hampering regional com- hydrodynamic–depositional processes responsible
parisons and development of holistic interpretations. for such large bioclastic accumulations.Downloaded from http://sp.lyellcollection.org/ by guest on August 24, 2021
J. Hendry et al.
Fig. 4. (a) Proposed three-fold division of pre-salt reservoir types in the South Atlantic comprising bioclastic (BIO),
clotted peloidal silica (CPS) and spherulite fibrous fan (SFF) (‘shrubs’) type facies associations, as named after the
principal components; modified after Wright and Barnett (2017); Wright 2021 pers. comm. Photomicrographs (i) and
(ii) are reproduced from Olivito and Souza (2020), (iii) is from Cazier et al. (2014), (iv) from Vieira de Luca et al.
(2017) (© AAPG 2017, reprinted by permission of the AAPG whose permission is required for further use), and (v)
and (vi) is from Wright (2021). (b) Diagram showing the particular stratigraphic and spatial distribution of these
facies associations on both conjugate margins of the South Atlantic, adapted from Saller et al. (2016).
The second reservoir type, referred to as the clot- (Moreira et al. 2007; Chaboureau et al. 2013)
ted peloidal silica system (CPS: Fig. 4a) (Hunt et al. (Fig. 4a), the youngest SFF pre-salt reservoirs are
2019), is typically characterized by seismic wedges characterized by crystalline ‘shrubs’ and spherulites,
and mounded geometries deposited on the flanks of associated with magnesian clays and dolomite, cor-
structural and volcanic highs in outer basin structural responding to the Barra Velha Formation in the San-
highs (Fig. 4b) (Saller et al. 2016; Hunt et al. 2019). tos Basin. The biogenic (microbial) v. abiotic origin
It comprises cycles of extensively silicified micro- of these has been hotly debated (see Muniz and
bialites, depositional breccias and dolomites. Older Bosence 2015; Wright and Barnett 2015) but on
parts of the CPS system are known to contain thin the Brazilian margin the consensus is that they are
coquina intervals, whereas above the basinward largely of abiotic origin and were precipitated by
extension of the pre-Alagoas unconformity (Fig. 4) chemical-driven processes within hypersaline–alka-
the youngest part of this system can also contain line lakes (Wright and Barnett 2015, 2017; Pietzsch
thin intervals with spherulite–fibrous fan compo- et al. 2018, 2020; Wright 2021). However, the
nents (Saller et al. 2016; Hunt et al. 2019). These synoptic relief of the carbonate accumulations and
CPS reservoirs contain abundant precompaction therefore the water depth at the time of formation
low-temperature silica cements (Fig. 4aiii–iv) (Saller has proved a source of continued discussion. A key
et al. 2016; Lapponi et al. 2019) and later hydrother- issue is whether seismically imaged geometries inter-
mal silica (Saller et al. 2016; Vieira de Luca et al. preted to represent prograding depositional slopes
2017; Lapponi et al. 2019). Thus, the CPS system (i.e. clinoforms: see Buckley et al. 2015; Minzoni
appears to be the distal equivalents of both the CPS et al. 2021) or microbial carbonate build-ups (i.e. Bar-
depositional system and the spherulite fibrous fan nett et al. 2018; Correa et al. 2019; Menezes de Jesus
(SFF) depositional system described below, and et al. 2019), are primary pre-salt depositional geome-
leads to some debate concerning its affinity to both tries or are alternatively seismic imaging artefacts, a
the BIO and SFF reservoir systems (Fig. 4a, b). result of post-depositional structural deformation or
Above a prominent unconformity, known as the igneous extrusive or intrusive bodies (Wright and
Pre-Alagoas unconformity on the Brazilian margin Barnett 2017, 2019; Wright and Rodriguez 2018).Downloaded from http://sp.lyellcollection.org/ by guest on August 24, 2021
Seismic characterization of carbonates
Aptian pre-salt deposits on the African margin geometry and clinoform progradation that is inter-
(Kwanza Basin) are broadly correlative to the preted to have been controlled by winds that varied
Barra Velha Formation but contain facies identified on a seasonal basis. These same preferred clinoform
as microbial in origin (Saller et al. 2016; Sabato orientations are recognized by Barnett et al. (2020)
Ceraldi and Green 2017), as well as shrubs and in the Campos Basin (Olivito and Souza 2020). Min-
spherulites similar to those of the Barra Velha. Lim- zoni et al. (2021) also infer regionally consistent
ited seismic data have been published but steep-sided accommodation patterns and platform downstepping
platforms and isolated to amalgamated mounds have reflecting long-term changes in lake level. These
been interpreted on the crests of basement fault conclusions follow from an underlying assumption
blocks (Saller et al. 2016). Whilst there are numerous that the carbonate depositional system behaves in a
modern and ancient alkaline–saline lake microbia- way similar to modern tropical-marine carbonate
lites that could serve as partial (small-scale) ana- platforms, with water-depth-dependent production
logues to these, direct outcrop analogues for the profiles, and facies controlled by water depth,
abiotic spherulite and shrub facies are very rare.(cf. accommodation and wave energy. Such assumptions
Mercedes-Martín et al. 2017). are common when interpreting apparent synoptic
Current debate concerning the interpretation of relief developed by lacustrine carbonate systems,
pre-salt seismic geometries is focused on five topics: and so it is certainly useful to consider if they
(1) Are clinoform geometries real or artefacts could apply in this case and, if they do, what interpre-
derived from a complex salt overburden, syndeposi- tations and predictions ensue.
tional tectonism or volcanism? (2) Was significant To date, most studies of the bioclastic pre-salt res-
synoptic depositional relief associated with deposi- ervoirs have been focused on the African Congo
tion of the shrub–spherulite systems? (3) How Basin and the Campos Basin of Brazil (e.g. Harris
should present-day geometries be restored to account 2000; Olivito and Souza 2020). Barnett et al.
for syn- and post-depositional deformation? (4) Is (2020) use well data from the Mero Field in the
it necessary to account for the scale difference northern Santos Basin to constrain seismic-scale cli-
between seismic geometries and those of modern/ noformal and mounded geometries. They demon-
ancient lacustrine analogues, and how might this strate how borehole-image-derived dips, corrected
be approached? (5) How to differentiate between to account for later structural tilt, show repeated
carbonates and precursor rift volcanics given the patterns of low–high dip values that correspond to
overlap in their acoustic properties and some seismic seismically imaged clinoforms. The preferred orien-
geometries/facies? tation of the clinoforms is interpreted to indicate
At one end of the spectrum of interpretations, wind-driven submarine currents, much as suggested
some workers consider the post-Alagoas hypersa- by Minzoni et al. (2021) but, in this case, reworking
line–alkaline lake system to be entirely shallow, shells from lower-energy settings into spits and bars
and that the observed seismic geometries are acqui- nucleated on basement ridges. Clearly, some consid-
sition artefacts, and/or are otherwise misleading erable relief was developed during deposition of the
and so have been misinterpreted (Wright and Barnett biogenic system (Barnett et al. 2020) and could pre-
2017; Wright and Rodriguez 2018). In contrast, oth- sumably have been inherited by the succeeding
ers conclude that the geometries are largely primary, hypersaline lacustrine system.
closely mirror those of tropical marine counterparts Reservoir property prediction during exploration
and that the individual fields represent a complex is closely associated with seismic-scale geometries
archipelago of isolated lacustrine carbonate plat- and interpreted seismic facies, and it should be
forms surrounded by deeper waters (e.g. Simo noted that understanding of seismic facies in non-
et al. 2019; Minzoni et al. 2021). These different marine carbonate depositional systems is still in its
positions also involve additional arguments related infancy, and so caution is required in direct compar-
to geochemical data and high-frequency correlations ison with and the use of the same terminology as
in the uppermost part of the pre-salt stratigraphy marine examples. A key consideration, alluded to
(Pietzsch et al. 2018), so there remains much to by Barnett et al. (2020), is the impact of both syn-
finally resolve. and post-depositional tectonic activity. This can
Minzoni et al. (2021) utilize a tapestry of high- result in misleading structures and seismic geome-
quality 3D seismic data tied to wells from the north- tries: for example, Brown (2011, p. 121):
ern Santos Basin to reveal a picture of isolated lacus-
trine carbonate platforms that nucleated on tectonic
In general, stratigraphic features, after being deposited
highs. In both the fresh saline BIO and overlying on a flat-lying surface, will be bent and broken by
hypersaline–alkaline SFF lacustrine systems, they later tectonic movements. Stratigraphy and structure
interpret clinoformal geometries to indicate water then become confused and the interpretive task comes
depths of up to several hundred metres. They recog- in separating them. The structure must be interpreted
nize regionally consistent asymmetry in platform before stratigraphy can be appreciated.Downloaded from http://sp.lyellcollection.org/ by guest on August 24, 2021
J. Hendry et al.
It is important that future studies utilize complete set of processes operated. Processes can take many
integration of interpretations of the tectonic evolu- forms: for example, stratigraphic (Burgess 2012) or
tion of the fields and basins to reach agreement on seismic (Janson et al. 2007; Janson and Fomel
exactly which aspects of seismic imaged geometries 2011). A forward model composed of a reasonable
are pre- and post-depositional. representation of well-understood physical processes
As a final comment, it is important to note that is already somewhat calibrated by the information
considerable efforts are underway to apply quantita- encoded in that process representation, so it should
tive seismic-inversion and rock-physics methodolo- output generally realistic results (Burgess 2012).
gies for the purpose of 3D inversion and reservoir Careful comparison between the forward-model out-
model building. Although this important aspect of put and seismic data, perhaps using the data to refine
seismic reservoir characterization in the pre-salt is the forward-model parameters to improve the match
not covered in this volume, it is beginning to appear between models and observations from a specific
in the literature (e.g. Ferreira et al. 2019, 2021; Penna outcrop or subsurface example, can then provide
et al. 2019; Dias et al. 2021). insight into what is the most realistic interpretation
of the seismic image.
If two forward models are combined, one to
Novel developments in seismic modelling of calculate stratigraphic products of depositional pro-
carbonates cesses and the other to simulate the seismic imaging
of those strata (e.g. Lecomte et al. 2015; Masiero
Although seismic data are without doubt extremely et al. 2020), a synthetic seismic image of a strati-
useful to image, understand and explore subsurface graphic forward model is produced (e.g. Antonatos
strata, we also know that interpretation of seismic 2018; Masiero et al. 2021). Comparing this syn-
images is often complicated by significant problems thetic seismic image with a seismic image of the
such as poor image quality, and resulting uncertainty real subsurface is a potentially powerful means to
about accuracy and uniqueness of interpretation of understand the formation and significance of
imaged geological features. Faced with this uncer- sequence-stratigraphic features imaged in the seis-
tainty, inverse- and forward-modelling methods are mic data, such as onlap or truncation. This is because
key tools to help make the critical link between the causal mechanism of the features is fully under-
‘real’ outcrop and interpreted subsurface carbonate stood from examination of the stratigraphic forward
geology (e.g. Lanteaume et al. 2018). Four papers model without having to make the usual unsubstan-
in this volume use various aspects of the inverse tiated seismic and sequence stratigraphic assump-
and/or forward modelling to explore how carbonate tions (see Thorne 1992 for an insightful discussion).
depositional and diagenetic processes are recorded in Mascolo and Lecomte (2021) employ detailed
carbonate strata and imaged by seismic data. digital outcrop data to construct synthetic seismic
Inverse methods use mathematical and statistical models of the Cretaceous Apulian carbonate plat-
techniques to recover information from observed form–slope–basin transition exposed across the
seismic data on subsurface physical properties (Bar- Maiella Mountain in central Italy. The aim of the
clay et al. 2008; Bosch et al. 2010). The simplest seismic modelling is to use the outcrop as an ana-
example takes the reflectivity of rock interfaces, as logue for subsurface platform margin and slope
measured more-or-less directly in seismic data, and strata elsewhere: for example, in the Adriatic Sea.
converts that to impendence, or other rock properties Synthetic seismic data are generated in both post-
such as porosity. Inverse modelling can also be used stack-time- and pre-stack-depth-migrated domains
to explore the seismic expression of major diagenetic to image the spatial distribution and depositional
features such as palaeokarst (see the introduction to architecture of the slope-failure escarpment and
Fournillon et al. 2021 below). Details of all the base-of-slope carbonate strata mapped in the out-
numerous and complex pre- and post-stack inversion crops. This approach is novel and pragmatic because
algorithms are beyond the scope of this paper but a the use of seismically imaged outcrops fills the scale
recent review can be found in Schuster (2017). A gap imposed by seismic resolution limits with criti-
key point is that the complex acoustic properties of cal predictive detail that could usefully reduce
carbonates mean that precise inversion workflows some key subsurface uncertainties.
tend to be case-specific depending, for example, on With a similar aim, Masiero et al. (2021) inte-
the rock fabric, fluid type, fracturing or karstification, grate two forward models to explore how carbonate
and on the desired outcome, so analysis via the platforms develop in and respond to synrift tectonic
inverse method is useful but rarely straight forward. settings. One is a novel 3D stratigraphic forward
Forward modelling, in contrast to inverse meth- model that predicts platform architectures and
ods, does not start with the original data but, instead, gross facies distributions in response to variables
calculates forward in time from some starting condi- such as fault geometry, displacement rate, tectonic
tion to determine what would be observed if a given subsidence, climate and eustasy, and the other is aDownloaded from http://sp.lyellcollection.org/ by guest on August 24, 2021
Seismic characterization of carbonates
seismic forward model to produce the corresponding determined to be carbonates can potentially indicate
acoustic response on 2D sections. The work adds reservoir quality, and acoustic impedance is the attri-
supporting evidence to existing models of synrift bute commonly used to target reservoir potential.
carbonate-platform evolution by reproducing the However, early diagenesis can have a major impact
fundamental platform morphologies and facies dis- on pore stiffness and P-wave attenuation, especially
tribution characteristics described in such conceptual early cementation in grainy limestones, generation of
models. However, some results diverge from the microporosity in muddy limestones and dolomitiza-
standard conceptual model predictions: for example, tion (e.g. Brigaud et al. 2010; Vanden Berg et al.
for the same slip rates, under high-transport regimes 2018; Salih et al. 2021). Significantly, oil emplace-
the platform is likely to prograde downdip, while in ment may have a lesser impact on pore stiffness
low-transport systems the platform margin quickly and, hence, VP than on total porosity because
backsteps towards the footwall crest. The modelling cementation may still take place in the water-wet
also suggests that backstepping platform margins microporous grains, increasing their rigidity. Whilst
may be difficult to identify on seismic images due numerous experimental and theoretical studies have
to bypass slopes with fringing aprons where grainy attempted to characterize carbonate rock physics in
sediments are mixed with fine-grained facies, gener- texturally simple cases, dual or even treble porosity
ating continuous high-amplitude seismic reflections systems occur in many reservoirs (e.g. microporos-
from the platform interior onto the slope. ity, interparticle and moldic porosity, fractures and
A somewhat different approach is adopted to connected vugs). Such reservoirs may be unique
explore a specific example of platform drowning cases requiring direct calibration from well data, or
by Gold et al. (2020). They derive a well-to-seismic at least careful geological assumptions about deposi-
tie across a 600 m-thick 4 myr duration drowning tional and diagenetic processes and their resultant
interval at the top of the Yawee Limestone carbonate geometrically categorized pore fabrics (e.g. Warrlich
platform on the southern margin of West Papua. The et al. 2010; Fournier et al. 2014, 2018).
well tie is then used to refine the seismic depth– Laboratory measurements have shown that the
velocity model and reprocess the seismic data. fluid effect on acoustic properties in carbonate
Improved seismic imagery is analysed with a seismic rocks is small in magnitude, and more complex in
inversion and stratigraphic forward modelling nature (i.e. less predictable) than in their siliciclastic
method to better understand and predict carbonate counterparts. Because of their anisotropic heteroge-
and shale facies distribution, porosity, and pore-fluid neous pore networks and complex diagenetic histo-
properties through the heterolithic drowning succes- ries, carbonate rocks may not follow simple rock
sion strata. Seismic inversion is discussed in more mechanical rules like the Gassmann equation
detail below. (Baechle et al. 2005, 2009; Vanorio et al. 2008;
Verwer et al. 2008), although this is still used as a
reference: for example, where homogeneous matrix
Seismic characterization of fluid flow and porosity is augmented by variable amounts of frac-
diagenesis in carbonates ture porosity (e.g. Hammond and Payne 2013). As
a result of these complexities, interpretation of the
Seismic inversion, the process of extracting reservoir seismic signal in carbonates can be challenging as
and fluid characteristics from seismic data, involves the interpreter has to decide whether seismic reflec-
making geologically informed assumptions about tions follow geological time lines, as assumed by
the physical (elastic) behaviour of the rock matrix the seismic stratigraphy principles (Eberli et al.
(see Avseth et al. 2014). Carbonate rocks have faster 2002), or whether reflection geometries are influ-
acoustic velocities, on average, than siliciclastic enced by diagenetic modifications that can cut
rocks, coupled with a very high potential for diage- across time lines (e.g. Fournier and Borgomano
netic modification. The acoustic properties of car- 2007; Warrlich et al. 2010; Teillet et al. 2020).
bonate rocks have previously been shown to be Liquid substitution in the pore space (brine to oil,
influenced by mineralogy (e.g. calcite v. dolomite), for instance) results in very small changes in imped-
total porosity, pore shape, amount of microporosity ance, making liquid direct hydrocarbon indicators
and, to a lesser extent, fluid content (Eberli et al. (DHIs) in carbonate system seismic data very diffi-
2003; Xu and Payne 2009; Weger et al. 2009). cult to detect (Rafavich et al. 1984; Adam et al.
This means that relationships between VP, VS, poros- 2006). As a consequence, the most common DHIs
ity and permeability, and their variations with burial in carbonates are created by gas–water contacts
depth, are much less tightly constrained than in sili- within high-porosity reservoirs, although these are
ciclastic sandstones. Empirical data show that some probably reliable only with a thick reservoir interval
limestones approximate to a common VP–porosity and/or lower impedance of the gas leg than overly-
transform (‘Wylie time-average relationship’), so ing shale, or with high-quality S-wave data and
low-velocity anomalies within strata independently VP/VS calibration from nearby or representativeYou can also read
