ExxonMobil (United Kingdom)

Carbonate (limestone and dolomite) reservoirs account for approximately 50% of oil and gas production worldwide. However, seismic responses in carbonate rocks are poorly understood. In addition, DHI ranking and AVO classification systems developed for clastic rocks are unlikely to be applicable to carbonate rocks. An accurate and physically sound carbonate rock physics model is needed to address these technical issues.

Joanna Ajdukiewicz joined Exxon Production Research Company in 1980. She was Reservoir Quality Assessment and Prediction team lead there from 1991 to 1995 and at Imperial Oil Research Centre in Calgary from 1995 to 1997. Subsequently, she has worked a variety of Exploration Company assignments in the North Sea, Gulf of Mexico, and Middle East. Her current interests are in predicting the distribution of early diagenetic controls on deep reservoir quality. Rob Lander develops diagenetic models for Geocosm LLC. He obtained his Ph.D. in geology from the University of Illinois in 1991, was a research geologist at Exxon Production Research from 1991 to 1993, and worked for Rogaland Research and Geologica AS from 1993 to 2000. He is also a research fellow at the Bureau of Economic Geology. To guess is cheap; to guess wrongly is expensive (Chinese proverb). Reservoir-quality predictive models will be a useful element of risk analysis until remote-sensing tools are invented that accurately measure effective porosity and permeability ahead of the bit. This issue of the AAPG Bulletin highlights recent advances in a new generation of reservoir quality models that have successfully predicted porosity and permeability in diverse siliclastic reservoirs under many different burial conditions. Most previous attempts at predrill reservoir quality prediction have relied on empirical correlations or on first-principle geochemical simulations that incorporate laboratory-derived input parameters (Wood and Byrnes, 1994). The new reservoir quality models differ from previous approaches in that, although incorporating theory-inspired algorithms, they include terms with values that are explicitly designed to be calibrated by, and tested against, data sets of high-quality petrographic analyses that are linked to thermal and effective-stress histories. Petrographic observations therefore provide essential constraints in these models on the types, timing, and rates of key geologic processes affecting sandstone pore systems. This approach avoids the pitfalls inherent …

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Abstract Seismic data from the Angolan margin and laboratory experiments on brittle-ductile models are used to study thin-skinned deformation above the salt at margin scale, which is characterized by extension upslope and contraction downslope. The initial geometry of the salt basin was wedging out, both landward and seaward, and the salt was entirely covered by sediments at the onset of gravity-driven deformation. Downslope contraction is accommodated by upslope extension. The upslope extensional domain is subdivided into three subdomains with (1) sealed tilted blocks, (2) growth fault and rollover systems, and (3) extensional diapirs. We interpret this particular arrangement of extensional structures as directly resulting from variations in mechanical coupling between brittle (sediments) and ductile (salt) layers. The downslope contractional domain is subdivided into three subdomains with (1) diapirs squeezed at late stage, (2) polyharmonic folds and thrust faults developed at early stage, and (3) folds and thrusts developed at late stage. This structural zoning results from the initiation of contraction at a distance from the salt toe and further migration of contraction immediately downslope and upslope.

Abstract Analogues, especially outcrop analogues, have played a central role in improving understanding of subsurface reservoir architectures. Analogues provide important information on geobody size, geometry and potential connectivity. The historical application of outcrop analogues for understanding geobody distributions in reservoirs is reviewed, from the pioneering work of the 1960s to the high-tech virtual outcrop methodologies of today. Four key types of analogue data are identified: hard data, which describe the dimensions and geometry of the geobody; soft data, which describe the conceptual relationships between different geobody types; training images, which record the dimensions, proportions and spatial relationship; and analogue production data, which are taken from direct subsurface production analogues. The use of these different data types at different stages of the geomodelling workflow is discussed and the potential sources of error considered. Finally, a review of geobody and analogue studies in different clastic environments is discussed with reference to selected previous work and the range of papers in the current volume.

Abstract Observations and hydrothermal experiments were used to derive new information about how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones. Samples of deeply buried, porous sandstones with different types of clay coats were split in two, coats removed from one of each pair of splits, and grain surfaces inspected with scanning electron microscopy. Quartz grains in a fluvial-deltaic sandstone buried to 115°C had no visible authigenic quartz on grain surfaces cleaned of diagenetic chlorite coats, though well-developed overgrowths occurred on nearby, naturally uncoated grains. However, in similar sandstones buried to ≥164°C, quartz-grain surfaces exposed by chlorite-coat removal were covered with small (∼5 μm), mainly anhedral, syntaxial quartz overgrowths. Similar overgrowths were observed under various detrital and diagenetic clay coats in porous eolian sandstones buried to temperatures up to 215°C. We conclude that clay coats may retard quartz nucleation at moderate temperatures, but at high temperatures, many coats permit quartz nucleation and preserve porosity by limiting cement growth. To investigate cement growth-limitation mechanisms, samples with coats removed were subjected to quartz-cementing conditions in a hydrothermal reactor. During experiments, the naturally occurring small overgrowths on clay-cleaned grains coalesced and grew, suggesting that clay particles in coats inhibit cement growth by forming barriers to early-overgrowth coalescence. Although the fraction of grain-surface coverage is the primary control on cement inhibition by coats, cement growth–interference textures vary with coat type, providing a mechanism by which coat composition may be a secondary control on inhibitory effectiveness. In deeply buried sandstones, quartz cement can fill significant microporosity within diagenetic chlorite coats, potentially affecting mechanical and petrophysical rock properties.

Abstract We have developed process-based models for early grain coats and their impact on deep reservoir quality in the Jurassic eolian Norphlet Formation, Alabama, with implications for exploration and development in other conventional and tight-gas continental reservoirs. The Norphlet, a major gas reservoir to depths of 21,800 ft (6645 m) and temperatures of 419°F (215°C), displays contrasting intervals of high and low reservoir quality within compositionally similar cross-bedded eolian sands. Study results show that grain coats formed soon after deposition are responsible for differences in deep Norphlet porosity of up to 20% and permeability up to 200 md. Three types of grain coats were identified in Norphlet dune sands, each formed in a different part of a shallow groundwater system, and each with distinctive impact on deep reservoir quality. Diagenetic chlorite coats, formed where dunes subsided into shallow hypersaline groundwater, preserve good deep porosity (to 20%) and permeability (to 200 md). Continuous tangential illitic coats, formed in the vadose zone of stabilized dunes exposed to periodic fresh-water influx, preserve good deep porosity (to 15%) associated with poor permeability (<1 md) due to linked formation of later high-temperature diagenetic illite. Discontinuous grain coats, formed in active dunes where grains were abraded by eolian transport, are associated at depth with tight zones of pervasive quartz cement, low porosity (<8%), and low permeability (<1 md). These concepts plus data from 60 wells were used to derive bay-wide predictive tight and porous-zone isopachs that can be used for well placement, geologic models, and field development.

Abstract The Eastern Barents, Kara, Laptev, East Siberian seas and the western Chukchi Sea occupy a large part of the Eurasian Arctic epicontinental shelf in the Russian Arctic. Recent studies have shown that this huge region consists of over 40 sedimentary basins of variable age and genesis which are thought to bear significant undiscovered hydrocarbon resources. Important tectonic events controlling the structure and petroleum geology of the basins are the Caledonian collision and orogeny followed by Late Devonian to Early Carboniferous rifting, Late Palaeozoic Baltica–Siberia collision and Uralian orogeny, Triassic and Early Jurassic rifting, Late Jurassic to Early Cretaceous Canada Basin opening accompanied by closure of the South Anyui Ocean, the Late Mesozoic Verkhoyansk–Brookian orogeny and Cenozoic opening of the Eurasia Oceanic Basin. The majority of the sedimentary basins were formed and developed in a rift and post-rift setting and later modified through a series of structural inversions. Using available regional seismic lines correlated with borehole data, onshore geology in areas with no exploration drilling, and recent Arctic-wide magnetic, bathymetry and gravity grids, we provide more confident characterization of the regional structural elements of the Russian Arctic shelf, and constrain the timing of basin formation, structural styles, lithostratigraphy and possible hydrocarbon systems and petroleum play elements in frontier areas.

Abstract Conventional reservoir modeling approaches are developed to account for uncertainty associated with sparse subsurface data but are not equipped for detailed reconstruction of high-resolution geologic data sets. We present a surface-based modeling procedure that enables explicit representation of heterogeneity across a hierarchy of length scales. Numerous surfaces are used to construct complex facies-body geometries and distributions prior to generating a grid, allowing sampled and conceptual data to be fully incorporated within field-scale models. Our approach is driven by the improved efficiency that surfaces introduce to reservoir modeling through their geologically intuitive design, rapid construction, and ease of manipulation. Cornerpoint gridding of the architecture defined by the surfaces reduces the number of cells required to represent complex geometries, thus preserving geologic detail and rendering upscaling unnecessary for fluid-flow simulations. The application of surface-based modeling is demonstrated by reconstructing the detailed three-dimensional facies architecture of a wave-dominated shoreface-shelf parasequence from a rich outcrop data set. The studied outcrop data set describes reservoir architecture in a generic analog for many shallow-marine reservoirs. The process of model construction has demonstrated the function of (1) shoreface-shelf clinoforms, (2) paleogeographic changes in shoreline orientation, and (3) storm-event-bed amalgamation in controlling facies architecture. These subtle geometric features cannot be accurately represented using conventional stochastic reservoir modeling algorithms, which results in poor estimation of facies proportions and associated hydrocarbon volumes in place. In contrast, the surface-based modeling approach honors all data and captures subtle geometric facies relationships, thus allowing detailed and robust reservoir characterization.

Research Article| July 01, 2011 Decoding downstream trends in stratigraphic grain size as a function of tectonic subsidence and sediment supply Alexander C. Whittaker; Alexander C. Whittaker † 1Department of Earth Science and Engineering, Royal School of Mines, Imperial College, London SW7 2AZ, UK †E-mail: a.whittaker@imperial.ac.uk Search for other works by this author on: GSW Google Scholar Robert A. Duller; Robert A. Duller 1Department of Earth Science and Engineering, Royal School of Mines, Imperial College, London SW7 2AZ, UK Search for other works by this author on: GSW Google Scholar Joshua Springett; Joshua Springett 2ExxonMobil, Leatherhead, Surrey KT22 8UX, UK Search for other works by this author on: GSW Google Scholar Rosie A. Smithells; Rosie A. Smithells 3Earth Science and Geography, School of Physical and Geographical Sciences, Keele University, William Smith Building, Keele, Staffordshire ST5 5BG, UK Search for other works by this author on: GSW Google Scholar Amy L. Whitchurch; Amy L. Whitchurch 1Department of Earth Science and Engineering, Royal School of Mines, Imperial College, London SW7 2AZ, UK Search for other works by this author on: GSW Google Scholar Philip A. Allen Philip A. Allen 1Department of Earth Science and Engineering, Royal School of Mines, Imperial College, London SW7 2AZ, UK Search for other works by this author on: GSW Google Scholar GSA Bulletin (2011) 123 (7-8): 1363–1382. https://doi.org/10.1130/B30351.1 Article history received: 14 Jun 2010 rev-recd: 06 Aug 2010 accepted: 08 Aug 2010 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alexander C. Whittaker, Robert A. Duller, Joshua Springett, Rosie A. Smithells, Amy L. Whitchurch, Philip A. Allen; Decoding downstream trends in stratigraphic grain size as a function of tectonic subsidence and sediment supply. GSA Bulletin 2011;; 123 (7-8): 1363–1382. doi: https://doi.org/10.1130/B30351.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Downstream grain-size fining in stratigraphy is driven primarily by selective deposition of sediment, and the long-term efficiency of this process is determined by: (1) the magnitude and characteristics of the input sediment supply; (2) the spatial distribution of subsidence rate, which creates accommodation for sediment preservation; and (3) the dynamics of sediment transport and deposition. A key challenge is to determine how these first two factors control the caliber and spatial distribution of deposits over time scales of 104–106 yr without incorporating sediment transport details that are largely unknowable for time-averaged stratigraphy in the geological past. We address this using grain-size data collected from fluvial conglomerates in the Eocene Pobla Basin, Spanish Pyrenees, a synorogenic basin where the timing of sediment deposition is well-constrained; the sediment budget is closed; and good exposure enables time lines within stratigraphy to be picked out unambiguously. For successive stratigraphic horizons, downstream trends in grain size and composition are derived for basin-filling sediment-routing systems with length scales of 6 and 40 km, respectively. Our data show that the rate of grain-size fining varies over time and with system length and can be linked to changes in source area. These results are contrasted with grain-size data from the Antist Group, a 60-km-long Oligocene system that mantles the Southern Pyrenees, where very slow rates of grain-size fining on the wedge top of this fold-and-thrust belt are observed. We apply a self-similarity–based selective deposition model to quantify the competing controls of tectonic subsidence and sediment supply on derived grain-size trends, and model results are compared with independent constraints on the Eocene–Oligocene evolution of the Pyrenees. Our results suggest that it is now possible to invert time-averaged grain-size trends in stratigraphy to gain quantitative information on the geological boundary conditions governing the evolution of sedimentary basins. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Regional grain size trends in fluvial successions can reveal important information regarding the dynamics of sediment routing systems. Self‐similar solutions for down‐system grain size fining have recently been proposed to explore how key variables, such as the spatial distribution of deposition, sediment discharge, and sediment supply characteristics, control spatial distribution of grain size in fluvial successions over time scales of 10 4 –10 6 years. We explore the sensitivity of these solutions to changes in key variables and assess their applicability to ancient fluvial successions. Several sensitivity analyses are presented to investigate the relative control of the key model variables on the spatial pattern of down‐system grain size fining in fluvial successions. Sensitivity analyses demonstrate that (1) an increase in the initial value of sediment discharge to a basin causes a decrease in the rate of grain size fining in fluvial successions, an effect that becomes nonlinear for large values of initial sediment discharge; (2) a short‐wavelength/high‐amplitude subsidence regime generates a greater rate of down‐system grain size fining and a long‐wavelength/lower‐amplitude subsidence regime generates a lesser rate of down‐system grain size fining in fluvial successions; and (3) an increase in the spread of grain sizes in the sediment supply generates a greater rate of down‐system grain size fining. We apply this modeling technique to grain size data sets collected from two time surfaces within conglomerates of the Upper Eocene Montsor Fan Succession of the Pobla Basin, Spanish Pyrenees. These data sets exhibit approximately self‐similar grain size distributions; further, the observed increase in down‐system grain size fining associated with smaller depositional system lengths provides support for the application of self‐similar solutions to fluvial successions. By applying these solutions to carefully collected grain size data from fluvial successions, we are able to relate explicitly the initial grain size supplied to the system, the spatial distribution of subsidence and the sediment discharge into the basin to the rate of grain size fining in fluvial successions. This method thus offers a powerful means of elucidating sediment routing system dynamics over time.

Abstract The use of digital outcrop models in combination with traditional sedimentological field data improves the accuracy and efficiency of qualitative and quantitative characterization of outcrop analogs for reservoir modeling purposes. In this article, we apply an innovative methodology of outcrop characterization to an Upper Triassic fluvial-dominated system, exposed in extensive outcrops with limited three-dimensional (3-D) exposure. Qualitative analysis of the study outcrop allows the subdivision of the formation into three architectural intervals. Each interval can be further subdivided into subintervals on the basis of architectural style. This subdivision provides information on reservoir compartmentalization, which is used for zonation of the geocellular model. Qualitative analysis also provides valuable information on reservoir facies distribution. A new technique termed “perpendicular projection plane” is presented as a tool for quantitative analysis of outcrops with reduced 3-D exposure. This technique improves the accuracy of apparent width measurements of geobodies exposed in outcrops, which are subparallel to paleoflow. The quantitative analysis provides a detailed data set of geobody dimensions to use as conditioning data for analog reservoir models. Statistical analysis of the dimensions provides empirical relationships to apply in subsurface analog systems to reduce uncertainty related to stochastic modeling approaches.

Outcrop analogue studies provide key information for reservoir modelling which is difficult to obtain from traditional subsurface datasets (i.e. seismic data, well data). Terrestrial laser scanners or LiDAR (light detection and ranging), combined with digital photography, provide a new technique to create high resolution 3D digital outcrop models (DOMs). These DOMs generate exhaustive information which is used to build more realistic three-dimensional facies-based geocellular models and populate the different model zones. This paper documents the use of an extensive dataset, which combines high resolution traditional field data and DOMs. We provide an accurate description of the workflow followed in the geocellular modelling of a fluvial-dominated continental formation. Geocellular facies association and connectivity models are well constrained by outcrop observations and a number of different techniques are used in the quality control of the final model. The study also qualitatively discusses the uncertainties identified during the workflow and proposes methods to reduce them. The workflow and results shown in this paper can be applied in similar analogue systems in order to help improve model building for subsurface reservoirs. © 2010 EAGE/Geological Society of London.

Abstract The paper summarizes the results of geological and geophysical studies of the Siberian Arctic Shelf (Laptev, East Siberian and Chukchi seas), which is one of the largest continental shelves on Earth. This region consists of as many as 22 significant sedimentary basins of variable age and genesis which are expected to bear significant undiscovered volumes of hydrocarbons. Two major groups of the basins are identified based on the age of the underlying crustal basement: (1) post-Hauterivian/Barremian basins resting on the Late Mesozoic folded basement; and (2) older (Late Palaeozoic to Early Mesozoic?) basins preserved outboard of the Late Mesozoic deformational front in the northern part of the East Siberian and Chukchi seas. At least two significant tectonic events caused the overall tectonic pattern of the shelf as well as formation and structural styles of its individual crustal domains and the sedimentary basins: (1) Late Mesozoic convergence and subsequent collision of the Arctic Alaska–Chukchi Microplate with the Verkhoyansk–Kolyma/Omolon margin of the North Asia Continent around 130–125 Ma (the Verkhoyansk–Brookian compressional event); and (2) a series of Cretaceous and Cenozoic extensional events related to the origin of the Arctic oceanic basins. Based on 2D regional multichannel seismic reflection data constrained by onshore geology, plate tectonic models and inter-regional correlations, as well as on gravity and magnetic grids, the structural styles, lithostratigraphy and possible hydrocarbon systems of the offshore sedimentary basins are considered.

Abstract Distributed acoustic sensing (DAS) using fiber-optic cables is a recent addition to seismic acquisition methods. However, a DAS “sensor” differs significantly from conventional, discrete sensing devices such as geophones or accelerometers. For one, DAS measures something akin to strain instead of particle velocity or acceleration. Other properties of the DAS system also aren't obvious at first. What is its instrument response, noise performance, and repeatability? How are DAS channels properly positioned, e.g., in case of a borehole deployment: depth calibrated? To better understand these issues and their impact on the DAS seismic method's application space, a field test was conducted in which three DAS vendors recorded the same survey using a borehole-installed fiber while recording simultaneously with a conventional downhole array. The results show that all DAS systems achieved good, repeatable signal integrity while exhibiting different noise characteristics. DAS noise can be addressed with well-established processing algorithms, but further benefits can be gained from DAS-specific algorithms. Where required, DAS seismic data can be processed to closely match the vector response of conventional geophones. DAS data converted in this way can assist in the up/down separation step without the need for dip filters. DAS VSP data can also be merged with conventional 3D and 4D seismic, adding value in situations such as undershooting of surface facilities in marine settings.

To investigate the impact of orogenic inheritance on the characteristics of the North Atlantic rift system we develop new mapping methods that highlight the first-order architecture and timing of rifts, as well as the distribution of heterogeneities inherited from the Palaeozoic Caledonian and Variscan orogenies. These maps demonstrate major differences in the behaviour of the North Atlantic rift system relative to the two orogens, with the Variscan front appearing to be an important boundary: the rift cuts through the Caledonian orogen and is parallel to its structural grain to the north, whereas it circumvents the core of the Variscides to the south. In addition, rifting is protracted and polyphase with break-up being magma-rich north of the Variscan front, as opposed to the south, where a single, apparently continuous extensional event led to magma-poor break-up in less than 50 myr. Also, the North Atlantic rift system reactivates sutures corresponding to large (>2000 km) former oceans, whereas sutures of small (<500–1000 km) oceanic basins are little affected in both the northern and southern North Atlantic. These observations point to a major influence of orogenic inheritance on the characteristics of rift systems.

The western Greater Caucasus formed by the tectonic inversion of the western strand of the Greater Caucasus Basin, a Mesozoic rift that opened at the southern margin of Laurasia. Subsidence analysis indicates that the main phase of rifting occurred during the Aalenian to Bajocian synchronous with that in the eastern Alborz and, possibly, the South Caspian Basin. Secondary episodes of subsidence during the late Tithonian to Berriasian and Hauterivian to early Aptian are tentatively linked to initial rifting within the western, and possibly eastern, Black Sea and during the late Campanian to Danian to the opening of the eastern Black Sea. Initial uplift, subaerial exposure, and sediment derivation from the western Greater Caucasus occurred at the Eocene-Oligocene transition. Oligocene and younger sediments on the southern margin of the former basin were derived from the inverting basin and uplifted parts of its northern margin, indicating that the western Greater Caucasus Basin had closed by this time. A predominance of pollen representing a montane forest environment (dominated by Pinacean pollen) within these sediments suggests that the uplifting Caucasian hinterland had a paleoaltitude of around 2 km from early Oligocene time. The closure of the western Greater Caucasus Basin and significant uplift of the range at approximately 34 Ma is earlier than stated in many studies and needs to be incorporated into geodynamic models for the Arabia-Eurasia region.

Inhaled administration is underutilised because the drug discovery process is viewed as challenging, risky, and expensive. However, unmet medical need continues to grow, and significant opportunities exist to discover novel inhaled medicines delivering the required lung concentrations while minimising systemic exposure. This profile could be achieved by a combination of properties, including lung retention and low oral bioavailability. Property-based rules exist for orally administered compounds, but there has been limited progress defining in silico predictors to guide the discovery of novel inhaled drugs. Recently, the use of informative cell- and tissue-based screens has greatly facilitated the identification of compounds with optimal characteristics for inhaled delivery. Here, we address opportunities for novel inhaled drugs, and the key challenges and uncertainties hampering progress.

The Western Black Sea (WBS) is now an area of active petroleum exploration, and several seismic surveys have been acquired to support the ongoing activity. Seismic imaging in the WBS is challenging due to the presence of complex overburden, such as canyons, shallow gas channels, and complex shallow stratigraphy. Such overburden complexities create amplitude dimming and wipeouts in the images and cause structural distortions of the reservoir units. Conventional velocity model building and seismic migration methods are not optimal for addressing such challenges. Full-wavefield inversion (FWI) technology was applied to overcome the complex seismic-imaging challenges in the WBS and to support the exploration and development program. To date, a suite of FWI products has been used to identify shallow hazards, derisk prospects, optimize well placement, perform accurate resource estimation, and support development planning. Unlike traditional seismic migration, generated FWI products are not simply depictions of the seismic reflectivity but rather are high-resolution volumes of subsurface properties, such as velocity and impedance that are directly interpretable. In contrast, FWI in most published examples is applied only on low-frequency data (typically less than 10 Hz) to enhance the velocity models needed for migration. Brute-force generation of high-resolution subsurface property volumes using FWI can be computationally prohibitive because of the high cost of the required finite-difference simulations. Many of these practical obstacles have been overcome through the use of advanced wave propagation and optimization algorithms and faster computer architectures. In the WBS, detailed subsurface property models generated with FWI have provided an accurate description of the overburden complexity and helped mitigate imaging distortions. The resulting imaging uplift has enabled improved structural definition, stratigraphic characterization of the reservoir, and quantitative interpretation. Predrill predictions using FWI volumes have shown excellent agreement with well results.

Abstract A total of 143 sedimentary successions that contain, or may be prospective for, hydrocarbons were identified in the Arctic Region north of 58–64°N and mapped in four quadrants at a scale of 1:11 000 000. Eighteen of these successions (12.6%) occur in the Arctic Ocean Basin, 25 (17.5%) in the passive and sheared continental margins of the Arctic Basin and 100 (70.0%) on the Circum-Arctic continents of which one (<1%) lies in the active margin of the Pacific Rim. Each succession was assigned to one of 13 tectono-stratigraphic and morphologic classes and coloured accordingly on the map. The thickness of each succession and that of any underlying sedimentary section down to economic basement, where known, are shown on the map by isopachs. Major structural or tectonic features associated with the creation of the successions, or with the enhancement or degradation of their hydrocarbon potential, are also shown. Forty-four (30.8%) of the successions are known to contain hydrocarbon accumulations, 64 (44.8%) are sufficiently thick to have generated hydrocarbons and 35 (24.5%) may be too thin to be prospective.