Introduction to the sedimentology of paralic reservoirs: recent advances

Paralic reservoirs reﬂect a range of clastic depositio- nal environments developed along or near coast-lines, including deltas, shoreline–shelf systems and estuaries. Such reservoirs provide the backbone of production in many mature basins around the world, and contribute signiﬁcantly to global con- ventional hydrocarbon production. Strata that host these reservoirs are shaped by a wide variety of dep- ositional processes and controls which reﬂect the upstream supply of sediment and water, the characteristics of the receiving basin, relative sea level, tectonic setting, and the internal dynamics of depositional systems. Consequently, they exhibit much variability in their stratigraphic architecture and sedimentological heterogeneity, which translates into complex patterns of reservoir distribution and reservoir performances that are challenging to pre-dict, optimize and manage. This Special Publication presents new research and developments in estab-lished approaches to exploration and production of paralic reservoirs. It arises from the conference and associated core workshop titled ‘Sedimentology of Paralic Reservoirs: Recent Advances and their Applications’, which was organized by the Petro- leum Group of the Geological Society of London and held in London from 18 to 21 May 2015.

Paralic reservoirs reflect a range of clastic depositional environments developed along or near coastlines, including deltas, shoreline-shelf systems and estuaries. Such reservoirs provide the backbone of production in many mature basins around the world, and contribute significantly to global conventional hydrocarbon production. Strata that host these reservoirs are shaped by a wide variety of depositional processes and controls which reflect the upstream supply of sediment and water, the characteristics of the receiving basin, relative sea level, tectonic setting, and the internal dynamics of depositional systems. Consequently, they exhibit much variability in their stratigraphic architecture and sedimentological heterogeneity, which translates into complex patterns of reservoir distribution and reservoir performances that are challenging to predict, optimize and manage. This Special Publication presents new research and developments in established approaches to exploration and production of paralic reservoirs. It arises from the conference and associated core workshop titled 'Sedimentology of Paralic Reservoirs: Recent Advances and their Applications', which was organized by the Petroleum Group of the Geological Society of London and held in London from 18 to 21 May 2015.

Volume organization
The papers in this Special Publication are arranged into three sections, each with a distinct theme. The aim of this introduction is to provide a broader context for the papers in these three sections.

Subsurface characterization of paralic reservoirs
The first group of papers explores the sedimentological character of paralic reservoirs using subsurface data. The papers consider the subdivision of paralic reservoirs into stratigraphic layers; the dimensions, geometries and distributions of depositional elements (cf. geobodies) within such layers; and facies characteristics within depositional elements. These aspects of reservoir sedimentology all influence fluid flow and hydrocarbon recovery. Stratigraphic layering of paralic reservoirs commonly defines the position of laterally extensive barriers to vertical flow (e.g. mudstone intervals) and the thickness of intervening flow units (sensu Ebanks 1987) that are bounded by such barriers. The spatial arrangement of depositional elements controls the lateral extent, continuity and connectivity of fluid-flow pathways within stratigraphic layers (e.g. Weber & Van Guens 1990), and influences the volumetric proportion of hydrocarbon-bearing rock that is contacted by fluids moving through the reservoir layers (i.e. the areal and vertical sweep efficiency). Facies characteristics determine the grain sizes, textures and rock properties (e.g. permeability, porosity) of depositional elements, and they influence in turn the efficiency with which hydrocarbons are displaced from swept volumes of the reservoir at the pore scale (i.e. displacement efficiency) (e.g. Weber 1986). Reynolds (2016) provides an overview and synthesis of paralic reservoirs, which explains their global significance and identifies their distinctive features across a range of scales. The impact of sedimentology on reservoir characterization and associated field-development strategy is emphasized throughout. This paper explains the overall motivation for previous and current research on paralic reservoirs, including the examples presented in the volume. The sedimentological character and stratigraphic architecture of the Lower Jurassic Cook Formation reservoir in the Knarr Field, offshore Norway, are documented by Churchill et al. (2016). In their study, the depositional environments and stratigraphic zonation of the reservoir are constrained by integrated sedimentological, micropalaeontological and palynological analyses. The reservoir is interpreted to record a temporal evolution in the depositional-process regime, from netregressive and tide-dominated in its lower part to net-transgressive and wave-dominated in its upper part. Heldreich et al. (2017) use a combination of core, wireline-log and 3D seismic data to develop a depositional model for the fluvio-deltaic, Upper Triassic Mungaroo Formation, a major exploration and production target in the North Carnarvon Basin, offshore Australia. Their analysis characterizes the geometries, dimensions and vertical facies successions of depositional elements to aid reservoir characterization and modelling, and discusses uncertainties in interpreting allogenic (external) and autogenic (internal) controls on the architecture of strata that contain these geobodies. Characterization of stratigraphic architectures that may have resulted from autogenic behaviours, such as avulsion of distributary channels in fluvio-deltaic strata, is attempted by Flood & Hampson (2016) using a combination of core-based facies and palaeosol analyses, and the application of spatial statistical tools to channelized sandbody distributions in upper coastal plain strata in the Middle Jurassic Brent Group reservoir, Brent Field, offshore UK.

Tidal heterogeneity in paralic systems
Heterogeneity in lithological character is a recurring theme in paralic reservoirs. Arguably, heterogeneity is at its greatest in reservoirs that were deposited under the influence of tidal currents, where intercalation of sandstones and mudstones across a wide range of scales is common (e.g. Nio & Yang 1991). Such intercalations have repeatedly been shown to have a significant impact on fluid flow (e.g. Jackson et al. 2005;Ringrose et al. 2005) and remain an area of active research (Massart et al. 2016). Accordingly, the second group of papers in the Special Publication reviews and extends the literature that describes tide-influenced paralic reservoirs and their outcrop analogues.
Longhitano & Steel (2016) present a generic depositional model for deltas that advanced into seaways or straits in which tidal currents were dominant, based on two well-documented outcrop examples. The model identifies diagnostic aspects of the plan-view geometries, three-dimensional stratal stacking patterns, and vertical successions of facies, palaeocurents and depositional-process indicators in such deltas. The model may allow reinterpretation and improved characterization of hithertoenigmatic sandstone reservoirs along the margins of elongate marine basins. Mellere et al. (2016) document the facies characteristics, stratigraphic architecture and palaeogeography of a series of stacked tide-dominated and tidally influenced deltas and estuaries in the Middle Jurassic Bryne and Sandnes formations, Søgne Basin, offshore Norway and Denmark, using core, wireline-log and 3D seismic data. These strata contain complex spatial patterns of reservoir distribution and character, which reflects their deposition in an evolving saltinfluenced rift basin. A facies analysis and stratigraphic architectural study of net-transgressive, early Miocene tide-dominated deltaic reservoirs from the Balingian province, offshore Sarawak, Malaysia is presented by Amir Hassan et al. (2016). Increased wave influence is characteristic of transgressive intervals in these reservoirs. Vertical and lateral stacking patterns of sequence stratigraphic units reflect tectonic controls on hinterland denudation and subsidence variations, respectively. Johnson et al. (2016) document the stratigraphic context and internal architecture of an estuarine outcrop analogue (from the Cretaceous Straight Cliffs Formation, Utah, USA) that exhibits variable tide and wave influence. The detailed bed-scale architecture of inclined heterolithic strata from a bayhead delta in the inner estuary is characterized in order to develop diagnostic criteria for bayhead delta depositional elements in subsurface reservoir data.
McLean & Wilson (2016) use a quantitative approach to outcrop data from the Plio-Pleistocene Erin Formation, Trinidad to document both tidal and river flood signatures in thinly bedded mudstones and sandstones -an approach that is readily, but rarely, transferred to subsurface core data.

Analogue studies
Understanding the distribution and character of paralic reservoirs often relies on insights from a range of analogues, which provide appropriate context or additional detail to aid interpretation of sparse subsurface data. Analogues take many forms: modern sedimentary systems, in which processes can be observed and linked directly to depositional product; sub-modern depositional systems that are imaged at high resolution and in three dimensions using shallow seismic data; analogous reservoirs rich in static and dynamic data; outcrops of ancient strata, which allow preserved stratigraphic architectures and facies characteristics to be observed and integrated across a wide range of spatial scales; and numerical and physical (e.g. flume tank) models that allow the underlying assumptions of our interpretations to be investigated experimentally. The final group of papers in this Special Publication presents a range of paralic reservoir analogues from studies of modern sediments and ancient outcrops. Lane et al. (2016) unravel temporal and spatial patterns of river-channel avulsion in a Holocene-modern fluvial megafan and associated mixed-influence delta (Mitchell River, Queensland, Australia) using a combination of mapping, vibracoring, trenching and topographical surveying. The resulting patterns of stratigraphic architecture and channelized sediment-body distribution are integrated with vertical facies successions and intra-sediment-body architectural panels in order to develop a generic model of depositional-element distribution within a regressive megafan delta. Lambiase et al. (2016) characterize sedimentation patterns in the modern Mahakam Delta, Borneo, which is undergoing transgression via retrogradational stacking and switching of delta lobes. Diagnostic core-based sedimentological criteria are presented for three sharp-based sandstone depositional elements (regressive fluvial distributary channels; transgressively back-filled fluvio-estuarine distributary channels; transgressive wave-dominated shorefaces) that appear similar in wirelinelog data but have distinctly different geometries, spatial distributions and sequence stratigraphic implications. The stratigraphic architecture of a thick, net-aggradational interval of paralic strata, characterized by high accommodation and high sediment supply, is described from outcrops of the Cretaceous Straight Cliffs Formation, Utah, USA by Mulhern & Johnson (2016). Transgressiveregressive cycles composed of regressive, wavedominated deltaic and shoreface deposits and transgressive, wave-dominated estuarine deposits can be correlated between successions of different thickness that underwent variable local tectonic subsidence and early compaction. Gomis-Cartesio et al. (2016) document the architecture of ancient wave-dominated and mixed-influence shelf-edge deltas and underlying gullies and slope channels at outcrop (Permian Waterford Formation, South Africa). The facies character, geometry and dimensions of depositional elements associated with the shelf edge are documented and synthesized into a model for sediment bypass from deltaic distributary channels onto the slope via gullies that become wider and shallower down depositional dip.

Recurring themes and future directions
Heterogeneity in paralic reservoirs is increasingly well understood at all scales. Reynolds (2016) highlights that (1) advances in imaging due to improvements in seismic data acquisition and processing, (2) widespread application of sequence stratigraphic concepts, (3) availability of robust facies models, (4) improved use and organization of analogue databases, and (5) increased integration of available data types have all played key roles in allowing heterogeneity from pore scale to reservoir scale to be described in paralic reservoirs. Consequently, reserve volumes and their distributions are estimated earlier in field life, allowing better and more timely development decisions. Despite this undoubted progress, the conference and the papers presented here amply demonstrate that progress continues in the sedimentological description of paralic reservoirs and that several recurring themes represent fruitful directions for future work: † The tectonostratigraphic context of paralic successions influences their subsidence patterns and preserved thickness, sediment input and routing, and potentially also the process regime (e.g. via suppression or amplification of waves and tides : Stride 1982;Yoshida et al. 2007;Ainsworth et al. 2008;Wells et al. 2010). Spatial and temporal variations in tectonostratigraphic context may result in the development of subtle stratigraphic trapping configurations, and complex spatial patterns of reservoir distribution and character. Integrated analysis of seismic, well-log, core and biostratigraphic data is essential to unravel the stratigraphic architecture of such paralic successions, and thereby to identify and de-risk paralic reservoir targets, even in mature basins ( demonstrated that the internal dynamics of sediment-routing systems may influence stratigraphic architectures across a range of spatial and temporal scales (e.g. Muto & Steel 1992;Kim et al. 2006;Muto et al. 2007). This key theoretical contribution has yet to be routinely incorporated into the sequence stratigraphic interpretation of subsurface and outcrop data. However, distinguishing allogenic (external) and autogenic (internal) controls on sedimentation is important in predicting the extent, continuity and degree of order in paralic stratigraphic architectures, particularly in subsurface settings where data are sparse (e.g. Wang et al. 2011; see also discussions in Flood & Hampson 2016;Lane et al. 2016;Heldreich et al. 2017).
The theory needs to be tested against more 'real world' datasets in order to further develop diagnostic tools and predictive models. † Facies analysis of paralic deposits has traditionally emphasized the use of 'end-member' examples, dominated by riverine, wave or tidal processes, at the expense of mixed-influence depositional systems in which two or more of these processes are significant. In recent years, the stratigraphic architecture of mixed-influence paralic deposits, and the geometry and distribution of the depositional elements that they contain, have been addressed in a new classification scheme (Ainsworth et al. 2011), and in case studies of modern and ancient reservoir analogues (e.g. Legler et al. 2014;Ainsworth et al. 2016;Johnson et al. 2016;Lambiase et al. 2016;Lane et al. 2016 Bohacs et al. 2014). As well as unconventional shale reservoirs, there is potential to reassess hydrocarbons stranded in the shaly 'waste zones' of paralic sandstone reservoirs as reserves in 'halo plays' that surround these sandstones.
We thank the Geological Society, in particular its Petroleum Group, and the SEPM (Society for Sedimentary Geology) for their support of the conference, the related core workshop and this Special Publication. The success of the conference and core workshop owed much to the generous financial help of sponsors BP, Statoil and Chevron, and to the support of Weatherford Laboratories, DEA and the British Geological Survey in releasing and displaying cores. The Petroleum Group of the Geological Society is thanked for supporting the reproduction of colour figures in this volume.
We are indebted to the authors of the 34 talks, 20 posters and four core displays presented at the conference and core workshop, and to the many attendees for their lively contributions to discussions. Bruce Ainsworth, Janok Bhattacharya and Ron Steel are thanked for their input as conference co-convenors, in addition to the four editors of this volume. We are grateful to the authors who have contributed papers to this volume, and to the reviewers who provided insightful and constructive comments that shaped the final form of these papers. Finally, we thank the staff of the Geological Society's Conference Office and Publishing House for their administrative and logistical support throughout, in particular Tamzin Anderson, Jo Armstrong, Laura Griffiths, Angharad Hills and Annie Sewell.