Sedimentology and depositional environments in the Mesozoic, West of Shetland

As part of Phase 3 of the Faroe-Shetland Consortium project on the UK sector of the Faroe-Shetland Basin, BGS completed detailed (10 cm-scale) sedimentary logging of 604.77 m of conventional core recovered from 19 wells, drilled between 1974 and 1995. The project originally focussed solely on Jurassic-aged sediments, as identified in BGS held well databases. In reality, the project examined a suit of sediments ranging from the Triassic, Jurassic and Lower Cretaceous. In addition, a suite of complementary palynological sampling was completed. The goal was to provide additional depositional environment information from palyno-facies analysis, along with age determination for the sediments. Together, the two new datasets form an integrated re-assessment of the Mesozoic-aged depositional environments located to the West of Shetland. The Triassic-aged deposits of the Papa Group record deposition in a fluvial environment, whereby the fluvial systems display meandering to braided characteristics. In places, particularly in the upper parts of the cored Triassic-aged strata, the sediments display evidence for nearshore, wave-working processes. The nearshore working may be evidence for a transgression event at the top of the cored Triassic interval; something which should be investigated further in the future. Within the Jurassic-aged succession, two main groups were analysed: the Skerry Group and the Humber Group. Sediments within the Skerry Group were deposited within a fully marine, shelfal environment (the Stack Skerry Formation) and a relatively deeper-water, outer shelf to bathyal environment (the Sule Skerry Formation). Late Jurassic-aged sediments from the Humber Group are represented by the Rona Member, the Solan Sandstone Member and the Ridge Conglomerate Member. BGS has subdivided the Rona Member into five “facies”, largely on the basis of depositional environment. These include: Rona R1 (Fluvial), Rona R2 (Fan Delta), Rona R3 (Marginal Marine), Rona R4 (Shoreface/Littoral) and Rona R5 (Shallow Marine). Deposits of the Rona R4 facies represent good hydrocarbon reservoir targets, whilst the Rona R3 facies has the potential to form a sourcing lithology. The Solan Sandstone Member, interpreted to be contemporaneous with the deposition of the Rona Member, was deposited in a deep-marine, turbidite fan system. The Kimmeridge Clay Formation is encountered in a number of wells, typically represented by a dark grey- to black-coloured, hemi-pelagic mud-prone succession. The Early Cretaceous-aged Cromer Knoll Group is also present in the core data, represented by: turbidite fan deposits, formed in a deep marine environment (204/19-1 & 205/21-1A); and by shoreface/littoral deposits representative of a marine setting (205/26a-2). These sediments appear to rest unconformably on the Kimmeridge Clay Formation

Tectonostratigraphy and the petroleum systems in the Northern sector of the North Falkland Basin, South Atlantic

The North Falkland Basin represents one of the frontier areas for hydrocarbon exploration in the South Atlantic. This study presents the results of new subsurface mapping using 2D seismic data in the north of the Falkland Islands offshore area, which has delineated a series of discrete grabens northwards of the main North Falkland Basin, referred collectively to as the Northern sector of the North Falkland Basin (NNFB). Six regionally significant seismic reflectors are interpreted within this data, dividing the sedimentary fill into six tectonostratigraphic packages, including: early syn-rift; late syn-rift; transitional unit; early post-rift; middle to late post-rift; and a sag unit. Structural interpretation of the 2D seismic data has led to the definition of four north-south orientated depocentres, namely: (1) the Eastern Graben, largest of the depocentres; 20 km wide by 45 km long, reaching depths of 3 km; (2) the Eastern Graben Splay, a smaller depocentre; 10 km wide by 20 km long, reaching depths of 2–2.5 km; (3) the Western Graben Splay, the smallest depocentre; 5 km in width and 20 km long, with a basin depth of 2 km and (4) the newly defined Phyllis Graben, which is 13 km wide and 30 km long, with a basin depth of 3 km. A network of NW-SE and NE-SW trending faults controls the development of these grabens, separated by a Western, Eastern and Intra-Basin high. These grabens represent a northern continuation of the Northern Falkland Basin to the south. Hydrocarbon discoveries to the south of this study area (e.g. Sea Lion, Casper, Beverley, Zebedee, Isobel Deep, and Liz) confirm a working petroleum system adjacent to the Northern sector. This study has identified a number of seismic anomalies, including amplitude brightening events, which potentially correspond to an extension of this petroleum system, indicating active migration pathways. The main targets, in terms of hydrocarbon interest in the northern sector, are likely to be stratigraphically trapped hydrocarbon accumulations, contained within vertically-amalgamated turbidite fan sandstone reservoirs, deposited within the early post-rift. A second, yet to be tested, syn-rift play, in which the trapping geometries are structural and the reservoirs are fluvial sandstones is also identified

The influence of complex palaeobathymetry on development of deep-lacustrine fan systems

Pre-existing complex palaeobathymetry often plays a key role in the spatial-temporal distribution and character of deepwater sedimentary systems. Particularly in deep-marine fan systems where their spatial-temporal association with complex syn-depositional palaeobathymetry have been widely investigated. When present in deep-marine settings, complex palaeobathymetry is known to affect flow-type, flow direction, and resultant fan distribution, which ultimately leads to atypical reservoir rock distribution. By contrast, far fewer studies explore the influencing controls of palaeobathymetry on deep-lacustrine sedimentary systems. This is important to investigate as deep-lacustrine basins have quite different allogenic and autogenic controls on flow types and resultant fan systems and fan lobes, which varies through different stages of basin configuration. To address this knowledge gap, this study documents and characterises a suite of deep-lacustrine sedimentary systems imaged in high-quality 3D seismic data from the rift-sag transitional and early post-rift phases of the North Falkland Basin, Falkland Islands. A range of multi-scalar seismo-geomorphological features are identified, including super systems, fan systems, fan lobes, and channel elements. The influence of palaeobathymetry on flows and resultant sedimentary features is evidenced by frontal and lateral structural confinement at the super system scale, and lateral confinement plus fan/flow deflection at the fan system, fan, and lobe scale. Offset stacking and compensational lobe-scale stacking geometries are developed in response to the type and scale of confinement. Palaeobathymetry, created as depositional relief by preceding fan deposits, is shown to progressively influence flow types and resultant spatial distribution of ensuing sedimentary systems. During periods of basin-fill where encircling palaeobathymetry ultimately controlled super system scale distribution, the ponding of flows and resultant fan features against intra-basinal highs formed thick packages of potentially coarse-grained sediments. As the basin filled-up and encircling topography exerted less control on super system scale distribution, flows were able to surmount the intra-basinal highs, leading to flow stripping processes. The combination of ponding and flow stripping processes resulted in the deposition and preservation of coarse-grained sediments immediately behind or on top of intra-basin structures. The results of this study provide key insights into the interaction of deep-lacustrine sedimentary systems and complex palaeobathymetry, which ultimately influences reservoir distribution

Clastic injectites, internal structures and flow regime during injection: the Sea Lion Injectite System, North Falkland Basin

This paper details and describes a suite of 143 sub‐seismic‐scale clastic injectites encountered within the early Cretaceous, early post‐rift of the deep‐lacustrine North Falkland Basin. The injectites, referred to here as the Sea Lion Injectite System, are encountered below, above and in between the hydrocarbon‐bearing, deep‐lacustrine turbidite sandstones of the Bleaker 15, Sea Lion North, Sea Lion, Casper and Beverley fans. Sedimentary structures are documented within the injectites including: planar laminations, mud‐clast imbrication and clast alignment. Clasts align along centimetre‐scale foresets formed through ripple‐scale bedform migration in a hydraulically‐open fracture. The style of flow within the injectite system is interpreted as initially through fluid turbulence during an open fracture phase, which was followed by a later stage where laminar flow dominated, most likely during the closing phase of the fracture system. The host rocks display evidence for ductile deformation, which along with ptygmatic folding of dykes and internally injected mud‐clasts, suggests a period of injection into relatively uncompacted sediments. Evidence for brittle fracturing, in the form of stepped margins may be indicative of a separate phase of emplacement into more‐compacted sediments. This variability in deformation styles is related to multi‐phased injection episodes into host strata at different stages of consolidation and lithification at shallow burial depths. Injectites have been identified in four stratigraphic groupings: above the Bleaker 15 Fan and within/above the Sea Lion North Fan; within the hydrocarbon‐bearing Sea Lion Fan; overlying the Sea Lion Fan; and above/below the hydrocarbon‐bearing Casper and Beverley fans. This spatial association with the hydrocarbon‐bearing fans of the North Falkland Basin is important, considering the ability of injectite networks to form effective fluid‐flow conduits in the subsurface. Consequently, the findings of this study will improve the characterization of sub‐seismic scale injectites (and therefore fluid conduits) within otherwise impermeable strata

A depositional model for deep-lacustrine, partially confined, turbidite fans: Early Cretaceous, North Falkland Basin

This paper presents a model of facies distribution within a set of early Cretaceous, deep‐lacustrine, partially confined turbidite fans (Sea Lion Fan, Sea Lion North Fan and Otter Fan) in the North Falkland Basin, South Atlantic. As a whole, ancient deep‐lacustrine turbidite systems are under‐represented in the literature when compared with those documented in marine basins. Lacustrine turbidite systems can form extensive, good quality hydrocarbon reservoirs, making the understanding of such systems crucial to exploration within lacustrine basins. An integrated analysis of seismic cross‐sections, seismic amplitude extraction maps and 455 m of core has enabled the identification of a series of turbidite fans. The deposits of these fans have been separated into lobe axis, lobe fringe and lobe distal fringe settings. Seismic architectures, observed in the seismic amplitude extraction maps, are interpreted to represent geologically associated heterogeneities, including: feeder systems, terminal mouth lobes, flow deflection, sinuous lobe axis deposits, flow constriction and stranded lobe fringe areas. When found in combination, these architectures suggest ‘partial confinement’ of a system, something that appears to be a key feature in the lacustrine turbidite setting of the North Falkland Basin. Partial confinement of a system occurs when depositionally generated topography controls the flow‐pathway and deposition of subsequent turbidite fan deposits. The term ‘partial confinement’ provides an expression for categorising a system whose depositional boundaries are unconfined by the margins of the basin, yet exhibit evidence of internal confinement, primarily controlled by depositional topography. Understanding the controls that dictate partial confinement; and the resultant distribution of sand‐prone facies within deep‐lacustrine turbidite fans, is important, particularly considering their recent rise as hydrocarbon reservoirs in rift and failed‐rift settings

The effect of breached relay ramp structures on deep‐lacustrine sedimentary systems

Fault relay ramps are important sediment delivery points along rift margins and often provide persistent flow pathways in deepwater sedimentary basins. They form as tilted rock volumes between en-echelon fault segments, which become modified through progressive deformation, and may develop through-going faults that ‘breach’ the relay ramp. It is well established that hinterland drainage (fluvial/alluvial systems) is greatly affected by the presence of relay ramps at basin margins. However, the impact on deepwater (deep-marine/lacustrine) subaqueous sediment gravity flow processes, particularly by breached relay ramps, is less well documented. To better evaluate the complex geology of breached relay settings, this study examines a suite of high-quality subsurface data from the Early Cretaceous deep-lacustrine North Falkland Basin (NFB). The Isobel Embayment breached relay-ramp, an ideal example, formed during the syn-rift and was later covered by a thick transitional and early post-rift succession. Major transitional and early post-rift fan systems are observed to have consistently entered the basin at the breached relay location, directed through a significant palaeo-bathymetric low associated with the lower, abandoned ramp of the structure. More minor systems also entered the basin across the structure-bounding fault to the north. Reactivation of basin-bounding faults is shown by the introduction of new point sources along its extent. This study shows the prolonged influence of margin-located relay ramps on sedimentary systems from syn-rift, transitional and into the early post-rift phase. It suggests that these structures can become reactivated during post-rift times, providing continued control on deposition and sourcing of overlying sedimentary systems. Importantly, breached relays exert control on fan distribution, characterised by laterally extensive lobes sourced by widespread feeder systems, and hanging walls settings by small-scale lobes, with small, often line-sourced feeders. Further characterising the likely sandstone distribution in these structurally complex settings is important as these systems often form attractive hydrocarbon reservoirs

Deep to shallow-marine sedimentology and impact of volcanism within the Middle Triassic Palaeo-Tethyan Semantan Basin, Singapore

The Middle Triassic Pulau Ayer Chawan Formation is a predominantly deep-marine, occasionally shallow-marine sedimentary succession, deposited in the Singapore sector of the Palaeo-Tethyan Semantan Basin. The formation provides an important record of the dynamic interplay between a siliciclastic sedimentary system and the products of an adjacent active volcanic arc. It is characterised by six sub-environments, including: deep-marine turbidite fan, deep-marine background sedimentation, subaqueous debris cone, shallow-marine, volcanically-sourced turbidite fan, and hyaloclastite mound or ridge. Turbidite fan deposits preserve the input of both siliciclastic and volcaniclastic sediments from the shelf, transported into the deep-marine environment by a suite of subaqueous sediment gravity flow processes, including: turbidity currents; mixed flow types (hybrid event beds); concentrated and hyper concentrated sediment gravity flows, and debris flows. Thick heterolithic successions of debrites were likely sourced through regular collapse of an unstable shelf. The presence of hybrid event beds, encountered within the deep-marine turbidite fans, supports a slope that was out-of-grade, and may have been actively retreating towards the hinterland. Together, these factors suggest regional-scale uplift of the eastern margins of the Semantan Basin during Triassic times, most likely facilitated through volcanic activity in the adjacent Palaeo-Tethys Sukhothai Arc. Evidence for contemporaneous, arc-related magmatism includes ubiquitous volcaniclastic sedimentary rocks within formation, including pyroclastic density current deposits and perhaps more-strikingly through the hyaloclastites of the Nanyang Member. The hyaloclastites formed through quenching of magmas delivered into the deep-marine setting from a series of sub-sea vents or mounds

Evolution of Fractal Pore Structure in Sedimentary Rocks

Geological processes alter pore spaces over time, and their analysis can shed light on the dynamic fractal structure and fluid flow of rocks over time. This study presents experimental evidence to illustrate that the pore fractal structure evolves with sedimentation, carbonate cementation, clay growth, and dissolution. It examines, describes and characterizes a suite of core samples from the Gaotaizi oil layer of the second and third members of the Qingshankou Formation, Songliao Basin, China. The effects of mechanical compaction and other diagenesis effects on fractal pore structure on sedimentary rocks are discussed. A schematic diagram is proposed that describes the impacts of these diagenetic processes on fractal pore structure at the microscopic scale in sedimentary rocks. This work links the state of diagenetic alteration and fractal pore structure, which can guide practical applications such as predicting the permeability of sedimentary rocks. Key Points Evolution of fractal dimension with diagenesis was revealed Effects of diagenesis on fractal upper and lower limits were discussed Effect mechanism of fractal pore structure was revealed in sedimentary rocks Plain Language Summary Mechanical compaction or chemical alteration process will change the pore space of the rock, including pore size and grain-pore interface properties. We present the evidence that geological processes alter the “roughness” amplitude of grain-pore interface (fractal pore structure) in sedimentary rock, and discuss the evolutionary mechanism of the “roughness” amplitude of grain-pore interface. This work links the state of diagenetic alteration and fractal properties of rocks, which can guide practical applications such as predicting permeability of sedimentary rocks for any historical period

Paleozoic to Cenozoic sedimentary bedrock geology and lithostratigraphy of Singapore

A new lithostratigraphical framework for Singapore is proposed, based on the analysis of c. 20,000 m of core recovered from 121 c. 205 m deep boreholes and augmented with 218 field localities from across Singapore. The new framework describes a succession dating from the Carboniferous to the Quaternary. New U-Pb detrital zircon dates and fossil analysis were used to constrain the ages of key sedimentary units. The oldest known sedimentary rocks in Singapore are found to be the deformed Carboniferous (Mississippian) Sajahat Formation. These are succeeded by the newly erected, Middle and Upper Triassic, marine to continental Jurong Group and Sentosa Group successions that accumulated in the southern part of the Semantan Basin. The Jurong Group comprises four formations: the Tuas Formation, the Pulau Ayer Chawan Formation, the Pandan Formation and the Boon Lay Formation. The Sentosa Group contains two formations: the Tanjong Rimau Formation and the Fort Siloso Formation. In Singapore, the depositional record during this time is related to late Permian to Triassic arc magmatism in the southern part of the forearc basin to the Sukhothai Arc. The Jurong and Sentosa groups were deformed and weakly metamorphosed during the final stages of the Late Triassic to Early Jurassic orogenic event, deformation that led to the formation of the syn-orogenic conglomerates of the Buona Vista Formation. Following this, two distinct Lower Cretaceous sedimentary successions overstepped the Jurong and Sentosa group strata, including the Kusu Formation and the Bukit Batok Formation, both deposited in the southern part of the Tembeling Basin. A series of Neogene to Quaternary formations overly the Mesozoic and Palaeozoic stratigraphy, including the Fort Canning Formation, Bedok Formation and the Kallang Group

Characterization and classification of the microporosity in the unconventional carbonate reservoirs: A case study from Hanifa Formation, Jafurah Basin, Saudi Arabia

Formation porosity is a key factor that exerts substantial control upon reserve estimates, and ultimately may affect the development viability of unconventional resources. Microporosity studies have largely focused on siliciclastic mudstones, with limited examples that assess porosity type and distribution in organic-rich calcareous mudstone successions. In this work, a comprehensive porosity characterization study was performed on the organic-rich carbonate-dominated Hanifa Formation of the Jafurah Basin, which is the largest unconventional basin in Saudi Arabia. An extensive experimental program, involving petrographical description, SEM analysis, mineralogical analysis using XRD and QEMSCAN, and geochemistry analysis was employed in order to characterize the samples and analyze the geological origins of the microporosity. The main constituent minerals in this study are calcite (87 wt%) and anhydrite (9 wt%), as well as 85%. Fundamentally, the findings of this study demonstrate that the Hanifa Formation in the Jafurah Basin has the potential for gas exploration and recovery