UK Stratigraphical Framework Series : Oxford Clay Formation

This report forms part of the UK Stratigraphical Framework Series (UKSFS) and provides an overview of the stratigraphy of the Oxford Clay of southern and eastern England. The work covers both outcrop and subcrop areas and is primarily based on new interpretations of borehole geophysical log and core sample data with reference to published materials and outcrop sections as required. The emphasis of the report is on regional structure, thickness and facies trends in the Oxford Clay Formation. It should provide a framework for site-specific geological investigations that require understanding of the Oxford Clay Formation

Recent scientific advances in the understanding of induced seismicity from hydraulic fracturing of shales

The Secretary of State for Business, Energy & lndustrial Strategy has commissioned the British Geological Survey to write a short report about seismic activity associated with hydraulic fracturing (HF) of shales to extract hydrocarbons. The specific terms of reference are available at https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/fi le/1066525/BGS_Letter.pdf. These ask six questions related to recent scientific research on the hazard and risk from induced seismicity during hydraulic fracturing of shale rocks. Our report considers the scientific advances in this area since 2019 that have been published in peer reviewed scientific journals as well as other recent studies commissioned by regulatory authorities. The main conclusions of our report in relation to each of the questions in the terms of reference are as follows: Forecasting the occurrence of large earthquakes and their expected magnitude remains a scientific challenge for the geoscience community. This is the case for both tectonic and induced earthquakes. (Questions 1 and 2) Methods to estimate the maximum magnitudes of induced earthquakes based on operational parameters and observed seismicity have been tested using data from both Hydraulic Fracturing (HF) operations and data from other industries. These methods have shown some applicability to guide operational decisions using real-time data. However, they do not currently account for the possibility of events that occur after operations have stopped or earthquakes on faults that extend outside the stimulated volume whose magnitude is not controlled by operational parameters alone. (Questions 1 and 2) Probabilistic methods widely applied to model and forecast tectonic earthquake sequences show some promise when modified to incorporate information about HF operations and appear capable of providing informative forecasts of the observed earthquake patterns. Operators could make forecasts for operations in new wells using either generic parameters or ones calibrated for operations in adjacent wells. Further testing of these methods may allow them to be further developed for operational scenarios. (Questions 1 and 2) Enhanced seismicity monitoring and measurement based on machine learning (ML) has been shown to reveal previously undetected earthquakes and hidden faults, essential for both more reliable earthquake forecasts and characterisation of fault reactivation potential. This can compensate for both limited numbers of seismic stations and faults that remain unmapped even by 3D exploration seismic data. (Questions 1 and 2) Widely used probabilistic methods to assess hazards and risks for tectonic earthquakes can also be applied to induced seismicity. However, there are important differences between how tectonic and induced seismicity evolves in space and time. Recent studies have suggested possible solutions, but further work is needed to develop these models and incorporate them in risk assessments. (Questions 1 and 2) Traffic light systems remain a useful tool for the mitigation of risks from induced seismicity. New research shows how red-light thresholds can be chosen to reduce the probability of the scenario to be avoided to a required level. This research recommends that there should be sufficient space between the amber and red-light thresholds to ensure that operators have an opportunity to modify operations to mitigate risks. (Questions 1 and 2) Induced seismicity has been observed in other industries related to underground energy production both in the UK and elsewhere. In the absence of a seismic building code in the UK, consistent risk targets, i.e., scenarios to be avoided, could be considered for all energy related industries that present a risk of induced earthquakes. (Question 3) Recent research using high quality exploration data that is available for some parts of the UK reveals localised structural and stress heterogeneity that could influence fault reactivation. However, it is not possible to identify all faults that could host earthquakes with magnitudes of up to 3 prior to operations, even with the best available data. (Questions 4 and 5). Recent research from the USA demonstrates the importance of geomechanical modelling to identify faults that are most likely to rupture during operations. This information can be used to assess risks prior to and during operations. However, these models require accurate mapping of sub-surface faults, robust estimates of stress state, and knowledge of formation pore pressures and the mechanical properties of sub-surface rocks. While this information is available in areas with unconventional hydrocarbon potential such as the Bowland Basin, more data is needed from other basins to apply this more widely (Questions 4 and 5). Limited exploration data from other basins with unconventional hydrocarbon potential of the UK means that there are significant gaps in our knowledge of sub-surface structure of potential shale resources in these places. (Questions 4 and 5) The rates of HF-induced seismicity in other countries where shale gas production has been ongoing for many years are observed to vary widely. The limited number of HF operations in the UK means that it is difficult to make a valid comparison of the rates of occurrence of induced seismicity with elsewhere. This underlines the importance of knowledge exchange in monitoring and operational practices. (Question 6) Our review focusses on recently published geoscience related to induced seismicity caused by HF of shales. Ongoing and future research may bring new insights that may reduce uncertainties and improve mitigation of risks. We did not consider socio-economic research on perception of risks or the benefits of shale gas. Similarly, we do not consider technological advances in hydraulic fracturing

Hydraulic fracturing: a review of theory and field experience

This report summarises the current state-of-the-art knowledge of the hydraulic fracturing process used by the shale gas/oil industry using open peer-reviewed literature and from government commissioned research reports. This report has been written to make statements on our knowledge of the following questions: • How do hydrofractures form? • How far do hydrofractures extend during stimulation? • What dictates where hydrofractures propagate? • How do hydrofractures interact with the existing fracture network? • Can the size and distribution of hydrofractures be controlled? Gaps in our knowledge have been highlighted, with the largest of these resulting from differences between North American and European shale rocks

Seasonality fluctuations recorded in fossil bivalves during the early Pleistocene: implications for climate change

Understanding the transformations of the climate system may help to predict and reduce the effects of global climate change. The geological record provides a unique archive that documents the long-term fluctuations of environmental variables, such as seasonal change. Here, we investigate how seasonal variation in seawater temperatures varied in the Mediterranean Sea during the early Pleistocene, approaching the Early-Middle Pleistocene Transition (EMPT) and the beginning of precession-driven Quaternary-style glacial–interglacial cycles. We performed whole-shell and sclerochemical stable isotope analyses (δ18O, δ13C) on bivalves, collected from the lower Pleistocene Arda River marine succession (northern Italy), after checking shell preservation. Our results indicate that seawater temperature seasonality was the main variable of climate change in the Mediterranean area during the early Pleistocene, with the Northern Hemisphere Glaciation (NHG) exerting a control on the Mediterranean climate. We show that strong seasonality (14.4–16.0 °C range) and low winter paleotemperatures (0.8–1.6 °C) were likely the triggers leading to the establishment of widespread populations of so called “northern guests” (i.e., cold water taxa) in the Mediterranean Sea around 1.80 Ma. The shells postdating the arrival of the “northern guests” record a return to lower seasonal variations and higher seawater paleotemperatures, with seasonality increasing again approaching the EMPT; the latter, however, is not associated with a corresponding cooling of mean seawater paleotemperatures, showing that the observed seasonality variation represents a clear signal of progressive climate change in the Mediterranean Sea

The Bowland Shale Formation in the Blacon Basin: syngenetic processes, stacking patterns and heat productivity

We conducted a high-resolution multi-disciplinary analysis of two core sections in the borehole Ellesmere Port-1, Cheshire, UK. Biostratigraphic analysis indicates the core sections are Kinderscoutian and late Arnsbergian-Chokerian in age, respectively. Both cores are assigned to the Bowland Shale Formation (Holywell Shale). Coupled core scan and discrete geochemical analysis enables interpretation of syngenetic processes at a high stratigraphic resolution. Both cores exhibit the classic cyclicity of limestones, calcareous to non-calcareous mudstones and siltstones, interpreted to represent sediment deposition during fourth-order sea level fluctuation. Machine learning of the well log data coupled to the core scan data enabled prediction of the key lithofacies through the entire Bowland Shale interval in Ellesmere Port-1. The machine predictions show the Bowland Shale is interfingered with three turbiditic leaves of the Cefn-y-fedw Sandstone Formation and contains at least 12 complete fourth-order cycles. The Bowland Shale exhibits high radiogenic heat productivity (RHP) in comparison to other sedimentary rocks, due primarily to relative enrichment in U under intermittently euxinic conditions. Thermal modelling, however, shows Bowland Shale RHP contributes a negligible source of additional heat at the scale of 100s m

Towards the middle Pleistocene continental glaciation: assessing seasonality in the early Pleistocene of the Mediterranean area

The Early Pleistocene is a time characterized by high frequency climatic oscillations, which has its lower and upper boundaries coinciding with two important climatic events: the Northern Hemisphere Glaciation and the Middle Pleistocene Transition. The Mediterranean area was strongly affected by the Early Pleistocene climatic changes; one of the most important biotic events is represented by the appearance of the boreal guests at the beginning of the Calabrian Stage, indicating a progressive climatic deterioration in the Mediterranean region. The Arda River marine succession, cropping out in Northern Italy, has been shown to cover this time interval; it is also very rich in macrofossils, and thus it represents an ideal setting to study the climatic oscillations of the Early Pleistocene and understand if and how seasonality varied in this interval using pristine bivalve shells as geochemical archives. We have undertaken bulk isotope analyses on 249 fossil bivalve shells of species of Glycymeris, Aequipecten and Arctica, collected in 141 beds of the Arda section. We have also studied the sclerochemistry of ten pristine bivalve shells belonging to species of Glycymeris and Arctica to reconstruct in detail the seasonality in seven discrete intervals, which we can correlate to the Atlantic Marine Isotope Stages. We show a progressive increase in temperature seasonality through the Early Pleistocene, mostly controlled by cold winters, which became more pronounced toward the Middle Pleistocene Transition. However the isotope data from the bulk shells do not indicate cooling of overall sea surface temperatures throughout the interval, except for a significantly cold event at the first occurrence of the cold taxon Arctica islandica. The increase in seasonality and the arrival of cold guests indicate that the Northern Hemisphere glaciations exerted a strong control on the Mediterranean climate during most of the Early Pleistocene

Cross-basin Mo and U analysis of the Upper Mississippian Bowland Shale UK

The Bowland sub-basin is a target for hydrocarbon exploration but to a large extent it remains unexplored. To determine the economic potential of the Bowland sub-basin, it is important to identify the oceanographic processes involved in the deposition of the Bowland Shale Formation in the Late Mississippian (ca. 330 Ma). Palaeoceanographic processes are known to be a major control on the development of hydrocarbon source rocks. This study investigates core (Preese Hall-1 and Becconsall-1Z) materials from the Upper Bowland Shale, and compares to previously published data (outcrop Hind Clough), all from the Bowland sub-basin, Lancashire, UK. The sedimentology and geochemistry of this formation was determined via a multi-technique approach including x-ray fluorescence (XRF), sedimentology, gamma ray spectra, x-ray diffraction (XRD) and RockEval(6)TM pyrolysis. Key trace metal abundances and enrichment factors were used to assess sediment provenance and to determine the bottom water redox conditions during the deposition of the Upper Bowland Shale. Our results support interpretations that contemporaneous anoxia developed in bottom waters in at least three sites in the Bowland sub-basin. In a comparison with the Fort Worth Basin (Barnett Shale, USA), the Bowland sub-basin was apparently less restricted and deposited under a much higher mean sediment accumulation rate compared to the Fort Worth Basin. Knowledge from this study improves future resource estimates of the Bowland Shale Formation, and challenges the early assumptions that the Barnett Shale is an analogue to the Bowland Shale

What can a shell tell? : the story of the multiproxy bivalve Arctica islandica

Fossil carbonate bivalve shells are archives which can tell us several stories. In this prospect, one of the main character of the Quaternary marine evolution is represented by the multitasking bivalve Arctica islandica; its performances range from the field of biostratigraphy, palaeoclimatology and sclerochronology, but these are only some of its numerous \u2018talents\u2019. Among bivalve molluscs, Arctica islandica is known as the Methuselah because of its remarkable lifespan of up to 500 years. It represents one of the most important boreal guests, which migrated from higher northern latitudes into the Mediterranean Sea in consequence of the climatic cooling leading to the Middle Pleistocene glaciations. For this reason, since the eighties, A. islandica was considered one of the main markers of the Pliocene-Pleistocene boundary (1.8 m.y.), remaining in office until 2010, when, the boundary was lowered at 2.6 m.y. After that it was used to mark the Gelasian-Calabrian boundary. However, A. islandica has its most significant impact in the field of palaeoclimate and palaeoenvironmental reconstructions, recording seawater changes in its shell in the form of variable growth increment widths and variable geochemical properties. It is in fact an excellent proxy for studies of ancient ocean conditions because it sensitively and faithfully records the primary seawater isotope composition with no vital effect in its shell layers. A sclerochemical study performed on A. islandica shells from the Arda River marine succession (Castell\u2019Arquato, Northern Italy) highlights the potential of this species in recording the past seasonal variation occurred during its lifespan. This study allowed to reconstruct in details the climate evolution of the Mediterranean area prior to the Middle Pleistocene continental glaciation, showing that seasonality was the main variable involved in the climate change. The establishment of widespread populations of cold guests in the Mediterranean Sea at about 1.8 m.y. was, in fact, triggered by extreme seasonality; after this episode, the analyzed shells record an increasing trend in seasonality approaching the Middle Pleistocene Transition and the beginning of the continental glaciations in the Northern Hemisphere, which is, however, decoupled from a concomitant cooling of the average seawater temperatures. The examples here presented emphasize the multitasking role of A. islandica during the Quaternary, making it a very powerful tool for numerous and diversified studies