Chalk: all we need is a fracture log!

Abstract

Chalk fracture logging is reviewed in the context of the broader geology needed to build conceptual ground models. Examples of drilling damaged core illustrate the many issues faced by core loggers including identification of marker beds (marl seams, hardgrounds, flint bands, fossil shell beds) and the ‘interpretations’ necessary to complete a fracture log. Stratabound fractures impart a special style of fracturing to each Chalk formation. Lithology is a key factor in development of fracture style where marl seams control inclined conjugate fracture sets, development of listric growth faults and interbed slides. Lateral changes in lithology and thickness and consequent controls on fracture evolution are related to intra-Chalk tectonic episodes and tectonic setting with onshore interpretations supported by offshore seismic profiles. Strike-slip faults are illustrated in the Chalk cliffs of the Sussex coast. Fracture log reports should highlight special features such as shear zones and use annotated core photographs to illustrate issues requiring discussion. Received 18 March 2021; revised 20 May 2021; accepted 24 May 2021 When asked to identify marl seams, flint bands and fossil shell beds as part of Chalk core logging a response from some contractors has been: ‘Should we be expected to recognize and log these features? Surely all we need is a fracture log?’ Fracture logging is an important part of a site investigation; however, the other aspects of the geology are just as important to developing a ground model and are identified as part of best practice in Chalk site investigation (CIRIA 574, Lord et al. 2002). At its simplest, how can boreholes be correlated on fracture logs alone and how can the location, scale and sense of faulting be identified or presence of dissolution features be recognized from fracture logs? How can geophysical logs and seismic reflectors be interpreted from fracture logs? It is also quite usual in site investigations to find features that are not common or may not have been recorded before and these need highlighting. Whilst sympathizing with the commercial issues of time andmoney, such considerations should not override the science and engineering. Core logging should be about gaining the maximum geological information possible, as the success of a construction project may depend on recognizing all aspects of the geology across a site, not just fractures. Fracture logging chalk can be the bane of a site investigation geologist’s life, time consuming, controversial, frequently requiring ‘interpretation’ where core is fragmented or fractures have been induced during drilling. Clients usually demand a numerical figure even in core intervals where drilling fragmentation has occurred and such demands can seem unreasonable. How should such intervals be recorded in terms of fracturing? In the last three decades since the forerunners of the CIRIA grades were first mooted during and after the Brighton 1989 Chalk Symposium and then developed to become an industry standard (Lord et al. 1994, 2002) there has been a continuous debate on how to recognize the grades in drill cores and how they integrate with ‘standard’ fracture logging. In the hard-nosed business of site investigations the time (and money) taken to complete fracture logs and apply CIRIA grades has sometimes detracted from the value of recording other aspects of the geology including details of the stratigraphy (marl seams, flint bands, nodular chalk beds and fossil bands). Part of the problem is the time allocated to core logging in contracts and the absence in the specifications of the requirements to log stratigraphical details. A further issue is what is expected from the core log results by the client and consultant and frequent differences of opinion between parties on the results especially with fracture logs and CIRIA grades. Sometimes this has led to costly relogging or rewriting the core logs. Sometimes these requests and revisions have come from ‘office-based’ staff who have not seen the core and who have little experience of the real problems associated with logging and are simply interpreting contract requirements without reference to ‘field’ staff. Good attempts have been made to overcome these difficulties on many contracts by running logging courses and having experts help with the logging so that all parties can agree with the results. Chalk is especially difficult as drill-core is frequently disturbed during borehole drilling. Core disturbance is most pronounced (1) in chalk with flints, (2) in nodular chalk beds or hardgrounds and (3) in intervals with fracture zones. Drillers’ records such as rate of progress, loss of flushing medium and interruptions caused by lithology are invaluable in helping to recognize intervals where such disturbances have occurred and the depth of potential fracture zones but are not always available. A requirement to keep these records should be a mandatory part of any contract (for some recent contracts there have been no drillers’ logs or comments in situations where these would have been invaluable). A concentration on the contractual side of core logging (and core sampling and testing), although very necessary, can lead to a loss of focus on the wider geological context and the requirement to log other aspects of the geology. These aspects include the style of fractures, which, in the Chalk, are characteristic of particular lithologies and formations, and the tectonic and geomorphological settings of the site being investigated. In addition, lithological features such as marl seams play a critical role in the mechanical behaviour of chalk including generation of fractures and fracture styles, slope instability and shortand long-term deformation. Not recognizing and logging even thin wispy marl seams reduces the engineering value of the site investigation. Other questions relating to fractures raised during major construction projects include: ‘Is there evidence for strike-slip faulting in the Chalk and how common is it?’ This paper attempts to answer some of these questions and provide a geological context for fractures, fracture logging and CIRIA grades to aid the extrapolation © 2021 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions. 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