Marine and Petroleum Geology 17 (2000) 601±618 Overburden deformation patterns and mechanisms of salt diapir penetration in the Central Graben, North Sea I. Davison a,*, G.I. Alsop b, N.G. Evans c, M. Safaricz a aDepartment of Geology, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK bCrustal Geodynamics Group, School of Geography & Geosciences, University of St. Andrews, Fife, Scotland, KY16 9ST, UK cBHP Petroleum, Neathouse Place, London, SW1V 1LH, UK Received 3 September 1999; received in revised form 10 February 2000; accepted 24 February 2000 Abstract Active and passive diapirism control the deformation and geometry of hydrocarbon traps in the overburden, and a more detailed understanding of this process will help reservoir prediction and hydrocarbon recovery. Cores studies of seven Central Graben diapirs indicate Zechstein salt penetrated Late Cretaceous chalk by extreme tectonic thinning with high-angle (>708 to bedding) normal faulting, tensile fracturing and pressure solution. Attenuation of the chalk signi®cantly weakens the overburden, allowing buoyancy forces to dome up the overburden. Doming created enough topography for downslope sliding of chalk slabs on slip planes parallel to bedding or, in the case of the Kyle diapir, for chaotic debris ¯ows of lithi®ed chalk. Signi®cant extensional bedding-parallel faults and slump folds are developed within Palaeocene shale on the diapiric ¯anks. Inter-granular slip in unconsolidated clastic material was probably the dominant deformation mechanism. Diapirs have penetrated the Palaeocene clastic sediments by maintaining topographic relief, so that unlithi®ed sediment continually slid o the crest, producing translated intact rafts up to several tens of metres in thickness. 7 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction of Cretaceous chalk, and Tertiary sandstones and shales are used to determine the nature and mechan- There has been a surge in salt-tectonics research in isms of penetration through these horizons. The defor- the past decade, but there has been relatively little pre- mation features within the hydrocarbon reservoir units vious work on the details of diapir penetration mech- are described and related to position on the diapir and anisms through the overburden strata using ®eld lithology. All the studied ®elds are currently in the evidence. Seismic and physical modeling studies (e.g. development stage (Birch, Crowle, Hayes & Nash, Shultz-Ela, Jackson & Vendeville, 1993) commonly in- 1998; Evans, Rorison & Sykes, 1999; Foster & Rattey, dicate marked thinning of units adjacent to, and sur- 1993), and this analysis is aimed at understanding the rounding the crest of salt diapirs, but natural reservoir deformation mechanisms and patterns, to attenuation mechanisms of the overburden have not help improve hydrocarbon recovery. The study covers been studied in detail. In order to address this pro- the Kyle and Ban diapirs in the Western Trough blem, seven salt-diapirs have been investigated in the where salt has almost broken through the chalk; and Central Graben of the North Sea (Fig. 1). Obser- Machar, Monan, Mungo, North and South Pierce dia- vations on core data through the overburden reservoirs pirs in the Eastern Trough of the Central Graben (Davison et al., 1999, Fig. 1). The Machar diapir is overlain by chalk, but the other four diapirs in the * Corresponding author. Earthmoves Ltd, 8 Cabrera Avenue, Vir- Eastern Trough have penetrated the chalk and the ginia Water, Surrey, GU25 4EZ, UK, Tel.: +44-0-1344-842-967; fax: +44-0-1344-845-781. Palaeocene sandstones and the reservoirs are trapped E-mail address: [email protected] (I. Davison). against the vertical salt stock. Two kilometers of cores 0264-8172/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S0264-8172(00)00011-8 602 I. Davison et al. / Marine and Petroleum Geology 17 (2000) 601±618 were examined and logged from Late Cretaceous to open fracturing are de®ned as zones where open frac- Palaeocene chalk and Palaeocene sandstone reservoirs tures are not separated by more than 10 cm of unde- and shales. Broadly similar deformation patterns were formed rock, measured along the core axis. In such found in the dierent diapir ®elds. zones there is a high probability that all fractures will All the diapirs are interpreted to have grown by form an interconnected network in three dimensions. downbuilding from Triassic through to Palaeocene Fractures are classi®ed here as tensile (Mode 1) frac- times, and were probably triggered by extensional tures, or small normal faults with displacements faulting during Triassic times (Smith et al., 1993; Hos- usually less than 10 mm. This de®nition of intercon- sack, 1995; Davison et al., 1999). Thin (0±300 m) nected fracture zones allows comparison of the poten- chalk sequences are present above the diapirs, which tial hydrocarbon productivity between dierent rock are surrounded by basinal areas with up to 1500 m of types and positions on an individual diapir and chalk. Owing to the strong lithological control on the between diapirs. The open fracture zones form later- nature and intensity of overburden deformation the ally connected open networks on a scale much greater chalk and Palaeocene sandstones and shales are than core observation reveals. All thickness measure- described separately. All depths referred to here, are ments of fractured zones noted here are measured drillers depths in metres, unless otherwise stated. perpendicular to the bedding dip. Natural rubble zones within the cores are due to brecciation associ- ated with faulting, and are also noted on the logs. 2. Deformation of chalk overburden Details of deformation from dierent lithologies are now presented in the necessary detail prior to discus- In the following discussion, interconnected zones of sion of the data. Fig. 1. Location map of diapirs in the study area. I. Davison et al. / Marine and Petroleum Geology 17 (2000) 601±618 603 2.1. Faulting and fracturing of chalk on Ban diapir 2a-6MST) shows evidence of pervasive minor fractur- ing with 46% of the core aected by open intercon- 2.1.1. Crest nected minor fractures, and 15% of the core is rubble The Ban diapir is elongate in a NW±SE direction zones associated with meso-scale (tens of meters dis- parallel to a large normal fault which probably con- placement) faulting (Fig. 2). Pure dilation (Mode 1) trolled its initiation (Davison et al., 1999). It is 4-km fractures are rare, and small extensional (relative to long and approximately 2.5-km wide. The southwes- bedding) faults are the main fracture type. The pattern tern ¯ank of the diapir contains most of the hydro- of fracturing is extremely heterogeneous, with zones of carbon reserve, and the chalk and Palaeocene intense minor fracturing up to 25±30 cm thick, devel- reservoirs have been lifted up like a ¯ap (Evans et al., oped within the fracture networks. Within such pock- 1999, Fig. 2) ets, fracture density ranges between 30 and 100 Chalk from the diapiric crest (wells 29/2a-6 and 29/ fractures/m (measured parallel to the core axis). The Fig. 2. Vertically-exaggerated cross section of Ban diapir showing summary core logs of zones of interconnected fracturing in the chalk and Palaeocene sandstone. 604 I. Davison et al. / Marine and Petroleum Geology 17 (2000) 601±618 largest zone of interconnected fracturing is 2-m wide 2.1.2. Shoulder and is adjacent to a brecciated (rubble) fault zone of Chalk from the SW shoulder of the Ban diapir unknown displacement. The development of thicker (well 29/2a-10) displays pervasive minor fracturing, interconnected fracture zones coupled with a typically comprising 40% of the core interval, coupled with closer spacing of fractures within zones, indicates that brecciation zones (10% of core) (Fig. 2). Late faults the Tor formation has undergone greater brittle exten- with associated open fracturing cut sedimentary sional deformation than the Eko®sk formation. How- slumps in the chalk and bedding-parallel closed hair- ever, the proportions of fractured core (46%) and line fractures. Seventy-®ve per cent of interconnected breccia zones (15%) remain similar in each case. open fracture zones have a spacing of less than 0.75 Fig. 3. (a) Cross-section and (b) map of Kyle diapir. (c)±(e) show cored zones of interconnected open fracturing (black intervals) in the chalk from two wells 29/2c-8z and 29/2c-8 and 29/2c-11. Grey shading shows zones where core was not available. I. Davison et al. / Marine and Petroleum Geology 17 (2000) 601±618 605 m, underlining the pervasive nature of brittle defor- 2.2.1. Crestal breccias mation. Chalk of the Tor formation is slightly less Well 29/2c-8 is located SW of the Kyle diapir crest, fractured, compared with the same unit adjacent to the where overburden dips steeply to the SW (Fig. 3). This diapiric crest (29/2a-6). On the southern diapir ¯ank, well penetrated salt breccias at the top of the diapir, 29/2a-7 chalk is much less aected by zones of open- overlain by anhydrite breccias, followed by mixed connected minor fracturing (12% of core) and breccia anhydrite and chalk breccias (Fig. 4), chalk breccias, zones (3%) (Fig. 2). and ®nally intact chalk (Fig. 5). No cap rock was observed at the top of the diapir. The anhydrite clasts 2.1.3. Lithlogical control on fracture systems on Ban reach up to 1 m in diameter and are internally tightly There are fewer interconnected fracture systems folded and sheared indicating they originally formed developed within minor quartz sandstone units within part of the diapir (Fig. 4). the Eko®sk chalk on the crest of the structure. Individ- The salt breccias could indicate that the salt was ual sandstone units may reach 1±2 m in thickness, actually exposed at the sea bed, and the salt clasts was with stacked sandstones up to 7 m in thickness.