Diagenetic characteristics of low permeability, non-reservoir chalks from the Central North Sea

Mallon, Anthony J.; Swarbrick, Richard E.

doi:10.1016/j.marpetgeo.2007.12.001

Cited by 41 publications

(29 citation statements)

References 22 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…2) are the main reservoir seals for the sub-Chalk reservoirs in the Central Graben, North Sea (Mallon & Swarbrick 2002Swarbrick et al 2010). These highly cemented and mechanically compacted Chalk units have the potential to seal high overpressure in the underlying highly pressured reservoirs (Mallon et al, 2005;Mallon & Swarbrick 2008;Swarbrick et al 2010). After the North Atlantic Ocean opened, the Eocene and younger Hordaland and Nordland groups, comprising up to 2500 m of predominantly siltstone and shale, were deposited.…”

Section: Geological Settingmentioning

confidence: 99%

Enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the Triassic Skagerrak, Central Graben, North Sea, UK

Stricker¹,

Jones²

2016

View full text Add to dashboard Cite

Current understanding of porosity preservation in deeply buried sandstone reservoirs tends to be focused on how diagenetic grain coatings of clay minerals and microquartz can inhibit macroquartz cementation. However, the importance of overpressure developed during initial (shallow) burial in maintaining high primary porosity during subsequent burial has generally not been appreciated. Where pore fluid pressures are high, and the vertical effective stress is low, the shallow arrest of compaction can allow preservation of high porosity and permeability at depths normally considered uneconomic. The deeply buried fluvial sandstone reservoirs of the Triassic Skagerrak Formation in the Central Graben, North Sea, show anomalously high porosities at depths greater than 3500 metres below sea floor (mbsf). Pore pressures can exceed 80 MPa in the upper part of the Skagerrak Formation at depths of 4000-5000 mbsf, where temperatures are above 1408C. The Skagerrak reservoirs commonly have high primary porosities of up to 35%, little macroquartz cement and variable amounts of diagenetic chlorite grain coats. This research sheds light on the complex controls on reservoir quality in the fluvial sandstones of the Skagerrak Formation by identifying the role of shallow overpressure in arresting mechanical compaction and the importance of chlorite detrital grain coatings in inhibiting macroquartz cement overgrowth as temperature increases during progressive burial.Gold Open Access: This article is published under the terms of the CC-BY 3.0 license.

show abstract

Section: Geological Settingmentioning

confidence: 99%

Enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the Triassic Skagerrak, Central Graben, North Sea, UK

Stricker¹,

Jones²

2016

View full text Add to dashboard Cite

show abstract

“…2). Mallon et al (2005Mallon et al ( , 2008 describe the deeply buried non-reservoir chalk as extensively cemented with fractional gross porosity from 0.04 to 0.18 (average 0.08), compared with 0.20-0.40 in the reservoir intervals at similar depths. The permeability of the reservoir chalk is in the milliDarcy to microDarcy range (10 215 -10 218 m 2 ), compared with nonreservoir chalk permeability in the nano-to microDarcy range (10 218 -10 221 m 2 ) depending on depth of burial.…”

Section: Characteristics Of Non-reservoir Chalkmentioning

confidence: 99%

“…These interpretations feature a rapid increase of overpressure at the base of the Chalk and through the Lower Cretaceous and Upper Jurassic shales, where present (Holm 1998a, b). We re-assess the data in light of new permeability for the Chalk (Mallon et al 2005;Mallon & Swarbrick 2008) from rare direct pressure measurements plus gas data. A high-quality database of 1248 wells was made available by IHS.…”

mentioning

confidence: 99%

Role of the Chalk in development of deep overpressure in the Central North Sea

Swarbrick¹,

Lahann

O’Connor³

et al. 2010

PGC

View full text Add to dashboard Cite

A high magnitude of overpressure is a characteristic of the deep, sub-Chalk reservoirs of the Central North Sea. The Upper Cretaceous chalk there comprises both reservoir and non-reservoir intervals, the former volumetrically minor but most commonly identified near the top of the Tor Formation. The majority of nonreservoir chalk has been extensively cemented with average fractional gross porosity of 0.08, and permeability in the nano-to microDarcy range (10 218 -10 221 m 2 ), and sealing properties comparable to shale. Hence deeply buried chalk is comparable to shale in preventing dewatering and allowing overpressure to develop. Direct pressure measurements in the Chalk are restricted to the reservoir intervals, plus in rare fractured chalk, but reveal that Chalk pressures lie on a pressure gradient which links to the Lower Cenozoic reservoir above the Chalk and the Jurassic/Triassic reservoir pressures below. Hence a pore pressure profile of constantly increasing overpressure with increasing depth is indicated. Mud weight profiles through the Chalk, by contrast, show many borehole pressures lower than those indicated by these direct measurements, implying wells are routinely drilled underbalanced. The Chalk is therefore considered the main pressure transition zone to high pressures in subChalk reservoirs. In addition to its role as a regional seal for overpressure, the Base Chalk can be shown to be highly significant to trap integrity. Analysis of dry holes and hydrocarbon discoveries relative to their aquifer seal capacity (the difference between water pressure and minimum stress) shows that the best empirical relationship exists at Base Chalk, rather than Base Seal/Top Reservoir, where the relationship is traditionally examined. Using a database of 65 wells from the HP/HT area of the Central North Sea, and extending the known aquifer gradients from the Fulmar reservoirs via Base Cretaceous to Base Chalk, leads to a risking threshold at 5.2 MPa (750 psi) aquifer seal capacity. Discoveries constitute 88% of the wells above the threshold and 36% below, with 100% dry holes where the aquifer seal capacity is zero (i.e. predicted breached trap). This relationship at Base Chalk can be used to identify leak points which control vertical hydrocarbon migration as well as assessing the risk associated with drilling high-pressure prospects in the Central North Sea.

show abstract

“…Characterization of the UK/Norwegian non‐reservoir samples included transmitted, cathodoluminescence (CL) and scanning electron microscopy (SEM), X‐ray diffraction, porosity, permeability analysis, mercury porosimetry and the quantification of the amount of insoluble residue within the sample (Mallon et al. 2005; Mallon & Swarbrick 2002b, in press) . Permeability measurements on this sample set were made using the TPD method. A total of 95 chalk core samples were sampled from four Danish offshore wells: [MFB‐7 (Dan Field); depth interval (2240–2440 m), Mona‐1 (3140–3440 m), West Lulu‐1 (2865–3690 m) and Svend T‐1 (2195–2347 m)] (Fig.…”

Section: Data Setsmentioning

confidence: 99%

How should permeability be measured in fine‐grained lithologies? Evidence from the chalk

Mallon¹,

Swarbrick²

2007

Geofluids

Self Cite

View full text Add to dashboard Cite

Two chalk data sets from the Central North Sea (UK ⁄ Norway and Denmark) with a similar lithological classification and porosity values (5-20%) have a difference in permeability of up to three orders of magnitude. The method of measuring permeability was different for the two data sets: samples from UK ⁄ Norway were measured by transient pulse decay (TPD), while the Danish samples were measured by routine core analysis. Petrological and petrophysical characterization of samples from the two data sets have revealed that all samples display a similar range of rock properties; the differences are not due to regional facies variations. It is likely that the low permeability values reported by TPD have more validity than routine core analyses in measuring lower porosity and permeability chalk (5-20% porosity, <0.01 mD). The fact that large sections of North Sea chalk potentially have much lower permeability than previously reported has widespread implications for petroleum migration and entrapment, and overpressure generation.

show abstract

Diagenetic characteristics of low permeability, non-reservoir chalks from the Central North Sea

Cited by 41 publications

References 22 publications

Enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the Triassic Skagerrak, Central Graben, North Sea, UK

Enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the Triassic Skagerrak, Central Graben, North Sea, UK

Role of the Chalk in development of deep overpressure in the Central North Sea

How should permeability be measured in fine‐grained lithologies? Evidence from the chalk

Contact Info

Product

Resources

About