The Zechsteinkalk reservoir of the Hewett field, southern North Sea
- Simon Petroleum Technology, Llandudno, Gwynedd, Wales (United Kingdom)
- Phillips Petroleum UK Ltd., Llandudno, Gwynedd, Wales (United Kingdom)
The Zechsteinkalk of Hewett field is characterized by complex porosity and permeability distribution. Facies and diagenesis together control porosity distribution, while production rates are enhanced by subseismic [open quotes]megafractures.[open quotes] Highest porosities occur in the upper part of the sequence where dolomitized shoal and perishoal oolitic and peloidal grainstones predominate. Intershoal and main shoal facies show no significant differences in average porosity. Tidal-flat muds developed at the shoreward margin of shoals have only very low porosity. Extreme porosity variation within perishoal facies is caused by differential diagenesis. Porosity has been created by dolomitization and leaching but has been occluded by early carbonate cementation and anhydrite cementation. Anhydrite is volumetrically the most significant effect, destroying 20% porosity in many intervals and completely cementing many fractures. Its distribution is highly heterogeneous. Models for the origin of the anhydrite explain and predict gross distribution trends, but superimposed smaller scale trends are related to depositional grain size and the local development of evaporitic environment over subaerially exposed grainstones. Cementation took place in at least three phases, pre-, syn-, and postfracturing. Natural fracturing occurs throughout the field. Fractures are typically subvertical, planar, dilational, and completely or partially cemented by anhydrite. The cementation history of the fractures indicates formation prior to the development of the structure. Although fracture aperture porosity occurs in a significant proportion of fractures recorded in core (30-40%), DST results indicate fracture-enhanced flow from only a small number of fractured zones where individual fractures are sufficiently closely spaced to form an interconnected system or [open quotes]megafracture.[close quotes] The megafractures trend northwest-southwest oblique to the primary fracture trend.
- OSTI ID:
- 6964895
- Report Number(s):
- CONF-9310237-; CODEN: AABUD2
- Journal Information:
- AAPG Bulletin (American Association of Petroleum Geologists); (United States), Vol. 77:9; Conference: American Association of Petroleum Geologists (AAPG) mid-continent section meeting, Amarillo, TX (United States), 10-12 Oct 1993; ISSN 0149-1423
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
03 NATURAL GAS
ANHYDRITE
ORIGIN
GEOLOGIC FRACTURES
CONCRETIONS
GEOLOGIC STRATA
POROSITY
RESERVOIR ROCK
PERMEABILITY
CARBONATES
CEMENTING
DIAGENESIS
DISTRIBUTION
FRACTURING
GEOLOGIC MODELS
GEOLOGIC STRUCTURES
JURASSIC PERIOD
NORTH SEA
ATLANTIC OCEAN
CARBON COMPOUNDS
GEOLOGIC AGES
GEOLOGIC DEPOSITS
MESOZOIC ERA
MINERALS
OXYGEN COMPOUNDS
SEAS
SULFATE MINERALS
SURFACE WATERS
020200* - Petroleum- Reserves
Geology
& Exploration
030200 - Natural Gas- Reserves
Geology
& Exploration