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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 5. (May)

First Page: 587

Last Page: 587

Title: Porosity Styles of the Midale Field in Williston Basin of Southeastern Saskatchewan: ABSTRACT

Author(s): John Kaldi

Article Type: Meeting abstract

Abstract:

The Midale oil field, southeastern Saskatchewan, lies on the northeastern flank of the Williston basin. The reservoir is in limestones, dolomites, and evaporites (mainly anhydrite) of the Midale Beds (Mississippian), that were deposited during a predominantly regressive episode on a shallow shelf.

The Midale Beds are divided into a lower, middle, and upper zone. Many of the characteristic pore types in these zones, as observed in thin section and under SEM, can be related to both original depositional environment and postdepositional diagenetic modifications.

The dominant styles of porosity in the fine-grained argillaceous limestones of the lower zone are secondary intraparticle, moldic, or microvuggy. These fabrics result from the preferential dissolution of cement or very fine shell debris. A lack of pore interconnections precludes these sediments from being effective reservoir rocks.

The most significant pore type in the middle zone is secondary intercrystalline porosity within fine-grained dolomite. This fabric is the result of solution of calcite or aragonite from between rhombs after incomplete dolomitization. A crinoidal grainstone with pervasive early diagenetic syntaxial rim cement forms a tight trap in the middle zone.

The upper zone consists of fractured calcareous microcrystalline dolomite. The main pore type is a non-fabric selective system of oblique to vertical microfractures, which may be associated with regional uplift or local salt solution. The presence of dolomite rhombs on fracture surfaces indicates that dolomitization was relatively late, postdating fracturing. The microfractures in the upper zone counteract the porosity-occluding effects of stylolitization and secondary anhydritization.

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