Stanley field, producing from the Mississippian Mission Canyon Formation in the east-central Williston basin, establishes the economic significance of shallow subsurface mechanical compaction of shallow-water marine limestone, and provides a quantitative measure of mechanical compaction with increasing depth of burial.

Figure

In the field the upper 65 m of the Mission Canyon is formed of subtidal marine and intertidal-supratidal island facies. Marine facies escaped pervasive early cementation and were significantly compacted. Evidence of mechanical compaction of marine facies in cores include: (a) horizontal aspect of mottling (burrows); (b) horizontal orientation of fossil fragments; (c) drag, drape, and penetration effects; (d) microstylolitic "horsetail" swarms; and (e) broken and crushed fossils (ostracods, trilobites, corals, and crinoids). Island facies, syndepositionally cemented by both fibrous marine (beach rocklike) and equant spar (freshwater) cements, show no evidence of significant mechanical compaction.

Compacted marine mudstone and wackestone form the updip seal for the field. Preserved primary interparticle porosity in compacted marine grainstone and packstone provides the principal reservoir facies.

Structure and isopach maps of various intervals of the Mission

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Canyon and overlying Charles Formations, in conjunction with cross sections through the field showing both formations, reveal timing of compaction and quantitative compensation by overlying deposits. Curves from 3 wells show percent compaction versus overburden. With 250 m of overburden, mechanical compaction is essentially complete and, on average, marine sections have been compacted 31%.

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