Abstract: Regional Porosity Patterns in the Sherwood Subinterval of the Mission Canyon Formation, Southwest to North-Central North Dakota

David M. Petty

The Sherwood subinterval of the Mission Canyon Formation consists of peritidal to shallow open-marine deposits in the area from southwest to north-central North Dakota. These formed along a northeast-southwest trending paleoshoreline which aggraded in north-central North Dakota and prograded to the northwest in southwest North Dakota.

Although shoal-island deposits (facies IIIa) can be mapped in a continuous belt from north-central to southwest North Dakota, dramatic differences in porosity and diagenesis occur along trend, and three regional diagenetic zones can be defined. Zone A in north-central North Dakota is characterized by the presence of good primary and some secondary porosity, while zone C in southwest North Dakota is characterized by the near absence of porosity. Zone B is transitional between zones A and C. The most important difference between zones A and C is the abundance of early equant calcite which tightly cemented most of the facies IIIa sediments in Zone C.

Dolomite porosity occurs in beds which parallel log markers and form an en-echelon pattern on the progradational portion of the Sherwood. This porosity exists mostly in dolomitized mudstones and wackestones of facies II (lagoonal deposits), IV (restricted-marine deposits) and V (open-marine deposits). Dolomite porosity is less common in north-central North Dakota where porous facies IIIa limestones separate facies IV and V deposits from the overlying evaporites of facies I. Brines generated in the sabkha-salina facies were probably diluted by ground water flow before significant dolomitization could occur in this area.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana