Deposition of Bluell Zone at Flaxton Field, Burke County, North Dakota

Dirk Anson Schwartz

The rocks of the Bluell zone at Flaxton field were deposited mainly in shallow sublittoral settings. Seven distinct lithofacies can be recognized in the Flaxton field area: (A) coated grain intraclastic packstone and grainstone, (B) skeletal peloidal intraclastic wackestone and packstone, (C) fenestral green algal mudstone and wackestone, (D) skeletal slightly intraclastic wackestone and packstone, (E) patterned dolomitic mudstone, (F) intraclastic peloidal wackestone, and (G) dolomudstone.

The depositional history at Flaxton field can be subdivided into four time units (T1-T4). During T1 (early Bluell time), deposition of the lower Bluell occurred in a shallow sublittoral environment in which topographic highs were subaerially exposed during occasional drops in the local strandline. Subenvironments during T1 include broad topographic highs, shallow sublittoral environments possessing lagoonlike conditions, and topographic lows, which experienced near-normal marine conditions. Deposition during T2 (middle Bluell time) was in a supralittoral environment during which significant amounts of silt-size quartz grains were transported into the study area by eolian processes. During T3 (middle to late Bluell time), shallow sublittoral and littoral deposition returned. Throughout most of T4 (late Bluell time), deposition was primarily through the subaqueous precipitation of primary dolomite and the deposition of silt-size quartz grains.

The diagenetic history of the Bluell zone at Flaxton field is very complex and has played an important role in trapping hydrocarbons at Flaxton field. This study also suggests that the majority of present-day structure in the Bluell zone at Flaxton field is not a result of the differential compaction of sediments, but instead is probably a result of tectonic movement.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988