Abstract: Depositional Sequences, Reservoir Heterogeneity and Paleogeographic Setting of Southern Williston Basin Waulsortian Mounds

INDEN, RICHARD F.

Waulsortian mounds in the southern Williston Basin nucleated on current-washed shallow water carbonate banks which were progressively drowned during regional early Lodgepole transgressive events. These banks represent the lateral equivalent of the black False Bakken Shale. They developed in response to underlying paleostructural patterns and basinal levels of anoxia Hardgrounds and early dissolution events concentrated at mound tops suggest that mound demise was the result of drastic changes in ocean chemistry during mound growth into shallow water environments. Whether mound growth was limited to paleostructural highs, or could occur in lows is equivocal. Evidence from facies evaluations and cross-sections subjected to decompaction routines indicate that both situations probably occurred.

The mounds consist primarily of stacked storm and or catastrophic I event (earthquake induced ?) debris sequences that were deposited on steep I slopes in deep subphotic environments. Individual units range from l to 20 ft thick and are ideally composed of basal intraclast lime rudstones and fining upward crinoid lime grainstones which grade upwards into cement-rich, bryozoan-pellet micobial boundstones containing stromatactis cavities. Each depositional unit is bound at top and base by erosional contacts which cut into lithified microbial boundstones, or crinoid grainstones. Vertical depositional sequences within the mounds are similar and are separated by correlative zones stained with iron oxide.

Sustained high-rate hydrocarbon production rates result from the fracture interconnection of porous lime grainstones. These have solution enlarged primary intergranular porosities upwards of 14 % (average less than 3 %) and permeabilities ranging upwards of 100md. Intraclast rudstones and microbial boundstones possess minor intergranular and separate vuggy porosities with less than 3 md permeability and serve as vertical (and lateral ?) seals within the system. Late stage dissolution resulting from early hydrocarbon migration events enhances reservoir quality, Enhanced primary recoveries may be obtained from these extremely heterogeneous mounds by horizontal/high angle lateral drilling.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado