Cretaceous of the Western Continental Interior: From Basin to Pore.

Up to 7Tcf of gas reserves are hosted within the Glauconitic sandstone (Mannville Group) in progradational shoreline successions informally called the Hoadley complex. To understand the controls on reservoir quality for this unit we have focused on three scales: the parasequence, depositional facies and microscale (grains and pores).

Parasequences within the Glauconitic are bounded by flooding and ravinement surfaces. Sandstones of progressively younger parasequences clinoform and downlap on older parasequences. Three parasequences (G1, G2 and G3) are readily identifiable and correlatable across the area. At the parasequence scale, fine to medium grained sandstones of the G3 with dominant detrital quartz (66 - 75%) have permeability values ( 10 - 200mD) one to three orders of magnitude higher than very fine to fine grained sands with significant detrital dolomite and rock fragments of the G2 (0.1 - 5mD). Within each parasequence, preferentially permeable facies coincide with the foreshore facies. Differences in detrital compositions of the recognized facies ( quartz-rich versus chert-rich ) have resulted in different diagenetic histories for the sandstones. The abundance of labile rock fragments in the upper and lower shoreface resulted in early reduction of permeabilities as a result of mechanical deformation, thereby preventing post-depositional leaching from creating significant diagenetic porosity. On the other hand, sandstones of the foreshore facies have developed significant diagenetic porosity as a result of favorable detrital composition (high quartz and low clay content). This manifests as microporosity in chert and the high abundance of dissolution features.

From mapping of permeability trends, it is obvious that the best well deliverability is found some distance seaward (NW) of the updip termination of the preferentially permeable G3 parasequence, within the foreshore facies not cannibalized by shoreface ravinement.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah