Reservoir Quality and Heterogeneity of Tidal Inlet Sandstones, Halfway Formation, Northeastern British Columbia, Canada

MUNROE, HUGH D., International Geoscience Consulting, Calgary, Alberta, Canada, and THOMAS F. MOSLOW, University of Alberta, Edmonton, Alberta, Canada

A subsurface investigation of the mid-to-late Triassic Halfway Formation in northeastern British Columbia has identified a series of wave-dominated tidal inlet sandstones associated with transgressive and prograding barrier island shoreline trends. Depositional models and facies reconstructions were based on sedimentologic analysis of approximately 60 cored sequences and 1200 well logs within the Halfway.

Tidal inlet sequences are very fine to coarse-grained quartzose sandstone ranging from 4.0 to 10.0 m in thickness. Facies with greatest reservoir quality are contained within the lower half of the sequence. Fine- to medium-grained stacked fining-upward units with scoured lower contacts and planar to trough cross-bedding characterize this facies. Molluscan shell molds and casts can comprise up to 60% of an inlet sequence. Porosity values as high as 25% are associated with these coquinas.

The upper half of most sequences is a planar to rippled laminated and burrowed (Skolithos) fine-grained sandstone of poorer reservoir quality. Where preserved, the uppermost 0.5 to 1.0 m is interbedded with backbarrier sandstones and mudstones that are wavy bedded and burrowed. The mudstones serve as an important stratigraphic seal. Bedding plane surfaces display symmetrical ripples, desiccation cracks, algal laminations, and root casts. These mudstones are interpreted to be of hyersaline lagoonal origin.

Tidal inlet sequences have a laterally discontinuous basal lag deposit (0.1 to 0.2 m thick) of poorly sorted, pebbly sandstone and angular mudstone rip-up clasts. The lags are formed by current scour and transport at the base of the inlet channel. As a result, the basal contact of the inlet sequence forms an erosional disconformity with underlying shoreface sandstones of the Halfway, or silty shelf sandstones of the Doig Formation. Overall, bed thickness, grain size, and frequency of cross-bed sets decrease upwards. Deposition in a laterally accreting channel by wave and tidal processes is indicated by the following: (1) an upward decrease in bed thickness, grain size and frequency of cross-bed sets; (2) the lack of a mud plug; and (3) the presence of associated backbarrier facies. Re ative to the paleostrandline, the orientation of Halfway tidal inlet deposits is a function of the: (1)

temporal landward or seaward migration of the barrier shoreline; (2) rate of sediment supply to the littoral zone; and (3) downdrift tidal inlet migration. The best modern analogs to the observed Halfway tidal inlet sequences and geometries are mixed energy, wave-dominated barrier shorelines of New Brunswick, Canada, and North Carolina, U.S.A.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)