Reservoir Architecture of Thin Bedded Marine Sandstone Reservoirs, Niobrara and Milk River Formations, Southern Canadian Plains and Northern Us Plains
Reservoir architecture studies of unconventional hydrocarbon accumulations comprised of thin bedded sandstone reservoirs have mainly focused on the gross architecture of such shale dominated successions and has often failed in explaining productivity variations within these commonly laterally extensive gas fields. Our detailed studies of gas fields in southern Canadian Plains comprised of thin bedded marine reservoirs have revealed that sweet spots within these lateral extensive shallow gas pools are often stratigraphically controlled by complex clinoform architecture controlling the distribution of reservoir facies. Clinoforms also in places provides stratigraphic traps within an apparent homogeneous succession, with large variations in gas saturations between clinoforms. Within individual clinoforms, vertical variations in reservoir characters are very subtle and can mainly be observed in cores as reservoir beds are typical less than 2cm thick, well below the resolution of common well logs. In contrast to conventional shoreface sandstone reservoirs, the highest permeabilities don’t always correspond with the sandiest intervals, as the degree of bioturbation increases and thereby content of mud within the matrix with upward shallowing. Within sandstones of the Medicine Hat Member, however, sand filled trace fossils provide permeability conduits between thin bedded sand beds and thereby increase gas productivity. Examples will be shown from the Cretaceous Medicine Hat Member and Milk River Formation from subsurface studies of producing shallow gas fields in southern Alberta and Saskatchewan. Examples will show the highly variable character of these reservoirs and demonstrate the difficulty in making a predictable exploration and evaluation model for these thin bedded reservoirs. Outcrops of correlative deposits of the Milk River Formation in the Little Rocky Mountains in northern Montana of shaly prodelta deposits of Eagle Formation provides insight in variation in the lateral extent of the thin sandstone beds, important for the reservoir modeling of reserves and well spacing within such thin bedded reservoirs. Our observations are likely applicable to other thin bedded sandstone reservoirs and to shale gas reservoirs where the thin sand and silt beds and laminae provide permeability conduits for drainage of the absorbed gas within the interbedded shales, such as in the Mancos shales.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009