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Abstract: Lathom “A” Pool: An Example of a Lower Cretaceous Compound Incised-Valley Reservoir from Western Continental Interior



The L. Cret. Glauconitic Fm. of the Western Canada Sedimentary Basin is characterised by a network of north-westward trending compound piedmont incised-valley systems (IVS) feeding lowstand to early transgressive shorelines to the north. The fills of these Glauconitic IVS exhibit a progressive northward change from i-low accommodation lowstand to early transgressive fluvial to fluvial-estuarine deposits, characterised by multiple erosive events, to ii- increasing accommodation where IVS are separated by coarsening-upward (highstand) shoreface parasequences resulting in full preservation of individual IVS. Individual IVS, from 1 to 5 km wide, are mappable in the subsurface for over 535km. Currently, detailed stratigraphic correlation, core analysis, petrographic, seismic and engineering data define at least 7 mappable IVS sequences which can be correlated to different interfluve surfaces.

The Lower Cretaceous Lathom “A” pool, discovered in 1968 and containing 25.7 MMBLS OOIP, is located in Twp 20 Rge 17 W4M. Lathom “A” is an example of one such compound piedmont IV reservoir. The Lathom “A” at present defines the northern terminus of oil-producing Glauconitic reservoirs along a 90km stretch of one L Cret. Glauconitic IVS. This reach contains more than 500 producing wells distributed through 122 pools with cumulative production to date in excess of 100MMBLS and 300 BCF of gas.

The L. Cret. stratigraphy in the Lathom “A” area rests on a basal sequence boundary between Mississippian carbonates and the L. Cret. Mannville Group with appreciable topographic relief. The Mannville Group consists, in ascending order, of the Detrital, undifferentiated “Basal Quartz”, Ostrocod Lst, Glauconitic, and Upper Mannville. Regionally, the Ostrocod-Glauconitic interval is further subdivided into i-the Ostrocod, composed of a basal, fine-grained quartz-rich incised-valley system overlain by multiple coarsening-upward quartz-rich shoreface deposits capped by a marlstone sequence (termed here Sequence 10); and ii- the Ostrocod is in turn overlain by 7 cycles of Glauconitic IV to shoreface deposits, capped in places by well developed paleosols.

The mineralogical composition of the Glauconitic sequences change from i- a basal sublithic IV and shoreface (Seq. 20), ii- a quartz-chert IV and shoreface (Seq. 30), iii- a sub-lithic IV and shoreface (Seq. 35), iv- a quartz-feldspar IV and shoreface (Seq. 40), v- multiple lithic IV and shorefaces (Seq. 50, 55 and 60). This progressive mineralogical change is most probably due to a combination of tectonic uplift in the hinterland coupled with fluvial system reorganization in the hinterland and eustatic changes associated with the northern Boreal seaway.

The Lathom “A” pool, typical of other reservoirs along the trend, produces from fluvial to fluvial-estuarine deposits associated with Sequences 30 and 40. Both sequences contain basal medium- to coarse-grained x-bedded fluvial sandstones, overlain by finer-grained, tidally-bedded and bioturbated estuarine sandstones and interbedded tidally-bedded sandstones and mudstones deposited in bayhead delta, distributary channel, tidal pointbar and central basin settings capped by paleosols. Sequence 50 and 60 lithic fluvial sandstones truncate beth Sequences 30 and 40 forming the stratigraphic erosional trap geometry for such pools.

Significant changes in permeability, porosity, and Sw between beth IV sequences and depositional facies directly controls producability, storage capacity and partitioning of hydrocarbons within the pool, thus allowing for better optimisation. When used in conjunction with regional studies, pool analogue studies of this type add valuable information both for further exploration along a trend and for planning further pool development.

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