Abstract: High Resolution Sequence Stratigraphy of Incised-Valley Systems: General Characteristics and Common Variants with Examples from the Western Interior Seaway

Brian A. Zaitlin

Incised valleys systems (IVS) created during a relative sea level fall and back filled during its subsequent rise constitute one of the most common scenarios of hydrocarbon-producing Cretaceous IVS reservoirs in the Western Canada sedimentary basin (WCSB). Theoretical considerations and observations of many modern incised-valley systems indicate that the stratigraphic organization of the entire valley fill, and of the estuarine deposits in particular, is predictable, and is controlled by changes in accommodation space. At the most general level, all valley fills can be classified as either "simple" or "compound," depending on whether they consist of a single depositional sequence or more than one sequence, respectively. Simple valley fills are most common in small valleys confined to low-gradient coastal plains (i.e., coastal-plain valleys), whereas compound systems are more common in larger incised-valleys that have their headwaters in a (mountainous) hinterland (i.e., piedmont valleys).

Any simple valley fill, or each sequence in a compound fill, can be subdivided longitudinally into three segments. Segment 1 is the most seaward part of the valley, lying between the lowstand and initial highstand shorelines; segment 2 is the middle section and corresponds to the dimensions of the estuary at the end of the transgression; and segment 3 is the most landward portion, lying landward of direct marine influence throughout its history. Estuarine deposits will be present in segments 1 and 2, but are absent, by definition, from segment 3. The idealized vertical succession of facies differs between segments 1 and 2 in relation to where the two segments lie with respect to the initial highstand shoreline (the shoreline location at the "turn-around" point between the transgressiv and highstand systems tracts). In segment 1, estuarine deposits overlie lowstand and transgressive fluvial deposits and typically exhibit a transgressive stacking of facies. Estuary-mouth facies are typically partially to completely removed by wave or tidal ravinement as the shoreline backsteps, and the remaining estuarine deposits are overlain by marine sands and/or muds. In segment 2, open-marine deposits are absent because this segment lies landward of the initial highstand shoreline. In the ideal case, transgressive estuarine deposits overlie lowstand and transgressive fluvial deposits, and are themselves capped by a regressive estuarine succession that forms as the estuary fills at the beginning of the highstand. The maximum flooding surface lies near the middle of the estuarine su cession and passes seaward into marine shales above the ravinement surface.

The following examples from the WCSB demonstrate how this approach has been used by PanCanadian Petroleum Limited in the characterization of lowstand to transgressive IVS reservoirs.

Lower Cretaceous Glauconitic Formation reservoirs along the Countess-Alderson (C-A) trend are interpreted to be part of a larger IVS trend extending from the Hoadley (LST) barrier system in central Alberta, southward for more than 535 km into the United States. The C-A part of this trend extends for more than 90 km through PanCanadian lands, and contains 122 pools with cumulative production in excess of 100 million bbl of oil and 200 bcf gas. Pool optimization/development studies integrating geological, petrological, 3-D geophysical, and reservoir engineering data have been used to better understand the geological controls influencing current production. The majority of pools produce from backstepping [LST to TST] fluvial and estuarine bayhead delta [BHD] and central basin [CB] deposi s, characteristic of segment 1.

The Countess "O" pool has produced 5.7 million bbl of oil in 22 years. Well-sorted quartzarenites are interpreted to have been deposited in a complex network of tidally influenced BHD distributary channels. To the south along trend, the Lake Newell project has defined 5 pools with 15.3 million bbl of recoverable oil in a compound IVS. The main reservoir consists of LST fluvial to LST-ET BHD distributary channel deposits overlain by CB mudstones in a series of retrogradationally stacked parasequences. The Johnson "B" pool, containing 4.3 million bbl, lies further south along trend, and is interpreted to be composed of a lower, coarse-grained chert and kaolinitic-rich "basal quartz" unit, incised by a Lower Glauconitic Formation IVS BHD complex.

In contrast to the segment 1 deposits described above, the (Lower Cretaceous) Lloydminster (Glauconitic-equivalent) IVS of the Senlac field lies within a separate paleovalley system to the east of the C-A trend. Senlac contains 84.3 million bbl OOIP and is located within a 25-km-long by 8-km-wide embayment along the north flank of a paleotopographic high. The distribution of facies is interpreted to represent a compound IVS, with LST fluvial deposits overlain by a complete wave-dominated sandy estuarine complex representing segment 2 of a larger north-south-trending IVS. The reservoir facies are associated with the sandy barrier (shoreface, tidal inlet, and flood-tidal delta deposits) located at the seaward end of the estuarine complex.

AAPG Search and Discovery Article #90948©1996-1997 AAPG Distinguished Lecturers