Douglas J. Cant

Sedimentological and Sequence Stratigraphic Organization of a Foreland Clastic Wedge, Mannville Group, Western Canada Basin

ABSTRACT The Aptian-Albian Mannville Group of the Western Canada foreland basin shows longitudinal paleoflow parallel to the basin axis, and therefore its sequence stratigraphic organization differs from that of either a passive margin or a transversely supplied foreland. A detailed isopach map of the lower transgressive systems tract shows that over most of the basin, the greater part of this unit was not deposited in accommodation generated primarily by flexural subsidence In the west, the basal Cadomin Conglomerate was shed during a period of erosion of the overthrust load and uplift of the foreland, and does not mark the initiation of overthrusting. The Cadomin was preserved in disequilibrium with respect to any sea-level-related base level. To the east, basal Mannville sediments were deposited in valleys forming a major drainage system cut into the underlying Paleozoic rocks or in a large but shallow cratonic basin overlying a thick Paleozoic salt undergoing solution. The valley fills are nested, cut into one another because of the lower rates of ormation of accommodation and also the narrow topographic depressions on the basal unconformity. None of these sediments were deposited in accommodation resulting from flexural subsidence. The upper highstand systems tract includes a number of unconformities, indicated in nonmarine to marginal-marine areas by incised valleys or abrupt, discontinuous facies shifts, and in shallow marine areas by falling to lowstand shoreface sandstones. In the area of higher subsidence rates near the overthrust belt, subaerial unconformities do not extend onto marine shelves; conformable surfaces with no evidence of subaerial exposure (Type 2 sequence boundaries) occur here. Eastward (parallel to the shoreline) in areas of lower subsidence rates, these pass into subaerial unconformities (Type 1 sequence boundaries) cutting across marine shelves, with falling and lowstand shoreface deposits at their distal edges. Traced much farther east into areas with the lowest subsidence rates, the Ty e 1 unconformities amalgamate, with the result that the gaps in the section are larger stratigraphically and represent longer time periods. The proportion of falling, lowstand, and transgressive sea-level deposits compared to highstand deposits is also greater cratonward. These stratigraphic and sedimentologic patterns are interpretable as the result of fluctuations in sea level generated by some unspecified extrabasinal mechanism, possibly eustasy, superimposed on the laterally varying foreland subsidence gradient.

Sequence Stratigraphic Analysis of Individual Depositional Successions: Effects of Marine/Nonmarine Sediment Partitioning and Longitudinal Sediment Transport, Mannville Group, Alberta Foreland Basin, Canada

In the Falher Member of the Mannville Group (Aptian-Albian) of western Canada, two shoreline successions contain the reservoir conglomerates for the giant Elmworth gas field. The Falher B succession has a basal sheetlike shoreface unit of hummocky cross-stratified sandstone that thins seaward and terminates about 30 km north (seaward) of the landward limit of the transgression. Another 25 km farther basinward, the succession shows a 20-30-m-thick sandstone, unattached to the prograding shoreface, and an overlying coarsening-upward shoreface succession with thin muds and coals, interpreted as back-barrier deposits. These basinward facies are the results of a relative sea level fall and the early stage of the subsequent rise. In the upper (Falher A) succession, immediately andward (south) of the barriers, fluvial valleys were incised into nonmarine mudstones and coals during the base-level fall. As relative sea level subsequently rose, in nonmarine areas the valleys were filled by estuarine and fluvial sands, then a widespread sheet of fine-grained nonmarine sediment was deposited. At the same time, the shoreline migrated back across the shelf. As it reached the original shorezone (structurally controlled), reworking of underlying deposits successively generated three gravelly barrier islands superimposed on the sandy shoreface succession. The conglomeratic reservoirs all rest above the unconformities, in the transgressive depositional system. Because this sequence is essentially a set of linked facies and not a composite of stacked individual facies successions, it is affected by sediment partitioning between facies. During relative sea level rise, little marine sedimentation occurred because sediment was trapped mainly in nonmarine areas. Conversely, during sea level fall, most deposition occurred in marine areas because of the absence of nonmarine accommodation. Westward (alongshore) toward the thrust belt, no falling or lowstand sea level succession developed. Instead, a wide regressive shoreface sandstone with a transgressive cap occurs. Subsidence rates were higher in this area, and relative sea level appears always to have risen, but at varying rates. The surface under the transgressive facies changes from a type 1 unconformity in eastern, lower subsidence areas to a type 2 unconformity in western, higher subsidence areas. Any two-dimensional sequence stratigraphic model, therefore, is inadequate to describe the lateral variation of the sequence and distribution of shoreface sandstones, because the subsidence gradient was not parallel to the direction of shoreface progradation.

Lithology-dependent diagenetic control of reservoir properties of conglomerates, Falher Member, Elmworth Field, Alberta

Conglomerates in the Falher Member of the Lower Cretaceous Spirit River Formation of Alberta are the reservoir rocks for the giant (2-3 Tcf or 5-8 × 1010 m3) Elmworth gas field. Three types of conglomerates are present: (a) unimodal--granules or pebbles lacking any matrix, (b) bimodal grain supported--a framework of pebbles with fine to medium sand in the interstices, and (c) bimodal sand supported--pebbles floating in fine to medium sandstone. The distribution of these three types is controlled by the original depositional environment. The granules and pebbles are composed of chert and silicified sedimentary and volcanic rock fragments. The sand grains are dominantly quartz. During diagenesis, very small (.01-0.1 mm) drusy quartz crystals form on the cherts and rock fragments, whereas quartz grains undergo heavy quartz overgrowth formation because of differences in nucleation and growth of quartz crystals. As a consequence, most unimodal conglomerates (low in quartz) have not experienced major reduction of porosity and permeability. Bimodal conglomerates, both sand and pebble supported, have lost significant amounts because of cementation of the quartz-rich matrix. Streaks and patches of kaolinite and calcite cements have also reduced porosity and permeability locally. Gas production from Falher wells, therefore, de ends not only on the amount of conglomerate, but also on the proportion of unimodal and bimodal types.

Diagenetic Traps in Sandstones

Diagenesis may be an important agent in trapping hydrocarbons in sandstones by formation of reservoirs or seals. Reservoirs are formed by the creation of secondary porosity during burial. Seals are formed by heavy cementation, which may allow sandstones to retain large hydrocarbon columns. Formation of a diagenetic trap requires that parts of the sandstone unit react differently from each other during burial. This can be caused by differences in (1) detrital mineralogy--resulting from grain size or depositional environment controls; (2) early diagenetic mineralogy--largely depositional environment controlled; (3) burial history--structural movement induced; and (4) fluid content--hydrocarbon or water saturation. Each of these factors can lead to differences in porosity and permeability of the sandstone sufficient to form reservoirs and seals. In the correct configuration, diagenetic traps may be formed. Basin-center gas accumulations result from diagenetic trapping in some instances.

Sedimentology and Petroleum Geology, Spirit River Formation (Lower Cretaceous), Deep Basin, Alberta: ABSTRACT

The Spirit River Formation is subdivided into three members in northwest Alberta. The basal Wilrich Member consists of two 50 to 100-m thick upward-coarsening cycles of marine shales, siltstones, and sandstones. The Falher Member consists of nonmarine clastics and coals in the southern part of the area. Around the Elmworth gas fields, it is composed of five transgressive and regressive cycles in which marine and nonmarine conditions alternated. Each cycle can be traced northward into a laterally extensive upward-coarsening marine cycle. The gas reservoirs are complexly interbedded fine conglomerates and sandstones. Conglomerates interpreted as fluvial deposits have sharp bases, moderate to poor sorting, some cross-bedding, and variable amounts of sandy matrix. Those interpreted as beach deposits are moderate to well-sorted, horizontally bedded, and may lack matrix entirely. A complete gradation exists between the types, which are closely interbedded. Shoreface and beach sandstones are fine grained, well sorted, burrowed, and have near horizontal laminations and truncation surfaces. On a large scale, this shore-zone complex is best considered a wave-dominated delta. The Notikewin Member is the final seaward progradation of this system. Most sandstones in the Falher have less than 6% porosity and 1 md permeability whereas the reservoirs may have 20% porosity, much of which is secondary, and several darcys permeability. Early cementation, then formation of secondary porosity in the delta complex followed by deep gas generation have created a combined stratigraphic-diagenetic trap. End_of_Article - Last_Page 557------------