West Central Alberta

Lower Mannville/Jurassic Study, West-Central Alberta

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Summary:
Location: T41-52, R5-16W5
Strata: Lower Mannville / Jurassic
Year of Study: 1998

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Introduction

An evaluation of cores, logs and tests from over 2900 wells has resulted in:

1. The development of criteria useful in discriminating Jurassic from Lower Cretaceous strata
2. An understanding of currently defined petroleum pools
3. The development of new exploration fairways

The main criteria used to define individual stratigraphic intervals are:

1. The Rock Creek is Bajocian in age, is a blanket deposit comprising marine coquinas and quartz arenites, contains diverse macro and trace fossil assemblages, has locally common phosphatic material, and has experienced a complex paragenetic history.
2. The Niton Sandstone is Oxfordian, is formed of marine quartz arenites that are commonly highly bioturbated, contains giant Teichichnus traces, has abundant clay, and exhibits low resistivity log responses. The thickest Niton beds are associated with relatively thin Rock Creek intervals.
3. The upper Fernie Formation is Kimmeridgian to Volgian, has a basal glauconitic shale of maximum marine flooding origin, and is overlain by highstand deposits of interbedded cherty sandstones and shale with Planolites burrows and synaeresis cracks, capped by thick, massive and muddy to cross-bedded fine-grained sandstones. The upper Fernie is widespread, and thickens to the west; a prominent ridge of upper Fernie sandstone, oriented northwest-southeast, occurs in the centre of the study area.
4. The Aptian Cadomin Member of the lower Mannville is a westward-coarsening and thickening quartz and chert pebble conglomerate and cross-bedded cherty sandstone, with subsidiary coal. It is restricted to the westernmost part of the study area.
5. The Aptian Ellerslie/Gething Member is a fining-upwards succession that contains cherty and quartzose sandstones, depending upon the composition of subcropping units. Crossbeds commonly display muddy drapes, rooted and burrowed (by Skolithos) zones and rare coquinoid interbeds formed of gastropod and bivalve shells. The thickest Ellerslie/Gething beds fill deep incisions upon Jurassic and Mississippian strata.
6. The Aptian-Albian Ostracod zone comprises a basal siliciclastic unit and an upper shale unit with concentrations of gastropods and thin-walled bivalves. The siliciclastic units contains poorly-sorted, tight, burrowed coquinoid sandstones with minor phosphate and current-rippled to cross-bedded lithic sandstones, with abundant secondary porosity created by the leaching of lithic grains. The thickest coarse-grained Ostracod beds are encountered in the vicinity of the deep incisions in Jurassic/Mississippian strata.

The general structure of the study area takes the form of a simple southwesterly dip. However, residual structure displays one prominent northeast-southwest trend, parallel to the principal stress direction in the basin, and coincident with the location of the underlying Nisku bank margin. A northwest-southeast residual trend in the west appears to influence gas pooling in Jurassic and Mannville strata at Peco.

Lower Mannville reservoir units in the study area are generally underpressured in the east, supranormally pressured in the west, and lack widespread aquifer development. The majority of oil pools occur in the eastern, shallow portions of the study area. A similar distribution of pressures and fluids is also observed within the Rock Creek-Niton and Viking units of west-central Alberta. The study area is located in a region of relatively high geothermal gradient but low heat flow. Associated with the highest pressures and temperatures is an abundance of secondary porosity within lithic sandstones, such that many deeply buried (i.e. over 3000 metres) Lower Mannville reservoirs have porosities between 15 and 20 percent, with permeabilities of several tens of millidarcies. Many wells with these petrophysical properties have been identified as bypassed pay opportunities.

The biggest potential hydrocarbon accumulations will be discovered within the thickest, most widespread, deeply-buried lithic sandstones. These will be encountered west of the northwest-striking residual structural high. In this area, thick, porous and permeable, gas-bearing, grain-supported Cadomin conglomerates are encountered (e.g. 46-18W5, 47-21W5). The optimum targets in the Ellerslie/Gething occur within basal sections of linear valley fills, particularly those tributary to the Spirit River Channel (i.e. located on the northeast margin), abutting impervious Fernie strata to the northeast. The thickest upper Fernie sandstones, where porous and permeable, may also be petroleum charged, if located updip of Mannville reservoirs, and upon structurally and paleotopographically high erosional remnants. The most promising interval for new large discoveries is the Ostracod. Prospective untested reservoir beds over 6 metres thick are located west, northwest, and southwest of the recent discoveries by Triumph in Twp. 45-10W5, where 3 to 4 metre thick productive zones occur. The thicker targets will be associated with Ellerslie/Gething valley fills. The western downdip extent of Ostracod sandstones is not presently known, but include productive beds in the large (174 BCF) Peco Field.