Rock Creek

Exploration Geology of the Middle and Upper Jurassic, West-Central Alberta


Summary:
Location: T41-55,R5-14W5
Strata: Rock Creek
Year of Study: 1995


Introduction

A subsurface study of Middle to Late Jurassic units (Poker Chip, Rock Creek, Niton) of west-central Alberta was undertaken in order to document lithofacies, interpret depositional environments, and suggest the primary controls on petroleum entrapment.

The Jurassic sequence of west-central Alberta is a laterally and vertically complex package of sandstones, coquinas, shales and mudstones, characterized by several internal disconformities and extreme facies contrasts.

Depositionally, the package is interpreted to represent predominantly subtidal shoal deposition upon a shallow shelf.

The most proximal deposits are located in the northeastern portion of the study area, but neither fluvial deposits nor subaerial exposure surfaces were recognized. The open marine basin was located far to the west and northwest. The study area is located to the east of, and between, two major Jurassic depocentres - the Nikanassin in the north and the Fernie-Kootenay in the south. The location of the study area between two major depocentres suggests that the development of Jurassic unconformities and the preservation or absence of post-Rock Creek Jurassic sediments were related to basin tectonics.

Although interbedded with coarse grained clastic units in the east, the Poker Chip Member mainly represents open marine deposition of shale and mudstone. Towards the tops of the muddiest zones, thin storm-generated sandstones are located near the contact with the overlying coarse units of the Rock Creek Member. The Poker Chip - Rock Creek contact appears gradational in the northwest (Wolf Creek) towards the regional shaleout of the Rock Creek - Niton package.

Although lateral correlations of individual beds are difficult within the Rock Creek Member, the basal deposit generally comprises a coquinoid sandstone which reflects high energy transport and deposition from both tidal and storm currents. The central part of the unit is variably muddy or sandy with both lithologic types displaying a limited marine trace fossil assemblage interpreted to represent stressed (i.e.; tidally and storm-influenced) environments. The uppermost Rock Creek lithologic type encountered throughout much of west-central Alberta is flaser bedded sandstone which represents the movement of sand sheets driven by poorly confined tidal currents.

Thick (up to 30m) multistory coarse grained sandstone bodies are locally encountered and can be mapped out with distinct pod shapes. These features are considered to be analogous to compound tidal ridge deposits which feather laterally into interbedded mudstones and sandstones. The tidal ridge deposits are interpreted to be in relatively close proximity to drowned river mouths or estuaries within a situation analogous to the modern North Sea Coast of Great Britain and Western Europe. In the northeast, several trends of channelized sandstones, interpreted to be subtidal estuarine channel fills, are found with southwesterly orientations. The thickest Rock Creek coarse grained units are found northwest of the Devonian Nisku bank margin which suggests a causal relation between Middle Jurassic deposition and deeper structures. Throughout much of the study region the Rock Creek is unconformably overlain by apparently Lower Cretaceous sandstones and shales. The Middle and Upper Jurassic strata are locally eroded to a significant degree and replaced by relatively thick Lower Cretaceous strata which form a series of elongate northeast-bysouthwest- trending valley fills. Many of these valleys were western tributaries of the large Edmonton Channel which was a major northwest-trending valley present during the Lower Cretaceous. The westernmost valleys drained into the Spirit River channel located further west beyond the study area. In the rest of the area the Rock Creek is overlain by younger Jurassic rocks which thicken to the north and northwest. The most significant post-Rock Creek Jurassic zone is a productive, blanket transgressive bioturbated muddy sandstone referred to as the Niton Sand". It is separated from older Jurassic deposits by an unconformity.

Most Rock Creek reservoir units can be classified as sandy coquinas, coquinoid sandstones or quartz arenites. Locally developed dolomitic units are encountered in close proximity to Lower Cretaceous valley fills. The sandy coquinas are comprised mainly of bioclastic debris and peloidslooids with quartz nuclei. Coquinoid sandstones comprise roughly equal mixtures of shell debris and quartz sand whereas the quartz arenites are formed of over 95 percent quartz with a minor chert and rock fragment component. The high quartz content of Rock Creek sandstones suggests that they were ultimately derived from the eastern craton.

Although the Rock Creek sandstones possess a simple mineralogy they have undergone a complex diagenetic history. The major cementing agents comprise silica1 calcite1 dolomite, siderite, pyrite, bitumen, clay, and minor barite. Silica cements mainly take the form of quartz overgrowths although silica replacement of shell material is also locally present. Quartz druze is present within some vugs and lines some fractures. Calcite cements include poikilotopic cements and micrite. Calcite also forms a grain-replacing material as well as a fracture-lining cement. Dolomite appears to be replacing mudstone or limy mud beds. Siderite is a locally developed cement and acts as a pore lining material. Sulfides (mainly pyrite) are also sporadically developed as cements and are most commonly observed at bed and unit contacts. Bitumen tends to be concentrated near muddy laminae, along fracture walls and as a pore lining. It is a major destroyer of reservoir quality in some wells.

Many Rock Creek sandstones are tight although secondary porosity is common in some carbonate cemented zones. Much significant porosity and permeability can be attributed to the development of fractures which were open prior to the development of some calcite and silica cements and hydrocarbon pooling. The shallowest units in the east and northeast, close to the subcrop edge, commonly display significant intergranular porosity, especially within eastern channelized Rock Creek sandstones and the blanket Niton Sandstone.

Jurassic structure within west-central Alberta takes the general form of a southwestern regional dip. At a local level Rock Creek sandstones display pervasive fractures which are better developed in areas of known Nisku reefal buildups (e.g. West Pembina, Wolf Creek). Fracturing can be observed within all Rock Creek lithofacies but is most commonly observed within flaser-bedded sandstones. Within this lithofacies mudstone flasers and interbeds have been highly compacted into a pseudo-stylolite appearance. Vertical fractures are well developed at the apices of the pseudo-stylolites.

For more information contact:

Leslie Sears
Petrel Robertson Consulting Ltd.
500, 736- 8th Avenue S.W.
Calgary, Alberta
T2P 1H4

Phone: (403) 218-1618
Fax: (403) 262-9135
lsears@petrelrob.com
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