Trapping Of Heavy Oil At Lloydminster
Authors: Vigrass L.W. 
Published: March 2010

Abstract
Most of the oil in the Lloydminister area occurs in the Sparky and General Petroleums (G.P.) sandstones. These sandstones are widespread tabular sheets which are probably marine delta-front deposits over much of the area. Detailed studies show that the bulk of the Sparky and G.P. oil is trapped downdip from gullies or channels that are filled with upper Marmville shaly rocks. The grillies are as much as 235 ft (72 m) deep and were formed by sub-aerial erosion on an imra-Mannville disconformity. The time involved in uplift, erosion, Subsidence and burial was short - probably less than a million years. The shaly fill in the gullies and the Mannville strata younger than Sparky are mainly fluviatile sediment derived from the Cordillera.
Recognition of the presence and nature of the disconformity is extremely important ill exploration, in development, and in evaluating heavy oil resources for enhanced recovery.
In the Lloydminster area, situated on the inter-provincial boundary between Saskatoon and Edmonton (Fig. 1), important reserves of medium and heavy oil occur in sandstones of the Lower Cretaceous Mannville Group. The oil area was discovered more than 50 years ago, but there have always been problems because of the nature of the oil; viscous asphaltic crude that is difficult to recover, troublesome to handle and hard to market. Interest in the area has increased because of the large amounts of oil-in-place and the potential for increasing recovery by thermal" or other exotic recovery methods. There have also been problems in deciphering the geology and defining the trapping mechanism, largely due to poor collection of geological data and lack of interset of most petroleum geologists. Most earlier geological workers, in published accounts at least, have interpreted the heavy oil sands as part of a delta complex, with the oil trapped in lenticular sand bodies or trapped behind distributary channels on the delta surface. Detailed log examination and mapping suggest a contrary interpretation: the important oil reservoirs are sheet-like sandstones, an important disconformity occurs within the Mannville and most of the oil is trapped downdip from gullies on this intra-Mannville erosion surface. Gullies on the unconformity are as much as 235 ft (72 m) deep and are filled with shaly beds and muddy sandstones.
This paper is dedicated mainly to providing evidence for the disconformity and showing examples of how the disconformity traps the bulk of the oil in the Lloydminster area. For additional details On stratigraphy, sedimentology and geological history, the reader is referred to Vigrass (1977).

Oil Fields of Lloydminster Area
Defined oil and gas pools of the map area (Fig. 2) are all in sandstones of the Mannville Group, the sandy sequence at the base of the Lower Cretaceous. Gas fields; stippled on Figure 2, are of minor importance and will not be discussed. Oil pools are of two classes: those that occur in the middle and lower part of the Mannville (Sparky or older) are shown solid, whereas those in the upper part of Mannville are shown by an outline only.