Reservoir Geology Of The Neilburg McLaren Pool, Lloydminster Region Of Saskatchewan
Authors: Vigrass L.W. , Richardson S. , Pappas E.S. 
Published: March 2010

The Neilburg McLaren reservoir zone consists of tabular Lower and Upper McLaren Sands separated by a thin coal. The two sandstone reservoirs have a combined thickness of less than 7 m. The main trapping mechanism for the Lower Sand is formed by erosional channels filled by shaly and cemented rocks. The reservoirs typically consist of very coarse silt (Lower McLaren) and very fine sandstone (Upper McLaren). Despite the fine grain size, some Neilburg wells produce heavy viscous oil at initial rates up to 11 m3/d (70 barrels/day). The Lower Sand, although it contains more clay-size material and is finer and less sorted than the Upper Sand, has a larger porosity (33%) and permeability in excess of one darcy.

Field operators report that large amounts of loose sand are produced with the fluids from the Lower Sand. A major objective of this study was to understand why this sand is produced. Our preliminary conclusion is that sand production at Neilburg is related to complete leaching of cementing material from the Lower Sand during digenesis. It is speculated that secondary porosity was also produced so that porosity and permeability were enhanced and oil productivity from the fine-grained reservoirs is anomalously high.

Introduction

This paper concerns the Neilburg McLaren Pool where sizeable reserves of heavy oil occur in tabular sandstones having an aggregate thickness of less than 7 m. The pool has a proclivity toward production of loose sand, but in many other respects it is reasonably typical of the numerous pools that produce from thin sheet like sandstones in the Lloydminster heavy oil belt.

Work leading to this report was done through a Sand Control Studies project conducted jointly by the University of Regina and the Saskatchewan Heavy Oil Laboratory (now SRC Petroleum Division). The work was carried out under the Canada / Saskatchewan Heavy Oil/Fossil Fuels Agreement. Major objective of the study was to develop an understanding of the basic mechanism of sand production in the Lloydminster heavy oil belt. Geological work done by Richardson led to a B.Sc. (Hon.) thesis(1) at the University of Regina. A progress report concerning mainly stratigraphy and trapping mechanism was published by Richardson and Vigrass(2) in 1984. Contract reports, with Pappas and Vigrass(3,4) as principal authors, have been submitted to the Saskatchewan Department of Energy and Mines. Research on digenesis of Lloydminster sands will be continued by Richardson as part of her M.Sc. studies at the University of Regina.

The reservoir zone at Neilburg consists of a McLaren Lower Oil Sand, which is topped by an underclay and thin coal seam, and a thin McLaren Upper Oil Sand. The pool occurs on a structural high but the main petroleum trap for the Lower Sand is formed by erosional channels filled with shaly and cemented rocks. The Lower and Upper Sands differ in several important aspects. Characteristically the Lower McLaren is very coarse silt and the Upper McLaren is very fine sandstone. Despite the small grain size, the Neilburg wells are prolific producers of viscous oil; initial production rates are as large as 11 m3/d (70 barrels/day) of oil with viscosity of about 0.5 Pa.s (500 centipoise).