Question 3 of 18: Arenite versus Wacke Sandstones and Reservoir Quality

Question 3: Arenite versus Wacke Sandstones and Reservoir Quality (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The Dott/Pettijohn boundary between the two is matrix content (primary depositional clay/silt, <0.03 mm): an arenite has <15% matrix — a clean, well-sorted, grain-supported framework produced by sustained current or wave winnowing that removed the fines before final deposition, later bound by sparry chemical cement rather than clay; a wacke has 15–75% matrix — a poorly sorted deposit in which sand grains "float" in an abundant primary clay/silt matrix because a rapid, unsorted depositional event (turbidity current, debris-flow-adjacent settling) delivered mud and sand together too quickly for winnowing to separate them.

The arenite is by far the better petroleum reservoir. Its low matrix content leaves open, interconnected primary intergranular pore space and correspondingly high permeability, and even where later cementation reduces porosity somewhat, the well-sorted grain framework typically retains usable storage and flow capacity. A wacke's abundant clay matrix already occupies and occludes the primary pore throats at deposition, giving low porosity and very low permeability even before burial; its poorly sorted, mud-rich fabric is also mechanically more ductile and compacts more readily under overburden, destroying what little pore space remained. Consequently arenites (clean sandstones) are the conventional clastic reservoir target, while wackes (greywackes) are treated as effective seals/baffles or, at best, poor, tight reservoirs requiring stimulation.