AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Resource Triangle – Parsing the Continuum of Continuous Accumulations
(1) Yegua Energy Associates, San Antonio, TX.
Resource plays -- those gas and oil plays which can produce conventional hydrocarbon fluids regionally on a well-by-well basis -- form a complex continuum which can be displayed on a triangle diagram. The end members of the triangle are the major reservoir constituents: Organic Matter, Silicate (Qz+Fsp), and Carbonate (Cc+Dol). The four major varieties of resource play can be visualized on the triangle; Coalbed methane plays near the organic matter apex, Tight sandstone plays near the silicate apex, Fractured carbonate plays near the carbonate apex, and Shale plays (gas and oil) in a zone above the S-C axis. Critical parameters for commercial reservoir development are low permeability (k<k-crit), low movable water (Sw<Sw-crit), low ductility (low clay content), and presence or absence of natural fractures. These critical parameters can be visualized and considered using the resource triangle.
The role of fracturing is critical and highly variable across all resource plays. The end member plays produce with natural fracture systems tapped by minimal hydrofrac treatments (cleat in coalbeds, fractures in carbonates and in sandstones or siltstones). The intermediate systems (shales) seem to have poor natural fracture networks that require massive creation of fractures to produce. Large fractures that tap into higher-permeability water sources can be destructive in any variety of resource play.
Resource plays may form a series of linked accumulations of different types. For example, the Eagle Ford oil and gas shale play is sandwiched between Austin and Buda fractured carbonate oil and gas plays; the Middle Bakken fractured carbonate oil play is sandwiched between Upper and Lower Bakken shale reservoirs; and the Piceance Basin (and other) tight gas sandstones have interbedded coalbed reservoirs.
Resource plays primarily consist of continuous hydrocarbon accumulations, but may include quasi-continuous reservoir intervals -- thick intervals of abundant, small ‘conventional’ accumulations which are always present but can still be produced on a well-by-well basis (Cotton Valley). Resource plays and tar sands (heavy oil) form the ‘neo-conventional’ class of hydrocarbon resources; true unconventional resources in today’s environment consist primarily of kerogen shales (‘oil shales’) and gas hydrates.