Abstract

Conventional oil source rocks (A.K.A. unconventional reservoirs) are formed by a series of processes which transform organic matter incorporated in sediments into oil and gas. Crude oil and most natural gas originate from hydrogen-rich organic matter such as plankton, algae and anaerobic bacteria preserved under anoxic depositional conditions. Very low temperature diagenesis liberates carbon dioxide from the organic matter which dissolves in interstitial water, forms organic acids, leaches carbonate minerals, and deposits calcite in pores and fractures.

Early catagenesis (~0.5% Ro) converts part of the hydrogen-rich kerogen to solid bitumen which extrudes from very organic-rich zones into fractures and rock matrix pores. Continued burial and heating successively forms crude oil, condensate, and wet gas, until ultimately only methane and graphite remain. Oil generation is a volume-increase reaction which results in greater source rock porosity and internal pressure. When the pressure exceeds geostatic, the rock ruptures, oil and gases are expelled, the fractures close, and the source rock returns to near pre-generation conditions. This process is repeated many times as the source rock passes through the oil generation maturity “window” (~0.6 to 1.4% Ro).

Primary oil migration out of the source rock is very inefficient and natural micro-fracturing of brittle rock due to tectonics and oil generation is essential. Oil production from unconventional reservoirs requires matrix porosity storage with pore sizes typically ranging from approximately 4 nm to several microns and permeability in the micro-Darcy range. Natural and stimulation fracturing provides the effective permeability. These tight unconventional reservoirs can occur within the source rock sequence itself (Eagle Ford, Wolfcamp,) adjacent to it (Middle Bakken, Sanish, Three Forks, and Lodgepole) or both (Woodford, Sycamore, Hunton, Viola). In very mature shale gas reservoirs, a nano-porosity network is contained primarily within the solid bitumen organic matter and the gas may be stored largely within these organic pores or adsorbed on the pore surfaces. Reservoir pressure and water saturation play important roles in ultimate recoveries from these tight, conventional oil source rock - unconventional reservoir sequences.