Components and processes affecting producibility and commerciality of shale resource systems

Abstract

Unconventional shale resource systems have provided North America with abundant energy supplies and reserves for present and future decades. This energy resource was largely overlooked until the new millennium. Prior 2000 only a few small oil and gas companies were pursuing such plays led by independent, Mitchell Energy and Development Corporation of The Woodlands, TX. Mitchell’s successful commercial development of the Mississippian Barnett Shale of the Ft. Worth Basin, Texas spawned an energy renaissance in North America. In the last decade the development of unconventional shale resource systems has been phenomenal in the United States with abundant new supplies of natural gas and oil. These resource systems are all associated with petroleum source rocks, either within the source rock itself or in juxtaposed, non-source rock intervals. Three key characteristics of these rocks, apart from being associated with organic-rich intervals, are their low porosity (less than 15%), ultra-low permeability (<0.1mD), and brittle or non-ductile nature. These characteristics play a role in the storage, retention, and the requirement of high-energy stimulation to obtain petroleum fl ow. Organic richness and hydrogen content certainly play a role in petroleum generation, but they also play a role in retention and expulsion fractionation of generated petroleum. Often a substantial portion of the porosity evolves from the decomposition of organic carbon that creates organoporosity in addition to any matrix porosity. Open fracture-related porosity is seldom important. Hybrid systems, i.e., organic-lean intervals overlying, interbedded, or underlying the petroleum source rock, are the best producing shale resource systems, particularly for oil due to their limited adsorptive affi nities and the retention of polar constituents of petroleum in the source rock. Paradoxically, the best shale gas systems are those where the bulk of the retained oil in the source rock has been converted to gas by cracking. Such conversion cracks the retained polar constituents of petroleum as well as saturated and aromatic hydrocarbons to condensate-wet gas or dry gas at high thermal maturity. Such high level conversion also creates the maximum organoporosity, while enhancing pore pressure. Bitumen (petroleum)-free total organic carbon (TOC) is comprised of two components, a generative and a non-generative portion. The generative organic carbon (GOC) represents the portion of organic carbon that can be converted to petroleum, whereas the non-generative portion does not yield any commercial amounts of petroleum due to its low hydrogen content. Organoporosity is created by the decomposition of the generative organic carbon as recorded in volume percent. In the oil window, this organoporosity is fi lled with petroleum (bitumen, oil and gas) and is diffi cult to identify, whereas in the gas window any retained petroleum has been converted to gas and pyrobitumen making such organoporosity visible under high magnifi cation microscopy. Production decline analysis shows the variable production potential of many North American shale gas and oil systems and their high decline rates. The most productive North American shale gas systems are shown to be the Marcellus and Haynesville shales, whereas the best shale oil systems are the hybrid Bakken and Eagle Ford systems.