Shale- and Mudstone-Hosted Oil and Gas

Oil and gas are commonly generated in source rock formations consisting primarily of fine-grained, highly impermeable mudstone containing a few percent of organic matter. Although commonly referred to as shale, source rocks are made up of mudstone and siltstone, with minor fine-grained sandstone, and grain-size equivalents among carbonate-rich and siliceous rocks. When hydrocarbons migrate out of the source rock to porous rock capped by an impermeable stratum, they form conventional oil and gas reservoirs. Oil and natural gas extracted directly from these petroleum source rocks and from nearby and also relatively impermeable sedimentary rocks constitute one of the fastest growing energy resources in the world. This chapter describes these tight oil and gas resources and the processes required to extract them from the rocks, including both hydraulic fracturing for mature and overmature source rocks, and surface and in situ retorting for immature source rocks (oil shale). For conventional reservoirs, horizontal well construction and hydraulic fracturing are, individually, relatively longstanding practices. The merging of these in long-reach horizontal legs and multistage hydraulic fracturing programs, as well as the monitoring of microseismic events related to fracturing, are newer developments. For oil shale, that is, rock formations containing mainly solid organic matter that must be heated to drive generation and mobilization of oil and gas from the rock, a variety of old, modified, and new technological solutions are producing oil and gas or are being tested to do so. Other chapters in this volume discuss aspects of unconventional resources and recovery methods, such as hydraulic fracturing and drilling technology. This chapter will focus on shale oil, tight oil, and shale gas and will discuss the technology used to extract these resources, with a shorter section on hydraulic fracturing.