If you are reading this blog you are almost certainly aware of the current level of excitement and enthusiasm surrounding shales. Drilling campaigns for oil and gas in shale plays are underway around the world and production from shale plays represents a substantial fraction of domestic production.
Shales are the most abundant types and volumes of rocks in sedimentary basins worldwide. Shales are the most abundant sources of hydrocarbons for oil and gas fields and (due to their low permeabilities) form the seal for many fields. A conventional oil or gas field needs a source, a reservoir (usually porous sandstones or carbonates such as limestone or dolomite), a trap (such as a structural closure, sealing fault or pinchout) and a seal. The source is necessary for hydrocarbons to exist and some pathway is necessary for theses hydrocarbons to migrate. The reservoir porosity was usually water filled at the time of deposition and entering hydrocarbons migrate through the porosity due to having lower density than water. The trap and seal are necessary to prevent continued migration of hydrocarbons and allow the reservoir to accumulate and store oil or gas. As a result, shales have been studied extensively by geologists and geochemists with most of the historical effort focused on their source rock potential. The fact that many shales still contain significant amounts of natural gas is no surprise to drillers and geologists. It is routine to observe natural gas “shows” while drilling through shales, occasionally in significant volumes. But while it may be clear to most readers what a porous sandstone or carbonate rock is like (the rocks that form most oil and gas reservoirs), shales remain somewhat of a mystery.
There are multiple definitions of what constitutes shale. One geologist told me that the way to differentiate between a siltstone and a shale was to put it in your mouth. If it tastes gritty, it is a siltstone. If it tastes smooth or oily, it is a shale. For the record, Baker Hughes does not recommend putting any rocks in your mouth. Shales are sedimentary rocks composed of clastics (portions of older rocks) comprising silts, muds and clays. Geological definitions of shales typically reference their grain size as being clay-like (less than 1/256th mm diameter particles). Silts are also defined based on grain sizes (between 1/16th and 1/256th mm) and are positioned between clays and sands. Very fine silts can be primarily composed of quartzitic materials lacking typical clay minerals. Clay minerals often include kaolinite, montmorillonite-smectite, illite and chlorite. There are dozens of “pure clays” with most clays being mixtures of multiple pure clays. One important clay in oilfield operations is bentonite, a typical weighting component of drilling mud. Another physical property often used in defining shales is their tendency to split into thin sheets or slabs (fissility). Muds are simply mixtures of water and very fine silt, clay and soil particles. Mudstones are hardened muds that may not show fissility unless dessicated. While most shales are clastics, significant carbonate components can be present in shales. The formations we refer to as shale gas or shale oil plays are often layers of shales and layers of silts and/or carbonates.
The depositional environment of shales that are ‘resource plays’ is typically a low energy one that allows very fine grained particles that will constitute future shales to be deposited. High energy deposits such as channel sands and turbidites will also lead to shale deposition. The low depositional environments associated with marine shales means that their geological and petrophysical properties change relatively slowly over large distances compared with the higher levels of heterogeneities in many sandstones and carbonates. Petrophysicists take advantage of this when analyzing logs of varying vintages and suppliers by normalizing log responses in marine shales.
Very fine grained organic material usually from plant materials are often deposited concurrently with the silt, mud and clay matter that will eventually form shales. Total Organic Carbon (TOC) is the weight percent of organic carbon in the shale (and does not include carbon from carbonates for example). Because many shales have relatively high organic carbon content, under certain conditions of heat and temperature they can serve as source rocks for oil and gas reservoirs. The organic content originated with living material, ultimately from plant, animal and bacterial matter such as algae, plankton or decomposed plant materials. Kerogen may be formed under the proper circumstances from this organic matter which can be broken down into lighter organic materials under heat and pressure. Most oil and gas produced originated with such shales as sources with a few exceptions.
Shales generally have extremely low permeability due to their very fine grain size. Permeabilities are so low in most shales that shales can form excellent seals for oil and gas fields. Drillers and mud loggers have long known that many shales contain significant quantities of natural gas as natural gas shows while drilling through shales are commonplace. While some shales are rich in heavy hydrocarbons (“oil shales”) this blog focuses on shales that are likely to contain natural gas or oil.
It is important to note that the type of shale production being discussed here is generally conventional oils or gas produced from relatively deeply buried shales that are produced in a manner roughly similar to conventional wells. We refer to this as shale oil and shale gas. The term oil shale is widely applied to petroleum extracted from shallow rocks with very high kerogen content. Many of the rocks referred to as oil shales are not actually shales and theses rocks are often physically mined rather than having the hydrocarbons produced through wellbores. Oil shales contain typically solid or ultraviscous hydrocarbon material in their pores. While some oil shales can be burned directly, most require some sort of extractive process coupled with upgrading to yield oil like hydrocarbons. Oil shales are not discussed in this blog.
Shale gas production has been known for over a century in the Devonian age formations in the Appalachian basin with more than 20,000 wells having been drilled in these shallow shales. Older (and deeper) Devonian shales include the Marcellus formation which is now a major focal point for the application of advanced technology. Given that we have known about these shales and their hydrocarbon potential for decades, if not more than a century, why has it taken so long to produce them? The answer is clear that the technology to drill, evaluate, complete and produce these shale plays has only been around for the last few years and many advances have been made in just the last few years.
In future blog posts we’ll take a look at the many shale plays in North American and around the world and the technology that is required to make them happen. Baker Hughes geoscientists and engineers have specialized software and capabilities to help understand the key variables in shales in order to optimize their evaluation, drilling, stimulation, development and recovery.
· Shale plays as “resource” plays.
· What does a shale need to be a commercial reservoir?
· What is a critically stressed fracture and how does it affect my shale play?
· So, what is different about hydraulic fracturing lately?