Companies drilling for oil and gas are engaged in an expensive game of chance. Given rising project costs and increasing pressure on investors to achieve acceptable levels of profitability, Producers have every reason to hedge their bets by reducing risk and pinpointing exactly where drilling has the highest chance of success. Seismic surveys are an essential tool in this endeavor when used to assess the potential of drilling locations and high-grade opportunities for development.
If you’ve been following activity in Oklahoma’s STACK play, you know that during much of the last few years, Producers have focused on appraisal work, i.e., finding the optimal drilling locations, well densities, etc. You can bet seismic surveys played a vital role in this task before drilling in earnest began. With the recent announcement of an upcoming seismic study in the Anadarko Basin core, I was reminded of the importance of seismic work in the exploration phase of the oil and gas lifecycle and thought it might be fitting to explore the topic of seismic surveys in this month’s article.
What is a Seismic Survey?
In an early February press release, TGS, a well-known geoscience data, and services company, announced plans for their “Canton 3D seismic survey targeting the high potential Mississippian Chester, Osage and Meramec intervals in the heart of the prolific SCOOP/STACK play fairway.” Sounds great, right? But, what does it mean? What is 3D Seismic? Why is it important enough to be announced in the Industry press? Most importantly, why is this potentially valuable information?
Geologists, geophysicists, and other geoscience professionals rely on seismic surveys to provide vital information about rock formations and other subsurface features. From analyzing survey results, it’s possible for scientists and engineers to determine rock properties, such as rock type and the degree to which it is naturally fractured. It may also be possible to identify oil and gas reservoirs as well as the boundaries between various formations and their thicknesses. Seismic surveys can be conducted on land and offshore. For this article, we’ll focus on land-based surveys.
A seismic survey is conducted by sending seismic waves from the earth’s surface into the subsurface along a predetermined line(s). Seismic waves are created either by a small explosive charge (like dynamite) set off in shallow holes or by large vehicles called vibroseis trucks that are equipped with heavy plates that send vibrations deep into the earth. Receivers on the surface called “geophones” receive the generated seismic waves reflected from the subsurface and transmit them to a recording system. Because geoscientists know how long it takes for sound waves to travel through and bounce back from different types of rocks and minerals careful interpretation of seismic data can help geophysicists and geologists create detailed cross-sections and three-dimensional maps of the subsurface including rock formations geological anomalies, and even hydrocarbon deposits of a surveyed area. It’s like the way bats, whales, and dolphins rely on echolocation to sense their environments by sending out and receiving sound waves. This echolocation sense creates a kind of “sound picture” of the animal’s location and what’s around them.
Kinds of seismic surveys
The various types of seismic surveys have expanded as computing power has increased and geoscientists have developed new instruments, software, and techniques to analyze subsurface features. Here’s a description of the main seismic survey types:
- 2D – The simplest, oldest data type. Seismic waves are sent downward along a single line of receivers providing a 2-dimensional “slice” of the earth’s the subsurface beneath the seismic line. Although limited in reach and scope, 2D surveys were the norm for much of the conventional development that’s taken place in previous decades.
- 3D – As computing power exponentially grew in the last 20+ years, 3D seismic has become the most common type of seismic survey conducted. The basic method is the same as 2D seismic but on a much larger scale. In a 3D survey, source and receiver points are arranged in dense grids over the area to be surveyed plus a border area to ensure adequate amounts of high-quality data are collected. An additional horizontal axis is captured creating the perception of depth and provides additional data points for a potentially more accurate geophysical survey. Since 3D seismic requires multiple parallel and perpendicular lines, the spacing of sources and receivers is set by the design and objectives of the study. The resulting dataset from 3D seismic provides a highly detailed and reliable model of the subsurface.
- 4D – 4D seismic introduces the element of time into 3D seismic surveys by repeatedly acquiring 3D seismic data of the same area over an extended time. This type of survey is often performed over conventional oilfields for secondary recovery purposes. The value of 4D is that its useful for monitoring resource depletion as well as discovering pockets of untapped reserves.
- 3C/4C – 3C/4C or Multi-component seismic measures sound waves both horizontally and vertically yielding extremely detailed and precise results. In turn, this data helps geoscientists more accurately describe and estimate reservoir characteristics.
How are 3D seismic surveys carried out?
Since 3D seismic is the most common type of seismic survey conducted (and the type TGS will be conducting), allow me to describe how these surveys are done.
Once an area has been identified for a seismic survey, a permitting process is undertaken to obtain landowner permission for seismic equipment, contractors, and crews to be on their property. Sometimes, additional permission will be sought if land clearing is needed within the survey boundary. Once the seismic project area has been permitted, the project design is finalized avoiding any unpermitted areas and providing sufficient area around the survey border to ensure high-quality data within the survey area. My good friend geophysicist, Gary Crews, offers the following analogy to better understand the need for collecting seismic data beyond the survey area. “One collects seismic imagery around the survey to provide a high-quality image in the survey area if you want to understand a picture of a grain of sand, you need a broader view to know if it is a sandbox or a beach.” 3D surveys must be conducted over a large area to provide sufficient data for accurate interpretation of the subsurface geology. These surveys commonly cover 50 to 100 square miles or more. In contrast, the planned Canton 3D seismic survey is expected to cover more than 450 square miles, according to TGS.
In preparation for gathering the three- dimensional (3D) seismic data, the survey crew establishes the study area’s grid. Source lines are established running one direction and receiver lines running a different direction. The source lines mark the points where either explosives or vibroseis vehicles will be placed. The receiver lines mark points where geophones (small devices in the ground that pick up reflected vibrations) will be placed to take readings when the explosives are set off, or the vibroseis vehicles are used. In the latter case, vibroseis trucks will move along the survey line stopping at prescribed intervals where they lower the vibrator to the ground and apply high-frequency sound waves. In either case, the reflected vibrations are sensed by the receivers (geophones) and recorded for subsequent processing and analysis. Using highly specialized software and robust workstations, technicians and analysts will translate and refine the raw seismic data from the field ultimately producing a detailed 3D model of the subsurface and a host of other analytical products.
What will the Canton 3D seismic study tell geoscientists about the STACK/SCOOP?
Seismic data provides extremely valuable data to help producers reduce risk while saving time and money. The Canton 3D will likely refine geoscientist’s overall understanding of the STACK/SCOOP fairway, but they’re not the only ones who will benefit. Drillers will be aided by the seismic data’s definition and delineation of their target shale formations and high-graded drilling locations. Completions will also see the benefit as they gain insight into the existence of natural fractures, which may open more easily during fracing. With permitting already underway on the Canton project and a projected start of the survey next quarter, scientists and engineers working the area may see processed data a year from now. With the completion of the Canton 3D, TGS will fill in a sizeable gap in their seismic data coverage over a large portion of the Anadarko Basin core.
- Sincere thanks to Gary Crews for his insights and technical review of this article.
- CGG Veritas
- Oklahoma Conservation Commission
Julie Parker has a decade of experience serving the Energy industry where she became an expert in the integration and application of geospatial technologies to exploration and production projects and workflows. Ms. Parker entered the industry in 2006 when she became the first GIS Director for Chesapeake Energy, a large independent producer of natural gas headquartered in Oklahoma City, Oklahoma with operations throughout the U.S. During her tenure at Chesapeake, Ms. Parker built and lead a robust, cross-functional GIS department that gained a reputation for developing and deploying leading-edge solutions for nearly all areas of the company.