Finding the field can be a noisy business

One of my early "scientific" memories was being taught that, after you see a lightening strike, if you start counting the time in seconds until you hear the thunder, then divide by five, this tells you how many miles away the storm is. This is because the speed of sound in air is around 1,100 ft/sec (I say around since it depends on temperature and humidity). In the same way it was fun to stand in a place where you could get an echo when you yelled really loudly.

From which you may surmise that it is time for another techie talk. For those new to the site, this is a post on occasional weekends where I try and simply explain some of the aspects of getting oil and gas out of the ground. The full list of previous posts on the topic is given at the end of the post, and you are asked to remember that this is a relatively simplified discussion that is meant to briefly describe the fundamental ideas.

In the last post I considered that we had drilled into a reservoir, and showed how roughly one might find out how much oil was there. But I did not mention how we found out how large the reservoir was (I used a 200 ft thick, block of rock that measured 1,000 ft in length and width to make the calculations easy and obvious). So this week perhaps we can talk about why we think the field is a thousand ft wide and long. (We ultimately physically measure the 200 ft dimension).

When oil was first used, the way you found it was by finding where it was seeping out of the ground. However most oil fields do not have this visible signature, because all the oil had been trapped in a reservoir rock. When oil is first formed it comes from a source rock, that will have contained the biological material (mainly algae and plankton) as well as the muds and sand that accumulate with the dying organisms. As this material sinks deeper into the earth the pressure of the overlying material, and the growing heat at greater depth will slowly turn it into initially oil, and if it gets hotter, into gas.

However as these fluids are formed, they can be squeezed out of the original rock, and being lighter, will then start to move up through the ground. They will continue to flow up until the nature of the rock above them stops them going any further. This has usually been because the rock above is impermeable, and so the oil, gas and often associated water, cannot continue flowing up to the surface. Over the course of time the rock that the oil and gas is passing through will have been tilted, and bent by the movement of the Earth's crust, so that, in a favorable case, when the oil gets stopped by an overlying impermeable layer (the cap rock) that rock and the reservoir rock will have been previously shaped so that the oil cannot just move over to the side to find another way up.

The result of the shaping of the ground has therefore been to form a trap for the rock. One might think of a series of hills underground where the oil has risen inside the hill and having reached the top cannot go further up or out because the rock above the hill is not permeable. The oil then lies in that hill, and forms an oilfield. (There are many other shapes that also help to trap the rock - the layers can have been tilted, for example, and then an impermeable rock, say a salt, or a granite) can have been pushed up through the layers forming a wall which stops the oil movement and forms a trap for it). There are some pictures of possible traps here.

So what we now need to do is to find a way of looking through the Earth so that we can see the shapes of the layers of the ground at different depths beneath our feet. It would be nice if we could use a form of radar for this and there is such a thing as Ground Penetrating Radar . However because the energy that this uses is very rapidly reduced as it moves through the ground, this technique is only really useful close to the surface, where it can be used to find water pipes, graveyards, and is proving very useful for archaelogy and other searches at shallow depths. It doesn't have enough power however to look as deep as we want to.

To look deeper we need to use a more powerful signal. And we also use the same ranging idea that helps see how far away a storm is. The speed at which sound moves, but in this case it is through the rocks. By setting off a very load noise - that used to be a stick of dynamite, or a gun but is now more often the impact of a heavy hammer on a plate. Heavy trucks are used that drive this signal into the ground. They come in a variety of sizes. At sea, the sound can be generated by an air gun

When the hammer hits the plate a sound wave is sent into the ground. This radiates out, in the same way that happens when you drop a pebble into a still swimming pool, except it does it in three dimensions. (Trying this in a coffee cup or sink doesn't work since I just discovered that it happens too fast to see what happens). When the wave that moves out hits a layer of rock that is different, then part of the wave energy bounces back rather than going deeper. By laying out a string of microphones (called geophones because they listen to the rock) instruments can hear when that part of the signal bounces back to the surface, and by listening at different points they can combine the signals to give what is known as a reflection Seismic survey of the underground layers. You can get information both from the speed of the wave, and the distance that it has traveled. This can then be used to provide a map. The initial signals are enhanced and analyzed by computer programs to provide pictures of the structure of the ground, whether on land, or at sea.

For many years the survey was carried out by a single line of geophones that were stretched along a single path, giving a two-dimensional (2D Seismic) slice through the ground. Increasingly the surveys now use a grid of geophones so that the pictures that are recovered give a picture that has 3 dimensions (3D Seismic) that makes it a lot easier to understand the information. (The technical term is interpret the data). Columbia University has an example that includes a fairly comprehensive set of information on the Gulf of Mexico as the pdf file entitled Prospectivity of the Ultra-Deepwater Gulf of Mexico. There is also an animated 4D example that shows how effective a tool it can be.

By shooting a survey repeatedly over a period of time, the intent is

4D seismic is a reservoir management tool based on carrying out a series of repeatable 3D seismic surveys at regular intervals over a producing reservoir in order to 'image' the movement of hydrocarbon fluids. The information derived from comparing different surveys over time will allow petroleum engineers to focus production drilling and other hydrocarbon recovery measures which can hopefully net millions of dollars from additional reserves.
By identifying where, for example, the oil:water interface is moving to, one can better site subsequent infill wells. And by knowing the thickness of the oil layer, and its relative position in the well, which one can then mark (or have the computer color into the projection) one can estimate what the size of the actual oilfield is that you have hit.

This is part of an ongoing weekend series on technical aspects of oilwell (and natural gas) drilling. Previous posts can be found at::
the drill
using mud
the derrick
the casing
pressure control
completing the well
flow to the well
working with carbonates
spacing your well
directional drilling 1
directional drilling 2
types of offshore drilling rigs
coalbed methane
workover rigs
Hydrofracing a well
well logging

As ever, if this is not clear, or if there is disagreement then please feel free to post, and I will try and respond.

what is the tried-and-true plane table method ?
It is a fairly basic method of drawing a local map, with the assumption that the curvature of the Earth can be neglected in the process.  You need a flat table, paper, and a special telescope (to make sure that you are accurately lined up on the target). The process itself is described in more detail here .  But in essence you start from two points and build your map out by drawing a line on the paper from where you are to points you want to identify, from both locations, and with the line between the two points accurately positioned on the paper.  Where the two lines from the initial points intersect give you the map position of the targets.
Basic Surveying.  I was trained as a Landscape Architect. But have had 5 years of intense Digital Surface Mapping, both of above ground land and Below surface land, in the Below Surface land It was the worlds Oceans and Coastlines.

 My Dad's Whole family was either into Oil Wells or Coal mines.  Basic knowledge on the subject has been drilled into me. (pun intended).

 Thanks for another Great Piece.

 One question,  I wonder how we could get a few 4D data spreads from Saudi Arabia?  Even if we had to do them ourselves!  (Yo, CIA goon,  Take this hammer and hit this plate real hard!  LOL,)

Actually there have been the odd references to 4D in some of the papers out of Aramco.  The only one I currently remember (and I haven't looked in a fair while) commented that it was still proving very difficult to detect the movement of the water:oil interface.
s/load noise/loud noise/g
This is a question I've been meaning to ask for a while and this seems as good a topic as any:  is there a map of the world that shows what areas have been explored for oil?  Not just where current fields are but what total land area has been explored and what has not.

Just something I've been wondering about since I started reading this site.  How much of the planet is actually left to explore for oil/NG?

"You want to compare brainpans? I won the Westinghouse prize when I was 12, big deal. Published at 19, so what. I got a double doctorate from MIT at 22, Chemistry and Geology. I taught at Princton for two and a half years. Why do I do this? Because the money's good, the scenery changes and they let me use explosives, ok?" -Rockhound (Steve Buscemi), "Armageddon" (1998)

Thanks for this series, HO.

Exsuburbanite, that is a good question, i've always wondered that myself. I am sure each company keeps a record of that, probably dating back to "day" one. I am sure there is a globe in someones office showing that, but a 3D model on the computer would be interesting to see too.

Speaking of 3D graphs, i like to go back to the plane table method, comparing the plane table method to 3D seismic graphs. While the 3D graphs may provide a better visual representation of a field's size, does the plane table method provide about the same information as 3D, or is it just rough data, whereas the 3D can provide more exact data? And how much more efficient is it to use the 3D graphing than the plane table method?

Oh, sorry I must have unintentionally misled you.  The plane table is used on the surface to provide locations of surface features (and these can include where, on the surface, there are hills, seeps etc).  Seismic surveys look down into the ground and give a picture of what the underground structures look like.  

Outside of the relatively local surveys done when one thinks there is an oilfield present, there is relatively little known of the rock beneath our feet.  I was going to post next about how we know where to look in the first place (gravimetric and magnetic surveys and the like) and they give a broader, though less detailed picture, and have been used much more extensively. But in most parts of the world the knowledge of the rock 100 ft below you is likely to be quite small, and such as it is, confined to relatively few folk.

This is a request for help.

Where can I get industry statsistics for oil and gas production in the US? I need annual production data by type of crude (light, medium, heavy, bitumen) and by production type (conventional, EOR, oil shale, ...).

Also data on operating costs and emissions would be good. I tried EIA and IPAA but there is only aggregated industry data.

I'm building a model of the US energy system for policy analysis.