Based in Mahone Bay, nova scotia, agile libre is an independent publisher of technical books about the subsurface.  

What's the use of fieldwork?

It’s a question that every manager will ask his staff sooner or later: what is the point of oil company geologists doing fieldwork? To answer this question I plan to take you on a virtual field trip, and to make things more interesting I have invited along some other technical specialists: a geophysicist, a petrophysicist, and a reservoir engineer.

Stepping from the vehicle we are confronted by a world-class outcrop. The first thing that strikes you is the scale, with maybe 100 metres of exposed rocks. Thin sandstone beds are stacked into thicker packages, and observing the outcrop as a whole one really gets a feel for the cyclicity of the rocks; in this case, a series of coarsening-up parasequences, separated by thick shales. There is a real fractal feel to the rocks, with sedimentary features and bedding at all scales.

Turning to our specialists we ask the geophysicist what a seismic line through the outcrop would show. With a vertical resolution of about 20 m she estimates that we might see three reflectors in our 100 m of vertical section, certainly no more than four, but indicates the series of dipping clinoforms visible at the top of the outcrop might be recognizable. Four reflectors representing maybe a thousand individual beds doesn’t sound like much, but her data can provide useful information where no wells have been drilled. Here she can see for herself what the seismic cannot image, and calibrate her intuition to nature — her next interpretation will be different.

Meanwhile, the reservoir engineer scratches his head and tells us that, when he scales up the three-dimensional geological model to simulate production in the subsurface, his cells are typically 50 m by 50 m by 1 m in thickness. He is somewhat shocked how much heterogeneity he is failing to model — everyone is learning something here.

Stepping closer it becomes clear that even the thinner sandstone beds are separated by individual layers of shale. The sandstone beds feature a variety of sedimentary structures, including cross-bedding, ripples, and a variety of trace fossils, and I explain how these can help us identify the sedimentary environment in which the rocks were formed, in this case a shallow marine setting. I ask the petrophysicist what might show up on log data from a well, and he gives us a fascinating summary on the potential resolution of gamma ray, sonic, and other logs. The gamma-ray log, for example, can resolve beds down to around 30 cm in thickness. He pulls out a formation micro-imager log (which uses resistivity measurements) to demonstrate how even burrows can be imaged using this tool. Conversation turns to another reservoir that has challenged the reservoir engineer. He is helping to develop a new oil-sands lease in Alberta, but despite the sands exhibiting high porosities production has been unpredictable. We hop back into the vehicle and head north. In a few kilometres, we reach the ancient shoreline, where thick estuarine deposits outcrop. Sandy tidal flat deposits are cut by a series of channels, and in places, the channels have amalgamated into thick, stacked sandstone intervals. Approaching the outcrop we can see that each channel sandbody is cut by numerous thin mudstone beds.

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I explain that these muds represent ebb tidal deposits, and that where they form baffles to flow, this can significantly affect production. The good news is that we can measure them in nature and model the subsurface better. A lightbulb goes off above the engineer’s head, and he starts taking photographs — something to show the boss when he gets back to the office. Here is a perfect analog to his troublesome reservoir, and the outcrop even shows some oil staining. We collect a few samples of sandstone to take back to the lab for analysis, to compare with core samples from the reservoir itself, a kilometre below the surface.

It is abundantly clear from our virtual trip that every outcrop provides a natural laboratory to explore rock geometries and properties. In addition, the rocks are a classroom in which everyone from the geologist to the technical specialist can learn about each others’ disciplines. So I suggest that you get yourself out there and start observing the rocks.

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