Well tops - an essential element for regional exploration

21 November 2016

Well tops or lithostratigraphic well information is often, if not always, an essential ingredient of regional exploration studies. Every Operator drilling a well in the UKCS produces a so-called composite well log, which shows the lithostratigraphic breakdown of the well on the left hand side (see figure below). Geologists use this information and compare it to other wells they are familiar with because it provides hints on thickness trends of important reservoir units. 

Any geologist working on the UKCS can use a database from the Oil and Gas Authority (OGA), which contain a wealth of lithostratigraphic and chronostratigraphic information for released wells in the UKCS. This is a good starting point for gaining insight in the spatial distribution of certain stratigraphic intervals. Let’s describe briefly what this database contains with regards to well tops. 

There are currently around 2400 lithostratigraphic names given to wells in the UKCS. The explanation for such a high number of names is that in many cases “local” names were used. A good example of a very local name is the “15/16-6 Sand” or “J166” well top, which is restricted to only one well. There are many examples of lithostratigraphic names which only occur a handful of times: “3rd Hot Shale”, “A1 Sandstone Unit”, “Palaeocene Sand Fmn. II Marker” to name but a few.  

It doesn’t take a lot of imagination to conclude that working with such a myriad of lithostratigraphic names is not ideal when it comes to creating distribution maps for specific intervals. What ultimately needs to be done is to standardise the names in such a way that units of the same nature (being it based on lithostratigraphy of chronostratigraphy) bear the same name.

For the Regional Mapping project we are currently working on with the OGA, we are using a well top database that was internally built over the years in a consistent way. About half of the 2000 exploration and appraisal wells drilled in the Moray Firth and the Central North Sea are already in this database and we keep on adding “new” wells as the project progresses. This ensures the maps we ultimately produce, and which will be made publicly available, will be wholly in line with the stratigraphic well interpretations.

  oga blog 2 vertical graph

This image shows part of the composite well log for the well 21/01a- 12 in the Central North Sea. The first two columns at the far left indicate the age of the interval drilled, followed by the depth and subsequently followed by the lithostratigraphic names (group and formation). 

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Project overview:

Lloyd's Register secured two major UKCS projects with the Oil and Gas Authority (OGA) for the UK Continental Shelf (UKCS), for the provision of Regional Exploration Maps and Regional and Field Support Engineering Services to help the OGA independently assess remaining undiscovered resources and improve geotechnical understanding. Full detail of the project win can be read here.

This is article two in a series of posts dedicated to our project for the OGA. To read the previous blog post click here.

Henk Kombrink 
Senior Geologist, LR