Interpreting the Drilling Logs

As drilling operations continue, the geologist monitors the drilling rate and records the geology of the formations penetrated by the drill-bit. Cuttings caught by the shale shaker are bagged for every ten feet of hole that is drilled. By examining the drill cuttings under a binocular microscope, the geologist determines the type of rock and looks for signs of potential oil-staining. This information is recorded in the drill-cuttings log. A mud logger may also be run to detect the presence of oil or gas in the drill cuttings or drilling mud. In Small County, the sedimentary succession consists of shales, anhydrites, limestones, dolomites, and sandstones. Oil is found in the pore spaces of potential reservoir rocks, which are mostly limestones, dolomites, and sandstones.

Limestones and dolomites are often collectively refereed to as "carbonates", because limestone is calcium carbonate and dolomite is calcium magnesium carbonate. Carbonates are potential hydrocarbon reservoir rocks with oil or gas held in pore spaces between the mineral crystals or grains, fractures, and "vugs" (larger pore spaces created by dissolution of the rock). Clastic rocks are siliceous sedimentary rocks made up of particles derived from weathering of pre-existing rocks. Clastics range in grain-size from conglomerates, through sandstones and siltstones, to shales. Clastics are also potential hydrocarbon reservoirs and sandstones generally have a higher pore volume than carbonate rocks. The pores are formed between the grains of sand in the formation. Anhydrite is a mineral with no porosity formed from the evaporation of sea-water and is calcium sulphate. Shales are formed from the compaction of mud and are usually not reservoir rocks because of the lack of effective porosity, but instead forms a seal or barrier to fluid flow.

The geologist makes a written record of what he or she sees in the cuttings under the microscope and draws a graphic strip log, using symbols that represent the rocktype and the relative proportion of the constituents. Care is taken to discriminate "cavings", that is fragments of rock from shallower formations higher in the hole that do not represent the rock being drilled. Note is also made of oil staining that is observed. The geologist has a "prognosis" which tabulates the estimated depth to key formations, based on subsurface mapping and other exploration methods. As drilling proceeds, the geologist picks the depths of the actual tops of these key formations based on the drilling rate and drill cuttings for comparison with the prognosis. If the prospect is drilled on an anticline, then formation tops that are encountered at more shallow depths than anticipated are further encouragement of a positive structure. Prospects for oil in a formation are even more likely if the formation is updip from existing production in the formation in neighboring wells. The formation tops will also be used later in subsurface correlation and the development of maps in further exploration.

A simple example of part of a geologist's report using this process is shown, where drilling time and drill-cuttings information is combined to prepare a geologic strip log together with formation tops.

The geologist may observe an oil show in the drill-cuttings (shown by the dark-green field of the cuttings at the top of the Foursquare Dolomite) or slight staining (pale green field). This staining is one indication that an interval could produce oil. In the Small County exercise, you will decide which intervals of the well to perforate for production based primarily on the staining in the drill cuttings log together with the resistivity log, which responds to the saturations of water and oil in the pore space.