Clay mineralogy of Tertiary formations in the northern Upper Rhine Graben – New insights from geothermal and hydrocarbon exploration
Löschan, Geraldine; Emmerich, Katja; Reinhold, Carsten; Reinecker, John
published: Jun 1, 2017
published online: Jul 31, 2017
ArtNo. ESP171016802002, Price: 29.00 €
Formations containing swellable clay minerals are often critical for drilling operations. In the Upper Rhine Graben, this problem especially applies to the Tertiary formations. For a more detailed characterisation of the specific formations and to assess potential correlations between borehole instabilities and clay mineralogy, 54 cutting samples from four different wells in the northern Upper Rhine Graben were subject to a clay mineralogical analysis by X-ray diffraction and measurements of the cation exchange capacity. The results show pure smectite only in the top section of the Hydrobia Beds (Miocene). With increasing depth, a transition to illite/smectite interstratified minerals with a decreasing share of smectite is observed, corresponding to a decreasing cation exchange capacity down to the Septarian Clay (Lower Oligocene). Whether this is a diagenetic effect or due to an initially higher smectite content in the Hydrobia Beds could not be finally clarified. Based on most recent temperature measurements, the maximum temperature for pure smectite in the Hydrobia Beds is at least 69 °C. One possible approach to explain the advanced illitisation within the Hydrobia Beds compared to older formations is considered the higher availability of potassium due to stronger dissolution of potassium feldspars resulting from higher water contents and slight overpressures. The results show that the given borehole instabilities cannot be correlated with the clay mineralogy. Most probably, they mainly result from specific stress conditions, high borehole inclinations and mechanical strains during drilling operations. However, the permanent share of smectite in interstratified minerals of up to 20 % may be a contributing factor in causing borehole instabilities.