Petrophysical and mechanical properties of the lower Silurian perspective oil/gas shales of Lithuania

Lithuania is situated in the central and eastern parts of the Baltic Sedimentary Basin. Several prospective Lower Palaeozoic gas shale formations are identified in the basin. Most of the published studies concern the organic chemistry and evaluation of gas potential, whereas only scarce data are provided on the petrophysical and mechanical properties of oil/gas shales. It is typical also for other basins, essentially concerning parameters like cation exchange capacity, swelling index, erodibility etc. Mineralogical, petrophysical and mechanical properties of the Lower Silurian organic rich shales of west Lithuania (within petroleum window area), considered as a most prospective body for unconventional oil/gas exploitation, are discussed in the present study. The most distinct feature due to anomalous organic matter (OM) content is the Middle Llandovery black shales that are compatible to the Llandovery “hot” shales defined in sedimentary basins of Africa and Arabian Peninsula. Brittleness index (BI) of shales was calculated from mineral composition, shales are classified as moderately brittle (predominate) to moderately ductile. It was stated that clay content and quartz content directly correlate with the BI, despite that they have a reverse effect on shale brittleness. The unconfined compressive strength and tensile strength also indicate low brittleness of shales that is accounted to high natural fracturing of the Lower Silurian shales. Low porosity, varying from 12 to 16% to less than 3%, have no discernible effect on shale strength properties. Cation exchange capacity measurements show low values that is related to high illite and detrital minerals content in shales. Organic matter (OM) content have noticeable statistically positive correlation with gamma ray intensity in organic most rich “hot” shales. Roller oven technique was applied to measure the shale erodibility. Experiments show that application of the tap water leads to high erodibility of shales, they are classified as the moderate to high erodibility rocks. Reversely, diesel considerably reduces erodibility index causing low erodibility of shales. As regards the acid solutions, the best effect in decreasing shale erodibility is measured when K2Cl (7%) solution is applied (moderate erodibility). Generally low erosion susceptibility is explained in terms of high content of detrital silt and predominance of illite and chlorite in the clay fraction. Cation suction time (CST) characterises the s